Subject: CVE-2021-47094: KVM: x86/mmu: Don't advance iterator after restart due to yielding

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

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 while also supporting yielding
after tdp_mmu_set_spte_atomic() fails.  Yielding after failing to set a
SPTE is very desirable as the "owner" of the REMOVED_SPTE isn't strictly
bounded, e.g. if it's zapping a high-level shadow page, the REMOVED_SPTE
may block operations on the SPTE for a significant amount of time.

The Linux kernel CVE team has assigned CVE-2021-47094 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 5.10 with commit faaf05b00aec and fixed in 5.15.12 with commit d884eefd75cc
	Issue introduced in 5.10 with commit faaf05b00aec and fixed in 5.16 with commit 3a0f64de479c
	Issue introduced in 5.10.30 with commit 3c7a18440638

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2021-47094
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	arch/x86/kvm/mmu/tdp_iter.c
	arch/x86/kvm/mmu/tdp_iter.h
	arch/x86/kvm/mmu/tdp_mmu.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/d884eefd75cc54887bc2e9e724207443525dfb2c
	https://git.kernel.org/stable/c/3a0f64de479cae75effb630a2e0a237ca0d0623c