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

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