CVE-2021-46933 (GCVE-0-2021-46933)

Vulnerability from cvelistv5 – Published: 2024-02-27 09:44 – Updated: 2025-05-04 07:00
VLAI?
Title
usb: gadget: f_fs: Clear ffs_eventfd in ffs_data_clear.
Summary
In the Linux kernel, the following vulnerability has been resolved: usb: gadget: f_fs: Clear ffs_eventfd in ffs_data_clear. ffs_data_clear is indirectly called from both ffs_fs_kill_sb and ffs_ep0_release, so it ends up being called twice when userland closes ep0 and then unmounts f_fs. If userland provided an eventfd along with function's USB descriptors, it ends up calling eventfd_ctx_put as many times, causing a refcount underflow. NULL-ify ffs_eventfd to prevent these extraneous eventfd_ctx_put calls. Also, set epfiles to NULL right after de-allocating it, for readability. For completeness, ffs_data_clear actually ends up being called thrice, the last call being before the whole ffs structure gets freed, so when this specific sequence happens there is a second underflow happening (but not being reported): /sys/kernel/debug/tracing# modprobe usb_f_fs /sys/kernel/debug/tracing# echo ffs_data_clear > set_ftrace_filter /sys/kernel/debug/tracing# echo function > current_tracer /sys/kernel/debug/tracing# echo 1 > tracing_on (setup gadget, run and kill function userland process, teardown gadget) /sys/kernel/debug/tracing# echo 0 > tracing_on /sys/kernel/debug/tracing# cat trace smartcard-openp-436 [000] ..... 1946.208786: ffs_data_clear <-ffs_data_closed smartcard-openp-431 [000] ..... 1946.279147: ffs_data_clear <-ffs_data_closed smartcard-openp-431 [000] .n... 1946.905512: ffs_data_clear <-ffs_data_put Warning output corresponding to above trace: [ 1946.284139] WARNING: CPU: 0 PID: 431 at lib/refcount.c:28 refcount_warn_saturate+0x110/0x15c [ 1946.293094] refcount_t: underflow; use-after-free. [ 1946.298164] Modules linked in: usb_f_ncm(E) u_ether(E) usb_f_fs(E) hci_uart(E) btqca(E) btrtl(E) btbcm(E) btintel(E) bluetooth(E) nls_ascii(E) nls_cp437(E) vfat(E) fat(E) bcm2835_v4l2(CE) bcm2835_mmal_vchiq(CE) videobuf2_vmalloc(E) videobuf2_memops(E) sha512_generic(E) videobuf2_v4l2(E) sha512_arm(E) videobuf2_common(E) videodev(E) cpufreq_dt(E) snd_bcm2835(CE) brcmfmac(E) mc(E) vc4(E) ctr(E) brcmutil(E) snd_soc_core(E) snd_pcm_dmaengine(E) drbg(E) snd_pcm(E) snd_timer(E) snd(E) soundcore(E) drm_kms_helper(E) cec(E) ansi_cprng(E) rc_core(E) syscopyarea(E) raspberrypi_cpufreq(E) sysfillrect(E) sysimgblt(E) cfg80211(E) max17040_battery(OE) raspberrypi_hwmon(E) fb_sys_fops(E) regmap_i2c(E) ecdh_generic(E) rfkill(E) ecc(E) bcm2835_rng(E) rng_core(E) vchiq(CE) leds_gpio(E) libcomposite(E) fuse(E) configfs(E) ip_tables(E) x_tables(E) autofs4(E) ext4(E) crc16(E) mbcache(E) jbd2(E) crc32c_generic(E) sdhci_iproc(E) sdhci_pltfm(E) sdhci(E) [ 1946.399633] CPU: 0 PID: 431 Comm: smartcard-openp Tainted: G C OE 5.15.0-1-rpi #1 Debian 5.15.3-1 [ 1946.417950] Hardware name: BCM2835 [ 1946.425442] Backtrace: [ 1946.432048] [<c08d60a0>] (dump_backtrace) from [<c08d62ec>] (show_stack+0x20/0x24) [ 1946.448226] r7:00000009 r6:0000001c r5:c04a948c r4:c0a64e2c [ 1946.458412] [<c08d62cc>] (show_stack) from [<c08d9ae0>] (dump_stack+0x28/0x30) [ 1946.470380] [<c08d9ab8>] (dump_stack) from [<c0123500>] (__warn+0xe8/0x154) [ 1946.482067] r5:c04a948c r4:c0a71dc8 [ 1946.490184] [<c0123418>] (__warn) from [<c08d6948>] (warn_slowpath_fmt+0xa0/0xe4) [ 1946.506758] r7:00000009 r6:0000001c r5:c0a71dc8 r4:c0a71e04 [ 1946.517070] [<c08d68ac>] (warn_slowpath_fmt) from [<c04a948c>] (refcount_warn_saturate+0x110/0x15c) [ 1946.535309] r8:c0100224 r7:c0dfcb84 r6:ffffffff r5:c3b84c00 r4:c24a17c0 [ 1946.546708] [<c04a937c>] (refcount_warn_saturate) from [<c0380134>] (eventfd_ctx_put+0x48/0x74) [ 1946.564476] [<c03800ec>] (eventfd_ctx_put) from [<bf5464e8>] (ffs_data_clear+0xd0/0x118 [usb_f_fs]) [ 1946.582664] r5:c3b84c00 r4:c2695b00 [ 1946.590668] [<bf546418>] (ffs_data_clear [usb_f_fs]) from [<bf547cc0>] (ffs_data_closed+0x9c/0x150 [usb_f_fs]) [ 1946.609608] r5:bf54d014 r4:c2695b00 [ 1946.617522] [<bf547c24>] (ffs_data_closed [usb_f_fs]) from [<bf547da0>] (ffs_fs_kill_sb+0x2c/0x30 [usb_f_fs]) [ 1946.636217] r7:c0dfcb ---truncated---
Severity ?
5.5 (Medium)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
CWE
  • CWE-476 - NULL Pointer Dereference
Assigner
References
Impacted products
Vendor Product Version
Linux Linux Affected: 5e33f6fdf735cda1d4580fe6f1878da05718fe73 , < f976dd7011150244a7ba820f2c331e9fb253befa (git)
Affected: 5e33f6fdf735cda1d4580fe6f1878da05718fe73 , < cc8c8028c21b2a3842a1e98e99e55028df275919 (git)
Affected: 5e33f6fdf735cda1d4580fe6f1878da05718fe73 , < 52500239e3f2d6fc77b6f58632a9fb98fe74ac09 (git)
Affected: 5e33f6fdf735cda1d4580fe6f1878da05718fe73 , < 33f6a0cbb7772146e1c11f38028fffbfed14728b (git)
Affected: 5e33f6fdf735cda1d4580fe6f1878da05718fe73 , < 240fc586e83d645912accce081a48aa63a45f6ee (git)
Affected: 5e33f6fdf735cda1d4580fe6f1878da05718fe73 , < 1c4ace3e6b8575745c50dca9e76e0021e697d645 (git)
Affected: 5e33f6fdf735cda1d4580fe6f1878da05718fe73 , < ebef2aa29f370b5096c16020c104e393192ef684 (git)
Affected: 5e33f6fdf735cda1d4580fe6f1878da05718fe73 , < b1e0887379422975f237d43d8839b751a6bcf154 (git)
Create a notification for this product.
    Linux Linux Affected: 4.0
Unaffected: 0 , < 4.0 (semver)
Unaffected: 4.4.298 , ≤ 4.4.* (semver)
Unaffected: 4.9.296 , ≤ 4.9.* (semver)
Unaffected: 4.14.261 , ≤ 4.14.* (semver)
Unaffected: 4.19.224 , ≤ 4.19.* (semver)
Unaffected: 5.4.170 , ≤ 5.4.* (semver)
Unaffected: 5.10.90 , ≤ 5.10.* (semver)
Unaffected: 5.15.13 , ≤ 5.15.* (semver)
Unaffected: 5.16 , ≤ * (original_commit_for_fix)
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          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nusb: gadget: f_fs: Clear ffs_eventfd in ffs_data_clear.\n\nffs_data_clear is indirectly called from both ffs_fs_kill_sb and\nffs_ep0_release, so it ends up being called twice when userland closes ep0\nand then unmounts f_fs.\nIf userland provided an eventfd along with function\u0027s USB descriptors, it\nends up calling eventfd_ctx_put as many times, causing a refcount\nunderflow.\nNULL-ify ffs_eventfd to prevent these extraneous eventfd_ctx_put calls.\n\nAlso, set epfiles to NULL right after de-allocating it, for readability.\n\nFor completeness, ffs_data_clear actually ends up being called thrice, the\nlast call being before the whole ffs structure gets freed, so when this\nspecific sequence happens there is a second underflow happening (but not\nbeing reported):\n\n/sys/kernel/debug/tracing# modprobe usb_f_fs\n/sys/kernel/debug/tracing# echo ffs_data_clear \u003e set_ftrace_filter\n/sys/kernel/debug/tracing# echo function \u003e current_tracer\n/sys/kernel/debug/tracing# echo 1 \u003e tracing_on\n(setup gadget, run and kill function userland process, teardown gadget)\n/sys/kernel/debug/tracing# echo 0 \u003e tracing_on\n/sys/kernel/debug/tracing# cat trace\n smartcard-openp-436     [000] .....  1946.208786: ffs_data_clear \u003c-ffs_data_closed\n smartcard-openp-431     [000] .....  1946.279147: ffs_data_clear \u003c-ffs_data_closed\n smartcard-openp-431     [000] .n...  1946.905512: ffs_data_clear \u003c-ffs_data_put\n\nWarning output corresponding to above trace:\n[ 1946.284139] WARNING: CPU: 0 PID: 431 at lib/refcount.c:28 refcount_warn_saturate+0x110/0x15c\n[ 1946.293094] refcount_t: underflow; use-after-free.\n[ 1946.298164] Modules linked in: usb_f_ncm(E) u_ether(E) usb_f_fs(E) hci_uart(E) btqca(E) btrtl(E) btbcm(E) btintel(E) bluetooth(E) nls_ascii(E) nls_cp437(E) vfat(E) fat(E) bcm2835_v4l2(CE) bcm2835_mmal_vchiq(CE) videobuf2_vmalloc(E) videobuf2_memops(E) sha512_generic(E) videobuf2_v4l2(E) sha512_arm(E) videobuf2_common(E) videodev(E) cpufreq_dt(E) snd_bcm2835(CE) brcmfmac(E) mc(E) vc4(E) ctr(E) brcmutil(E) snd_soc_core(E) snd_pcm_dmaengine(E) drbg(E) snd_pcm(E) snd_timer(E) snd(E) soundcore(E) drm_kms_helper(E) cec(E) ansi_cprng(E) rc_core(E) syscopyarea(E) raspberrypi_cpufreq(E) sysfillrect(E) sysimgblt(E) cfg80211(E) max17040_battery(OE) raspberrypi_hwmon(E) fb_sys_fops(E) regmap_i2c(E) ecdh_generic(E) rfkill(E) ecc(E) bcm2835_rng(E) rng_core(E) vchiq(CE) leds_gpio(E) libcomposite(E) fuse(E) configfs(E) ip_tables(E) x_tables(E) autofs4(E) ext4(E) crc16(E) mbcache(E) jbd2(E) crc32c_generic(E) sdhci_iproc(E) sdhci_pltfm(E) sdhci(E)\n[ 1946.399633] CPU: 0 PID: 431 Comm: smartcard-openp Tainted: G         C OE     5.15.0-1-rpi #1  Debian 5.15.3-1\n[ 1946.417950] Hardware name: BCM2835\n[ 1946.425442] Backtrace:\n[ 1946.432048] [\u003cc08d60a0\u003e] (dump_backtrace) from [\u003cc08d62ec\u003e] (show_stack+0x20/0x24)\n[ 1946.448226]  r7:00000009 r6:0000001c r5:c04a948c r4:c0a64e2c\n[ 1946.458412] [\u003cc08d62cc\u003e] (show_stack) from [\u003cc08d9ae0\u003e] (dump_stack+0x28/0x30)\n[ 1946.470380] [\u003cc08d9ab8\u003e] (dump_stack) from [\u003cc0123500\u003e] (__warn+0xe8/0x154)\n[ 1946.482067]  r5:c04a948c r4:c0a71dc8\n[ 1946.490184] [\u003cc0123418\u003e] (__warn) from [\u003cc08d6948\u003e] (warn_slowpath_fmt+0xa0/0xe4)\n[ 1946.506758]  r7:00000009 r6:0000001c r5:c0a71dc8 r4:c0a71e04\n[ 1946.517070] [\u003cc08d68ac\u003e] (warn_slowpath_fmt) from [\u003cc04a948c\u003e] (refcount_warn_saturate+0x110/0x15c)\n[ 1946.535309]  r8:c0100224 r7:c0dfcb84 r6:ffffffff r5:c3b84c00 r4:c24a17c0\n[ 1946.546708] [\u003cc04a937c\u003e] (refcount_warn_saturate) from [\u003cc0380134\u003e] (eventfd_ctx_put+0x48/0x74)\n[ 1946.564476] [\u003cc03800ec\u003e] (eventfd_ctx_put) from [\u003cbf5464e8\u003e] (ffs_data_clear+0xd0/0x118 [usb_f_fs])\n[ 1946.582664]  r5:c3b84c00 r4:c2695b00\n[ 1946.590668] [\u003cbf546418\u003e] (ffs_data_clear [usb_f_fs]) from [\u003cbf547cc0\u003e] (ffs_data_closed+0x9c/0x150 [usb_f_fs])\n[ 1946.609608]  r5:bf54d014 r4:c2695b00\n[ 1946.617522] [\u003cbf547c24\u003e] (ffs_data_closed [usb_f_fs]) from [\u003cbf547da0\u003e] (ffs_fs_kill_sb+0x2c/0x30 [usb_f_fs])\n[ 1946.636217]  r7:c0dfcb\n---truncated---"
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          "url": "https://git.kernel.org/stable/c/cc8c8028c21b2a3842a1e98e99e55028df275919"
        },
        {
          "url": "https://git.kernel.org/stable/c/52500239e3f2d6fc77b6f58632a9fb98fe74ac09"
        },
        {
          "url": "https://git.kernel.org/stable/c/33f6a0cbb7772146e1c11f38028fffbfed14728b"
        },
        {
          "url": "https://git.kernel.org/stable/c/240fc586e83d645912accce081a48aa63a45f6ee"
        },
        {
          "url": "https://git.kernel.org/stable/c/1c4ace3e6b8575745c50dca9e76e0021e697d645"
        },
        {
          "url": "https://git.kernel.org/stable/c/ebef2aa29f370b5096c16020c104e393192ef684"
        },
        {
          "url": "https://git.kernel.org/stable/c/b1e0887379422975f237d43d8839b751a6bcf154"
        }
      ],
      "title": "usb: gadget: f_fs: Clear ffs_eventfd in ffs_data_clear.",
      "x_generator": {
        "engine": "bippy-1.2.0"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2021-46933",
    "datePublished": "2024-02-27T09:44:00.758Z",
    "dateReserved": "2024-02-25T13:45:52.720Z",
    "dateUpdated": "2025-05-04T07:00:37.229Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.1",
  "vulnerability-lookup:meta": {
    "vulnrichment": {
      "containers": "{\"adp\": [{\"title\": \"CVE Program Container\", \"references\": [{\"url\": \"https://git.kernel.org/stable/c/f976dd7011150244a7ba820f2c331e9fb253befa\", \"tags\": [\"x_transferred\"]}, {\"url\": \"https://git.kernel.org/stable/c/cc8c8028c21b2a3842a1e98e99e55028df275919\", \"tags\": [\"x_transferred\"]}, {\"url\": \"https://git.kernel.org/stable/c/52500239e3f2d6fc77b6f58632a9fb98fe74ac09\", \"tags\": [\"x_transferred\"]}, {\"url\": \"https://git.kernel.org/stable/c/33f6a0cbb7772146e1c11f38028fffbfed14728b\", \"tags\": [\"x_transferred\"]}, {\"url\": \"https://git.kernel.org/stable/c/240fc586e83d645912accce081a48aa63a45f6ee\", \"tags\": [\"x_transferred\"]}, {\"url\": \"https://git.kernel.org/stable/c/1c4ace3e6b8575745c50dca9e76e0021e697d645\", \"tags\": [\"x_transferred\"]}, {\"url\": \"https://git.kernel.org/stable/c/ebef2aa29f370b5096c16020c104e393192ef684\", \"tags\": [\"x_transferred\"]}, {\"url\": \"https://git.kernel.org/stable/c/b1e0887379422975f237d43d8839b751a6bcf154\", \"tags\": [\"x_transferred\"]}], \"providerMetadata\": {\"orgId\": \"af854a3a-2127-422b-91ae-364da2661108\", \"shortName\": \"CVE\", \"dateUpdated\": \"2024-08-04T05:17:43.071Z\"}}, {\"title\": \"CISA ADP Vulnrichment\", \"metrics\": [{\"cvssV3_1\": {\"scope\": \"UNCHANGED\", \"version\": \"3.1\", \"baseScore\": 5.5, \"attackVector\": \"LOCAL\", \"baseSeverity\": \"MEDIUM\", \"vectorString\": \"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H\", \"integrityImpact\": \"NONE\", \"userInteraction\": \"NONE\", \"attackComplexity\": \"LOW\", \"availabilityImpact\": \"HIGH\", \"privilegesRequired\": \"LOW\", \"confidentialityImpact\": \"NONE\"}}, {\"other\": {\"type\": \"ssvc\", \"content\": {\"id\": \"CVE-2021-46933\", \"role\": \"CISA Coordinator\", \"options\": [{\"Exploitation\": \"none\"}, {\"Automatable\": \"no\"}, {\"Technical Impact\": \"partial\"}], \"version\": \"2.0.3\", \"timestamp\": \"2025-04-22T16:12:25.365291Z\"}}}], \"problemTypes\": [{\"descriptions\": [{\"lang\": \"en\", \"type\": \"CWE\", \"cweId\": \"CWE-476\", \"description\": \"CWE-476 NULL Pointer Dereference\"}]}], \"providerMetadata\": {\"orgId\": \"134c704f-9b21-4f2e-91b3-4a467353bcc0\", \"shortName\": \"CISA-ADP\", \"dateUpdated\": \"2024-07-05T15:20:37.728Z\"}}], \"cna\": {\"title\": \"usb: gadget: f_fs: Clear ffs_eventfd in ffs_data_clear.\", \"affected\": [{\"repo\": \"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\", \"vendor\": \"Linux\", \"product\": \"Linux\", \"versions\": [{\"status\": \"affected\", \"version\": \"5e33f6fdf735cda1d4580fe6f1878da05718fe73\", \"lessThan\": \"f976dd7011150244a7ba820f2c331e9fb253befa\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"5e33f6fdf735cda1d4580fe6f1878da05718fe73\", \"lessThan\": \"cc8c8028c21b2a3842a1e98e99e55028df275919\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"5e33f6fdf735cda1d4580fe6f1878da05718fe73\", \"lessThan\": \"52500239e3f2d6fc77b6f58632a9fb98fe74ac09\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"5e33f6fdf735cda1d4580fe6f1878da05718fe73\", \"lessThan\": \"33f6a0cbb7772146e1c11f38028fffbfed14728b\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"5e33f6fdf735cda1d4580fe6f1878da05718fe73\", \"lessThan\": \"240fc586e83d645912accce081a48aa63a45f6ee\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"5e33f6fdf735cda1d4580fe6f1878da05718fe73\", \"lessThan\": \"1c4ace3e6b8575745c50dca9e76e0021e697d645\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"5e33f6fdf735cda1d4580fe6f1878da05718fe73\", \"lessThan\": \"ebef2aa29f370b5096c16020c104e393192ef684\", \"versionType\": \"git\"}, {\"status\": \"affected\", \"version\": \"5e33f6fdf735cda1d4580fe6f1878da05718fe73\", \"lessThan\": \"b1e0887379422975f237d43d8839b751a6bcf154\", \"versionType\": \"git\"}], \"programFiles\": [\"drivers/usb/gadget/function/f_fs.c\"], \"defaultStatus\": \"unaffected\"}, {\"repo\": \"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\", \"vendor\": \"Linux\", \"product\": \"Linux\", \"versions\": [{\"status\": \"affected\", \"version\": \"4.0\"}, {\"status\": \"unaffected\", \"version\": \"0\", \"lessThan\": \"4.0\", \"versionType\": \"semver\"}, {\"status\": \"unaffected\", \"version\": \"4.4.298\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"4.4.*\"}, {\"status\": \"unaffected\", \"version\": \"4.9.296\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"4.9.*\"}, {\"status\": \"unaffected\", \"version\": \"4.14.261\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"4.14.*\"}, {\"status\": \"unaffected\", \"version\": \"4.19.224\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"4.19.*\"}, {\"status\": \"unaffected\", \"version\": \"5.4.170\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"5.4.*\"}, {\"status\": \"unaffected\", \"version\": \"5.10.90\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"5.10.*\"}, {\"status\": \"unaffected\", \"version\": \"5.15.13\", \"versionType\": \"semver\", \"lessThanOrEqual\": \"5.15.*\"}, {\"status\": \"unaffected\", \"version\": \"5.16\", \"versionType\": \"original_commit_for_fix\", \"lessThanOrEqual\": \"*\"}], \"programFiles\": [\"drivers/usb/gadget/function/f_fs.c\"], \"defaultStatus\": \"affected\"}], \"references\": [{\"url\": \"https://git.kernel.org/stable/c/f976dd7011150244a7ba820f2c331e9fb253befa\"}, {\"url\": \"https://git.kernel.org/stable/c/cc8c8028c21b2a3842a1e98e99e55028df275919\"}, {\"url\": \"https://git.kernel.org/stable/c/52500239e3f2d6fc77b6f58632a9fb98fe74ac09\"}, {\"url\": \"https://git.kernel.org/stable/c/33f6a0cbb7772146e1c11f38028fffbfed14728b\"}, {\"url\": \"https://git.kernel.org/stable/c/240fc586e83d645912accce081a48aa63a45f6ee\"}, {\"url\": \"https://git.kernel.org/stable/c/1c4ace3e6b8575745c50dca9e76e0021e697d645\"}, {\"url\": \"https://git.kernel.org/stable/c/ebef2aa29f370b5096c16020c104e393192ef684\"}, {\"url\": \"https://git.kernel.org/stable/c/b1e0887379422975f237d43d8839b751a6bcf154\"}], \"x_generator\": {\"engine\": \"bippy-1.2.0\"}, \"descriptions\": [{\"lang\": \"en\", \"value\": \"In the Linux kernel, the following vulnerability has been resolved:\\n\\nusb: gadget: f_fs: Clear ffs_eventfd in ffs_data_clear.\\n\\nffs_data_clear is indirectly called from both ffs_fs_kill_sb and\\nffs_ep0_release, so it ends up being called twice when userland closes ep0\\nand then unmounts f_fs.\\nIf userland provided an eventfd along with function\u0027s USB descriptors, it\\nends up calling eventfd_ctx_put as many times, causing a refcount\\nunderflow.\\nNULL-ify ffs_eventfd to prevent these extraneous eventfd_ctx_put calls.\\n\\nAlso, set epfiles to NULL right after de-allocating it, for readability.\\n\\nFor completeness, ffs_data_clear actually ends up being called thrice, the\\nlast call being before the whole ffs structure gets freed, so when this\\nspecific sequence happens there is a second underflow happening (but not\\nbeing reported):\\n\\n/sys/kernel/debug/tracing# modprobe usb_f_fs\\n/sys/kernel/debug/tracing# echo ffs_data_clear \u003e set_ftrace_filter\\n/sys/kernel/debug/tracing# echo function \u003e current_tracer\\n/sys/kernel/debug/tracing# echo 1 \u003e tracing_on\\n(setup gadget, run and kill function userland process, teardown gadget)\\n/sys/kernel/debug/tracing# echo 0 \u003e tracing_on\\n/sys/kernel/debug/tracing# cat trace\\n smartcard-openp-436     [000] .....  1946.208786: ffs_data_clear \u003c-ffs_data_closed\\n smartcard-openp-431     [000] .....  1946.279147: ffs_data_clear \u003c-ffs_data_closed\\n smartcard-openp-431     [000] .n...  1946.905512: ffs_data_clear \u003c-ffs_data_put\\n\\nWarning output corresponding to above trace:\\n[ 1946.284139] WARNING: CPU: 0 PID: 431 at lib/refcount.c:28 refcount_warn_saturate+0x110/0x15c\\n[ 1946.293094] refcount_t: underflow; use-after-free.\\n[ 1946.298164] Modules linked in: usb_f_ncm(E) u_ether(E) usb_f_fs(E) hci_uart(E) btqca(E) btrtl(E) btbcm(E) btintel(E) bluetooth(E) nls_ascii(E) nls_cp437(E) vfat(E) fat(E) bcm2835_v4l2(CE) bcm2835_mmal_vchiq(CE) videobuf2_vmalloc(E) videobuf2_memops(E) sha512_generic(E) videobuf2_v4l2(E) sha512_arm(E) videobuf2_common(E) videodev(E) cpufreq_dt(E) snd_bcm2835(CE) brcmfmac(E) mc(E) vc4(E) ctr(E) brcmutil(E) snd_soc_core(E) snd_pcm_dmaengine(E) drbg(E) snd_pcm(E) snd_timer(E) snd(E) soundcore(E) drm_kms_helper(E) cec(E) ansi_cprng(E) rc_core(E) syscopyarea(E) raspberrypi_cpufreq(E) sysfillrect(E) sysimgblt(E) cfg80211(E) max17040_battery(OE) raspberrypi_hwmon(E) fb_sys_fops(E) regmap_i2c(E) ecdh_generic(E) rfkill(E) ecc(E) bcm2835_rng(E) rng_core(E) vchiq(CE) leds_gpio(E) libcomposite(E) fuse(E) configfs(E) ip_tables(E) x_tables(E) autofs4(E) ext4(E) crc16(E) mbcache(E) jbd2(E) crc32c_generic(E) sdhci_iproc(E) sdhci_pltfm(E) sdhci(E)\\n[ 1946.399633] CPU: 0 PID: 431 Comm: smartcard-openp Tainted: G         C OE     5.15.0-1-rpi #1  Debian 5.15.3-1\\n[ 1946.417950] Hardware name: BCM2835\\n[ 1946.425442] Backtrace:\\n[ 1946.432048] [\u003cc08d60a0\u003e] (dump_backtrace) from [\u003cc08d62ec\u003e] (show_stack+0x20/0x24)\\n[ 1946.448226]  r7:00000009 r6:0000001c r5:c04a948c r4:c0a64e2c\\n[ 1946.458412] [\u003cc08d62cc\u003e] (show_stack) from [\u003cc08d9ae0\u003e] (dump_stack+0x28/0x30)\\n[ 1946.470380] [\u003cc08d9ab8\u003e] (dump_stack) from [\u003cc0123500\u003e] (__warn+0xe8/0x154)\\n[ 1946.482067]  r5:c04a948c r4:c0a71dc8\\n[ 1946.490184] [\u003cc0123418\u003e] (__warn) from [\u003cc08d6948\u003e] (warn_slowpath_fmt+0xa0/0xe4)\\n[ 1946.506758]  r7:00000009 r6:0000001c r5:c0a71dc8 r4:c0a71e04\\n[ 1946.517070] [\u003cc08d68ac\u003e] (warn_slowpath_fmt) from [\u003cc04a948c\u003e] (refcount_warn_saturate+0x110/0x15c)\\n[ 1946.535309]  r8:c0100224 r7:c0dfcb84 r6:ffffffff r5:c3b84c00 r4:c24a17c0\\n[ 1946.546708] [\u003cc04a937c\u003e] (refcount_warn_saturate) from [\u003cc0380134\u003e] (eventfd_ctx_put+0x48/0x74)\\n[ 1946.564476] [\u003cc03800ec\u003e] (eventfd_ctx_put) from [\u003cbf5464e8\u003e] (ffs_data_clear+0xd0/0x118 [usb_f_fs])\\n[ 1946.582664]  r5:c3b84c00 r4:c2695b00\\n[ 1946.590668] [\u003cbf546418\u003e] (ffs_data_clear [usb_f_fs]) from [\u003cbf547cc0\u003e] (ffs_data_closed+0x9c/0x150 [usb_f_fs])\\n[ 1946.609608]  r5:bf54d014 r4:c2695b00\\n[ 1946.617522] [\u003cbf547c24\u003e] (ffs_data_closed [usb_f_fs]) from [\u003cbf547da0\u003e] (ffs_fs_kill_sb+0x2c/0x30 [usb_f_fs])\\n[ 1946.636217]  r7:c0dfcb\\n---truncated---\"}], \"cpeApplicability\": [{\"nodes\": [{\"negate\": false, \"cpeMatch\": [{\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"4.4.298\", \"versionStartIncluding\": \"4.0\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"4.9.296\", \"versionStartIncluding\": \"4.0\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"4.14.261\", \"versionStartIncluding\": \"4.0\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"4.19.224\", \"versionStartIncluding\": \"4.0\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"5.4.170\", \"versionStartIncluding\": \"4.0\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"5.10.90\", \"versionStartIncluding\": \"4.0\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"5.15.13\", \"versionStartIncluding\": \"4.0\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"5.16\", \"versionStartIncluding\": \"4.0\"}], \"operator\": \"OR\"}]}], \"providerMetadata\": {\"orgId\": \"416baaa9-dc9f-4396-8d5f-8c081fb06d67\", \"shortName\": \"Linux\", \"dateUpdated\": \"2025-05-04T07:00:37.229Z\"}}}",
      "cveMetadata": "{\"cveId\": \"CVE-2021-46933\", \"state\": \"PUBLISHED\", \"dateUpdated\": \"2025-05-04T07:00:37.229Z\", \"dateReserved\": \"2024-02-25T13:45:52.720Z\", \"assignerOrgId\": \"416baaa9-dc9f-4396-8d5f-8c081fb06d67\", \"datePublished\": \"2024-02-27T09:44:00.758Z\", \"assignerShortName\": \"Linux\"}",
      "dataType": "CVE_RECORD",
      "dataVersion": "5.1"
    }
  }
}


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Sightings

Author Source Type Date

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.



Detection rules are retrieved from Rulezet.