CVE-2026-52969 (GCVE-0-2026-52969)
Vulnerability from cvelistv5
Published
2026-06-24 16:28
Modified
2026-07-15 02:48
Severity ?
7.0 (High) - CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
Summary
In the Linux kernel, the following vulnerability has been resolved: KVM: Reject wrapped offset in kvm_reset_dirty_gfn() kvm_reset_dirty_gfn() guards the gfn range with if (!memslot || (offset + __fls(mask)) >= memslot->npages) return; but offset is u64 and the addition is unchecked. The check can be silently bypassed by a u64 wrap. The dirty ring backing those entries is MAP_SHARED at KVM_DIRTY_LOG_PAGE_OFFSET of the vcpu fd, so the VMM can rewrite the slot and offset fields of any entry between when the kernel pushes them and when KVM_RESET_DIRTY_RINGS consumes them. On reset, kvm_dirty_ring_reset() re-reads the values via READ_ONCE() and feeds them straight back into this check; only the flags handshake is treated as the handover, the slot/offset payload is taken on trust. Crafting two entries entry[i].offset = 0xffffffffffffffc1 entry[i+1].offset = 0 makes the coalescing loop in kvm_dirty_ring_reset() compute delta = (s64)(0 - 0xffffffffffffffc1) = 63 which falls in [0, BITS_PER_LONG), so it folds entry[i+1] into the existing mask by setting bit 63. The trailing kvm_reset_dirty_gfn() call then sees offset = 0xffffffffffffffc1 and __fls(mask) = 63; the sum is 0 in u64 and the bounds check passes. That offset propagates into kvm_arch_mmu_enable_log_dirty_pt_masked() unchanged. On the legacy MMU path -- kvm_memslots_have_rmaps() == true, i.e. shadow paging, any VM that has allocated shadow roots, or a write-tracked slot -- it reaches gfn_to_rmap(), which indexes slot->arch.rmap[0][] with a near-U64_MAX gfn. That is an out-of-bounds load of a kvm_rmap_head, followed by a conditional clear of PT_WRITABLE_MASK in whatever the loaded pointer points at. The path is reachable from any process holding /dev/kvm. Range-check offset on its own first, so the addition cannot wrap. memslot->npages is bounded well below U64_MAX, so once offset < npages holds, offset + __fls(mask) (with __fls(mask) < BITS_PER_LONG) stays in range.
References
Impacted products
Vendor Product Version
Linux Linux Version: fb04a1eddb1a65b6588a021bdc132270d5ae48bb
Version: fb04a1eddb1a65b6588a021bdc132270d5ae48bb
Version: fb04a1eddb1a65b6588a021bdc132270d5ae48bb
Version: fb04a1eddb1a65b6588a021bdc132270d5ae48bb
Version: fb04a1eddb1a65b6588a021bdc132270d5ae48bb
Version: fb04a1eddb1a65b6588a021bdc132270d5ae48bb
Version: fb04a1eddb1a65b6588a021bdc132270d5ae48bb
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          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nKVM: Reject wrapped offset in kvm_reset_dirty_gfn()\n\nkvm_reset_dirty_gfn() guards the gfn range with\n\n\tif (!memslot || (offset + __fls(mask)) \u003e= memslot-\u003enpages)\n\t\treturn;\n\nbut offset is u64 and the addition is unchecked.  The check can be\nsilently bypassed by a u64 wrap.\n\nThe dirty ring backing those entries is MAP_SHARED at\nKVM_DIRTY_LOG_PAGE_OFFSET of the vcpu fd, so the VMM can rewrite the\nslot and offset fields of any entry between when the kernel pushes\nthem and when KVM_RESET_DIRTY_RINGS consumes them.  On reset,\nkvm_dirty_ring_reset() re-reads the values via READ_ONCE() and feeds\nthem straight back into this check; only the flags handshake is\ntreated as the handover, the slot/offset payload is taken on trust.\n\nCrafting two entries\n\n\tentry[i].offset   = 0xffffffffffffffc1\n\tentry[i+1].offset = 0\n\nmakes the coalescing loop in kvm_dirty_ring_reset() compute\n\n\tdelta = (s64)(0 - 0xffffffffffffffc1) = 63\n\nwhich falls in [0, BITS_PER_LONG), so it folds entry[i+1] into the\nexisting mask by setting bit 63.  The trailing kvm_reset_dirty_gfn()\ncall then sees offset = 0xffffffffffffffc1 and __fls(mask) = 63;\nthe sum is 0 in u64 and the bounds check passes.\n\nThat offset propagates into kvm_arch_mmu_enable_log_dirty_pt_masked()\nunchanged.  On the legacy MMU path -- kvm_memslots_have_rmaps() ==\ntrue, i.e. shadow paging, any VM that has allocated shadow roots, or\na write-tracked slot -- it reaches gfn_to_rmap(), which indexes\nslot-\u003earch.rmap[0][] with a near-U64_MAX gfn.  That is an\nout-of-bounds load of a kvm_rmap_head, followed by a conditional\nclear of PT_WRITABLE_MASK in whatever the loaded pointer points at.\nThe path is reachable from any process holding /dev/kvm.\n\nRange-check offset on its own first, so the addition cannot wrap.\nmemslot-\u003enpages is bounded well below U64_MAX, so once offset \u003c\nnpages holds, offset + __fls(mask) (with __fls(mask) \u003c BITS_PER_LONG)\nstays in range."
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    "cveId": "CVE-2026-52969",
    "datePublished": "2026-06-24T16:28:48.085Z",
    "dateReserved": "2026-06-09T07:44:35.375Z",
    "dateUpdated": "2026-07-15T02:48:28.368Z",
    "state": "PUBLISHED"
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}


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