CVE-2021-47094 (GCVE-0-2021-47094)

Vulnerability from cvelistv5 – Published: 2024-03-04 18:10 – Updated: 2025-05-04 07:04
VLAI?
Title
KVM: x86/mmu: Don't advance iterator after restart due to yielding
Summary
In the Linux kernel, the following vulnerability has been resolved: KVM: x86/mmu: Don't advance iterator after restart due to yielding After dropping mmu_lock in the TDP MMU, restart the iterator during tdp_iter_next() and do not advance the iterator. Advancing the iterator results in skipping the top-level SPTE and all its children, which is fatal if any of the skipped SPTEs were not visited before yielding. When zapping all SPTEs, i.e. when min_level == root_level, restarting the iter and then invoking tdp_iter_next() is always fatal if the current gfn has as a valid SPTE, as advancing the iterator results in try_step_side() skipping the current gfn, which wasn't visited before yielding. Sprinkle WARNs on iter->yielded being true in various helpers that are often used in conjunction with yielding, and tag the helper with __must_check to reduce the probabily of improper usage. Failing to zap a top-level SPTE manifests in one of two ways. If a valid SPTE is skipped by both kvm_tdp_mmu_zap_all() and kvm_tdp_mmu_put_root(), the shadow page will be leaked and KVM will WARN accordingly. WARNING: CPU: 1 PID: 3509 at arch/x86/kvm/mmu/tdp_mmu.c:46 [kvm] RIP: 0010:kvm_mmu_uninit_tdp_mmu+0x3e/0x50 [kvm] Call Trace: <TASK> kvm_arch_destroy_vm+0x130/0x1b0 [kvm] kvm_destroy_vm+0x162/0x2a0 [kvm] kvm_vcpu_release+0x34/0x60 [kvm] __fput+0x82/0x240 task_work_run+0x5c/0x90 do_exit+0x364/0xa10 ? futex_unqueue+0x38/0x60 do_group_exit+0x33/0xa0 get_signal+0x155/0x850 arch_do_signal_or_restart+0xed/0x750 exit_to_user_mode_prepare+0xc5/0x120 syscall_exit_to_user_mode+0x1d/0x40 do_syscall_64+0x48/0xc0 entry_SYSCALL_64_after_hwframe+0x44/0xae If kvm_tdp_mmu_zap_all() skips a gfn/SPTE but that SPTE is then zapped by kvm_tdp_mmu_put_root(), KVM triggers a use-after-free in the form of marking a struct page as dirty/accessed after it has been put back on the free list. This directly triggers a WARN due to encountering a page with page_count() == 0, but it can also lead to data corruption and additional errors in the kernel. WARNING: CPU: 7 PID: 1995658 at arch/x86/kvm/../../../virt/kvm/kvm_main.c:171 RIP: 0010:kvm_is_zone_device_pfn.part.0+0x9e/0xd0 [kvm] Call Trace: <TASK> kvm_set_pfn_dirty+0x120/0x1d0 [kvm] __handle_changed_spte+0x92e/0xca0 [kvm] __handle_changed_spte+0x63c/0xca0 [kvm] __handle_changed_spte+0x63c/0xca0 [kvm] __handle_changed_spte+0x63c/0xca0 [kvm] zap_gfn_range+0x549/0x620 [kvm] kvm_tdp_mmu_put_root+0x1b6/0x270 [kvm] mmu_free_root_page+0x219/0x2c0 [kvm] kvm_mmu_free_roots+0x1b4/0x4e0 [kvm] kvm_mmu_unload+0x1c/0xa0 [kvm] kvm_arch_destroy_vm+0x1f2/0x5c0 [kvm] kvm_put_kvm+0x3b1/0x8b0 [kvm] kvm_vcpu_release+0x4e/0x70 [kvm] __fput+0x1f7/0x8c0 task_work_run+0xf8/0x1a0 do_exit+0x97b/0x2230 do_group_exit+0xda/0x2a0 get_signal+0x3be/0x1e50 arch_do_signal_or_restart+0x244/0x17f0 exit_to_user_mode_prepare+0xcb/0x120 syscall_exit_to_user_mode+0x1d/0x40 do_syscall_64+0x4d/0x90 entry_SYSCALL_64_after_hwframe+0x44/0xae Note, the underlying bug existed even before commit 1af4a96025b3 ("KVM: x86/mmu: Yield in TDU MMU iter even if no SPTES changed") moved calls to tdp_mmu_iter_cond_resched() to the beginning of loops, as KVM could still incorrectly advance past a top-level entry when yielding on a lower-level entry. But with respect to leaking shadow pages, the bug was introduced by yielding before processing the current gfn. Alternatively, tdp_mmu_iter_cond_resched() could simply fall through, or callers could jump to their "retry" label. The downside of that approach is that tdp_mmu_iter_cond_resched() _must_ be called before anything else in the loop, and there's no easy way to enfornce that requirement. Ideally, KVM would handling the cond_resched() fully within the iterator macro (the code is actually quite clean) and avoid this entire class of bugs, but that is extremely difficult do wh ---truncated---
Severity ?
7.1 (High)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H
CWE
Assigner
References
Impacted products
Vendor Product Version
Linux Linux Affected: faaf05b00aecdb347ffd1d763d024394ec0329f8 , < d884eefd75cc54887bc2e9e724207443525dfb2c (git)
Affected: faaf05b00aecdb347ffd1d763d024394ec0329f8 , < 3a0f64de479cae75effb630a2e0a237ca0d0623c (git)
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    Linux Linux Affected: 5.10
Unaffected: 0 , < 5.10 (semver)
Unaffected: 5.15.12 , ≤ 5.15.* (semver)
Unaffected: 5.16 , ≤ * (original_commit_for_fix)
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Advancing the iterator\\nresults in skipping the top-level SPTE and all its children, which is\\nfatal if any of the skipped SPTEs were not visited before yielding.\\n\\nWhen zapping all SPTEs, i.e. when min_level == root_level, restarting the\\niter and then invoking tdp_iter_next() is always fatal if the current gfn\\nhas as a valid SPTE, as advancing the iterator results in try_step_side()\\nskipping the current gfn, which wasn\u0027t visited before yielding.\\n\\nSprinkle WARNs on iter-\u003eyielded being true in various helpers that are\\noften used in conjunction with yielding, and tag the helper with\\n__must_check to reduce the probabily of improper usage.\\n\\nFailing to zap a top-level SPTE manifests in one of two ways.  If a valid\\nSPTE is skipped by both kvm_tdp_mmu_zap_all() and kvm_tdp_mmu_put_root(),\\nthe shadow page will be leaked and KVM will WARN accordingly.\\n\\n  WARNING: CPU: 1 PID: 3509 at arch/x86/kvm/mmu/tdp_mmu.c:46 [kvm]\\n  RIP: 0010:kvm_mmu_uninit_tdp_mmu+0x3e/0x50 [kvm]\\n  Call Trace:\\n   \u003cTASK\u003e\\n   kvm_arch_destroy_vm+0x130/0x1b0 [kvm]\\n   kvm_destroy_vm+0x162/0x2a0 [kvm]\\n   kvm_vcpu_release+0x34/0x60 [kvm]\\n   __fput+0x82/0x240\\n   task_work_run+0x5c/0x90\\n   do_exit+0x364/0xa10\\n   ? futex_unqueue+0x38/0x60\\n   do_group_exit+0x33/0xa0\\n   get_signal+0x155/0x850\\n   arch_do_signal_or_restart+0xed/0x750\\n   exit_to_user_mode_prepare+0xc5/0x120\\n   syscall_exit_to_user_mode+0x1d/0x40\\n   do_syscall_64+0x48/0xc0\\n   entry_SYSCALL_64_after_hwframe+0x44/0xae\\n\\nIf kvm_tdp_mmu_zap_all() skips a gfn/SPTE but that SPTE is then zapped by\\nkvm_tdp_mmu_put_root(), KVM triggers a use-after-free in the form of\\nmarking a struct page as dirty/accessed after it has been put back on the\\nfree list.  This directly triggers a WARN due to encountering a page with\\npage_count() == 0, but it can also lead to data corruption and additional\\nerrors in the kernel.\\n\\n  WARNING: CPU: 7 PID: 1995658 at arch/x86/kvm/../../../virt/kvm/kvm_main.c:171\\n  RIP: 0010:kvm_is_zone_device_pfn.part.0+0x9e/0xd0 [kvm]\\n  Call Trace:\\n   \u003cTASK\u003e\\n   kvm_set_pfn_dirty+0x120/0x1d0 [kvm]\\n   __handle_changed_spte+0x92e/0xca0 [kvm]\\n   __handle_changed_spte+0x63c/0xca0 [kvm]\\n   __handle_changed_spte+0x63c/0xca0 [kvm]\\n   __handle_changed_spte+0x63c/0xca0 [kvm]\\n   zap_gfn_range+0x549/0x620 [kvm]\\n   kvm_tdp_mmu_put_root+0x1b6/0x270 [kvm]\\n   mmu_free_root_page+0x219/0x2c0 [kvm]\\n   kvm_mmu_free_roots+0x1b4/0x4e0 [kvm]\\n   kvm_mmu_unload+0x1c/0xa0 [kvm]\\n   kvm_arch_destroy_vm+0x1f2/0x5c0 [kvm]\\n   kvm_put_kvm+0x3b1/0x8b0 [kvm]\\n   kvm_vcpu_release+0x4e/0x70 [kvm]\\n   __fput+0x1f7/0x8c0\\n   task_work_run+0xf8/0x1a0\\n   do_exit+0x97b/0x2230\\n   do_group_exit+0xda/0x2a0\\n   get_signal+0x3be/0x1e50\\n   arch_do_signal_or_restart+0x244/0x17f0\\n   exit_to_user_mode_prepare+0xcb/0x120\\n   syscall_exit_to_user_mode+0x1d/0x40\\n   do_syscall_64+0x4d/0x90\\n   entry_SYSCALL_64_after_hwframe+0x44/0xae\\n\\nNote, the underlying bug existed even before commit 1af4a96025b3 (\\\"KVM:\\nx86/mmu: Yield in TDU MMU iter even if no SPTES changed\\\") moved calls to\\ntdp_mmu_iter_cond_resched() to the beginning of loops, as KVM could still\\nincorrectly advance past a top-level entry when yielding on a lower-level\\nentry.  But with respect to leaking shadow pages, the bug was introduced\\nby yielding before processing the current gfn.\\n\\nAlternatively, tdp_mmu_iter_cond_resched() could simply fall through, or\\ncallers could jump to their \\\"retry\\\" label.  The downside of that approach\\nis that tdp_mmu_iter_cond_resched() _must_ be called before anything else\\nin the loop, and there\u0027s no easy way to enfornce that requirement.\\n\\nIdeally, KVM would handling the cond_resched() fully within the iterator\\nmacro (the code is actually quite clean) and avoid this entire class of\\nbugs, but that is extremely difficult do wh\\n---truncated---\"}], \"cpeApplicability\": [{\"nodes\": [{\"negate\": false, \"cpeMatch\": [{\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"5.15.12\", \"versionStartIncluding\": \"5.10\"}, {\"criteria\": \"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"5.16\", \"versionStartIncluding\": \"5.10\"}], \"operator\": \"OR\"}]}], \"providerMetadata\": {\"orgId\": \"416baaa9-dc9f-4396-8d5f-8c081fb06d67\", \"shortName\": \"Linux\", \"dateUpdated\": \"2025-05-04T07:04:03.480Z\"}}}",
      "cveMetadata": "{\"cveId\": \"CVE-2021-47094\", \"state\": \"PUBLISHED\", \"dateUpdated\": \"2025-05-04T07:04:03.480Z\", \"dateReserved\": \"2024-02-29T22:33:44.300Z\", \"assignerOrgId\": \"416baaa9-dc9f-4396-8d5f-8c081fb06d67\", \"datePublished\": \"2024-03-04T18:10:45.240Z\", \"assignerShortName\": \"Linux\"}",
      "dataType": "CVE_RECORD",
      "dataVersion": "5.1"
    }
  }
}


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Detection rules are retrieved from Rulezet.