[arm64-v6.1.112-generic] KVM: x86: Fix shadow paging use-after-free due to unexpected role (CVE-2026-53359)#263
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commit 0cb2af2ea66ad8ff195c156ea690f11216285bdf upstream. The shadow MMU computes GFNs for direct shadow pages using sp->gfn plus the SPTE index. This assumption breaks for shadow paging if the guest page tables are modified between VM entries (similar to commit aad885e77496, "KVM: x86/mmu: Drop/zap existing present SPTE even when creating an MMIO SPTE", 2026-03-27). The flow is as follows: - a PDE is installed for a 2MB mapping, and a page in that area is accessed. KVM creates a kvm_mmu_page consisting of 512 4KB pages; the kvm_mmu_page is marked by FNAME(fetch) as direct-mapped because the guest's mapping is a huge page (and thus contiguous). - the PDE mapping is changed from outside the guest. - the guest accesses another page in the same 2MB area. KVM installs a new leaf SPTE and rmap entry; the SPTE uses the "correct" GFN (i.e. based on the new mapping, as changed in the previous step) but that GFN is outside of the [sp->gfn, sp->gfn + 511] range; therefore the rmap entry cannot be found and removed when the kvm_mmu_page is zapped. - the memslot that covers the first 2MB mapping is deleted, and the kvm_mmu_page for the now-invalid GPA is zapped. However, rmap_remove() only looks at the [sp->gfn, sp->gfn + 511] range established in step 1, and fails to find the rmap entry that was recorded by step 3. - any operation that causes an rmap walk for the same page accessed by step 3 then walks a stale rmap and dereferences a freed kvm_mmu_page. This includes dirty logging or MMU notifier invalidations (e.g., from MADV_DONTNEED). The underlying issue is that KVM's walking of shadow PTEs assumes that if a SPTE is present when KVM wants to install a non-leaf SPTE, then the existing kvm_mmu_page must be for the correct gfn. Because the only way for the gfn to be wrong is if KVM messed up and failed to zap a SPTE... which shouldn't happen, but *actually* only happens in response to a guest write. That bug dates back literally forever, as even the first version of KVM assumes that the GFN matches and walks into the "wrong" shadow page. However, that was only an imprecision until 2032a93 ("KVM: MMU: Don't allocate gfns page for direct mmu pages") came along. Fix it by checking for a target gfn mismatch and zapping the existing SPTE. That way the old SP and rmap entries are gone, KVM installs the rmap in the right location, and everyone is happy. Fixes: 2032a93 ("KVM: MMU: Don't allocate gfns page for direct mmu pages") Fixes: 6aa8b73 ("kvm: userspace interface") Reported-by: Alexander Bulekov <bkov@amazon.com> Reported-by: Fred Griffoul <fgriffo@amazon.co.uk> Cc: stable@vger.kernel.org Signed-off-by: Sean Christopherson <seanjc@google.com> Link: https://patch.msgid.link/20260503201029.106481-1-pbonzini@redhat.com/ Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Sasha Levin <sashal@kernel.org> (cherry picked from commit e9d4ea13aa2b6400bb10ec64b370ba3dadcd22f0)
commit 81ccda30b4e83d8f5cc4fd50503c44e3a33abfeb upstream.
Commit 0cb2af2ea66ad ("KVM: x86: Fix shadow paging use-after-free due
to unexpected GFN") fixed a shadow paging mismatch between stored and
computed GFNs; the bug could be triggered by changing a PDE mapping from
outside the guest, and then deleting a memslot. The rmap_remove()
call would miss entries created after the PDE change because the GFN
of the leaf SPTE does not match the GFN of the struct kvm_mmu_page.
A similar hole however remains if the modified PDE points to a non-leaf
page. In this case the gfn can be made to match, but the role does not
match: the original large 2MB page creates a kvm_mmu_page with direct=1,
while the new 4KB needs a kvm_mmu_page with direct=0. However,
kvm_mmu_get_child_sp() does not compare the role, and therefore reuses
the page.
The next step is installing a leaf (4KB) SPTE on the new path which
records an rmap entry under the gfn resolved by the walk. But when
that child is zapped its parent kvm_mmu_page has direct=1 and
kvm_mmu_page_get_gfn() computes the gfn for the 4KB page as
sp->gfn + index instead of using sp->shadowed_translation[] (or sp->gfns[]
in older kernels). It therefore fails to remove the recorded entry.
When the memslot is dropped the shadow page is freed but the rmap
entry survives, as in the scenario that was already fixed. Code that
later walks that gfn (dirty logging, MMU notifier invalidation, and
so on) dereferences an sptep that lies in the freed page, causing the
use-after-free.
Fixes: 2032a93 ("KVM: MMU: Don't allocate gfns page for direct mmu pages")
Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit b1337aae5e194324e4810d561764e7793f8b3864)
Avoids collisions when multiple users build on the same host. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Signed-off-by: Christoph Ostarek <christoph@zededa.com>
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See: #268 (comment) |
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Backport of upstream fix 81ccda30b4e83d8f5cc4fd50503c44e3a33abfeb ("KVM: x86: Fix shadow paging use-after-free due to unexpected role") together with its prerequisite 0cb2af2ea66ad8ff195c156ea690f11216285bdf ("KVM: x86: Fix shadow paging use-after-free due to unexpected GFN"). Both picked from the linux-6.1.y stable backports e9d4ea13aa2b6 and b1337aae5e194.
Also includes "Makefile.eve: make linuxkit tmp dir user-specific" to avoid collisions when multiple users build on the same host.
Fixes: 2032a93 ("KVM: MMU: Don't allocate gfns page for direct mmu pages")
CVE: CVE-2026-53359