CVE-2021-47094

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—