Sean Christopherson ce25681d59 KVM: x86/mmu: Protect marking SPs unsync when using TDP MMU with spinlock ... Add yet another spinlock for the TDP MMU and take it when marking indirect shadow pages unsync. When using the TDP MMU and L1 is running L2(s) with nested TDP, KVM may encounter shadow pages for the TDP entries managed by L1 (controlling L2) when handling a TDP MMU page fault. The unsync logic is not thread safe, e.g. the kvm_mmu_page fields are not atomic, and misbehaves when a shadow page is marked unsync via a TDP MMU page fault, which runs with mmu_lock held for read, not write. Lack of a critical section manifests most visibly as an underflow of unsync_children in clear_unsync_child_bit() due to unsync_children being corrupted when multiple CPUs write it without a critical section and without atomic operations. But underflow is the best case scenario. The worst case scenario is that unsync_children prematurely hits '0' and leads to guest memory corruption due to KVM neglecting to properly sync shadow pages. Use an entirely new spinlock even though piggybacking tdp_mmu_pages_lock would functionally be ok. Usurping the lock could degrade performance when building upper level page tables on different vCPUs, especially since the unsync flow could hold the lock for a comparatively long time depending on the number of indirect shadow pages and the depth of the paging tree. For simplicity, take the lock for all MMUs, even though KVM could fairly easily know that mmu_lock is held for write. If mmu_lock is held for write, there cannot be contention for the inner spinlock, and marking shadow pages unsync across multiple vCPUs will be slow enough that bouncing the kvm_arch cacheline should be in the noise. Note, even though L2 could theoretically be given access to its own EPT entries, a nested MMU must hold mmu_lock for write and thus cannot race against a TDP MMU page fault. I.e. the additional spinlock only _needs_ to be taken by the TDP MMU, as opposed to being taken by any MMU for a VM that is running with the TDP MMU enabled. Holding mmu_lock for read also prevents the indirect shadow page from being freed. But as above, keep it simple and always take the lock. Alternative #1, the TDP MMU could simply pass "false" for can_unsync and effectively disable unsync behavior for nested TDP. Write protecting leaf shadow pages is unlikely to noticeably impact traditional L1 VMMs, as such VMMs typically don't modify TDP entries, but the same may not hold true for non-standard use cases and/or VMMs that are migrating physical pages (from L1's perspective). Alternative #2, the unsync logic could be made thread safe. In theory, simply converting all relevant kvm_mmu_page fields to atomics and using atomic bitops for the bitmap would suffice. However, (a) an in-depth audit would be required, (b) the code churn would be substantial, and (c) legacy shadow paging would incur additional atomic operations in performance sensitive paths for no benefit (to legacy shadow paging). Fixes: a2855afc7e ("KVM: x86/mmu: Allow parallel page faults for the TDP MMU") Cc: stable@vger.kernel.org Cc: Ben Gardon <bgardon@google.com> Signed-off-by: Sean Christopherson <seanjc@google.com> Message-Id: <20210812181815.3378104-1-seanjc@google.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>		2021-08-13 03:32:14 -04:00
..
acrn	docs: acrn: Introduce ACRN	2021-02-09 10:58:18 +01:00
kvm	KVM: x86/mmu: Protect marking SPs unsync when using TDP MMU with spinlock	2021-08-13 03:32:14 -04:00
uml	docs: virt: user_mode_linux_howto_v2.rst: fix a literal block markup	2020-10-16 07:28:20 +02:00
guest-halt-polling.rst	docs: virt: guest-halt-polling.txt convert to ReST	2020-02-12 20:10:08 +01:00
index.rst	docs: acrn: Introduce ACRN	2021-02-09 10:58:18 +01:00
ne_overview.rst	nitro_enclaves: Add overview documentation	2020-09-22 13:58:41 +02:00
paravirt_ops.rst