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Subject[PATCH 2/5] KVM: x86/mmu: Map TDP MMU leaf SPTE iff target level is reached
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Map the leaf SPTE when handling a TDP MMU page fault if and only if the
target level is reached. A recent commit reworked the retry logic and
incorrectly assumed that walking SPTEs would never "fail", as the loop
either bails (retries) or installs parent SPs. However, the iterator
itself will bail early if it detects a frozen (REMOVED) SPTE when
stepping down. The TDP iterator also rereads the current SPTE before
stepping down specifically to avoid walking into a part of the tree that
is being removed, which means it's possible to terminate the loop without
the guts of the loop observing the frozen SPTE, e.g. if a different task
zaps a parent SPTE between the initial read and try_step_down()'s refresh.

Mapping a leaf SPTE at the wrong level results in all kinds of badness as
page table walkers interpret the SPTE as a page table, not a leaf, and
walk into the weeds.

------------[ cut here ]------------
WARNING: CPU: 1 PID: 1025 at arch/x86/kvm/mmu/tdp_mmu.c:1070 kvm_tdp_mmu_map+0x481/0x510
Modules linked in: kvm_intel
CPU: 1 PID: 1025 Comm: nx_huge_pages_t Tainted: G W 6.1.0-rc4+ #64
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
RIP: 0010:kvm_tdp_mmu_map+0x481/0x510
RSP: 0018:ffffc9000072fba8 EFLAGS: 00010286
RAX: 0000000000000000 RBX: ffffc9000072fcc0 RCX: 0000000000000027
RDX: 0000000000000027 RSI: 00000000ffffdfff RDI: ffff888277c5b4c8
RBP: ffff888107d45a10 R08: ffff888277c5b4c0 R09: ffffc9000072fa48
R10: 0000000000000001 R11: 0000000000000001 R12: ffffc9000073a0e0
R13: ffff88810fc54800 R14: ffff888107d1ae60 R15: ffff88810fc54f90
FS: 00007fba9f853740(0000) GS:ffff888277c40000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 000000010aa7a003 CR4: 0000000000172ea0
Call Trace:
<TASK>
kvm_tdp_page_fault+0x10c/0x130
kvm_mmu_page_fault+0x103/0x680
vmx_handle_exit+0x132/0x5a0 [kvm_intel]
vcpu_enter_guest+0x60c/0x16f0
kvm_arch_vcpu_ioctl_run+0x1e2/0x9d0
kvm_vcpu_ioctl+0x271/0x660
__x64_sys_ioctl+0x80/0xb0
do_syscall_64+0x2b/0x50
entry_SYSCALL_64_after_hwframe+0x46/0xb0
</TASK>
---[ end trace 0000000000000000 ]---
Invalid SPTE change: cannot replace a present leaf
SPTE with another present leaf SPTE mapping a
different PFN!
as_id: 0 gfn: 100200 old_spte: 600000112400bf3 new_spte: 6000001126009f3 level: 2
------------[ cut here ]------------
kernel BUG at arch/x86/kvm/mmu/tdp_mmu.c:559!
invalid opcode: 0000 [#1] SMP
CPU: 1 PID: 1025 Comm: nx_huge_pages_t Tainted: G W 6.1.0-rc4+ #64
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
RIP: 0010:__handle_changed_spte.cold+0x95/0x9c
RSP: 0018:ffffc9000072faf8 EFLAGS: 00010246
RAX: 00000000000000c1 RBX: ffffc90000731000 RCX: 0000000000000027
RDX: 0000000000000000 RSI: 00000000ffffdfff RDI: ffff888277c5b4c8
RBP: 0600000112400bf3 R08: ffff888277c5b4c0 R09: ffffc9000072f9a0
R10: 0000000000000001 R11: 0000000000000001 R12: 06000001126009f3
R13: 0000000000000002 R14: 0000000012600901 R15: 0000000012400b01
FS: 00007fba9f853740(0000) GS:ffff888277c40000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 000000010aa7a003 CR4: 0000000000172ea0
Call Trace:
<TASK>
kvm_tdp_mmu_map+0x3b0/0x510
kvm_tdp_page_fault+0x10c/0x130
kvm_mmu_page_fault+0x103/0x680
vmx_handle_exit+0x132/0x5a0 [kvm_intel]
vcpu_enter_guest+0x60c/0x16f0
kvm_arch_vcpu_ioctl_run+0x1e2/0x9d0
kvm_vcpu_ioctl+0x271/0x660
__x64_sys_ioctl+0x80/0xb0
do_syscall_64+0x2b/0x50
entry_SYSCALL_64_after_hwframe+0x46/0xb0
</TASK>
Modules linked in: kvm_intel
---[ end trace 0000000000000000 ]---

Fixes: 63d28a25e04c ("KVM: x86/mmu: simplify kvm_tdp_mmu_map flow when guest has to retry")
Cc: Robert Hoo <robert.hu@linux.intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/mmu/tdp_mmu.c | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index b740f38fedcc..e2e197d41780 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1170,7 +1170,7 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
goto retry;

if (iter.level == fault->goal_level)
- break;
+ goto map_target_level;

/* Step down into the lower level page table if it exists. */
if (is_shadow_present_pte(iter.old_spte) &&
@@ -1192,8 +1192,8 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
r = tdp_mmu_link_sp(kvm, &iter, sp, true);

/*
- * Also force the guest to retry the access if the upper level SPTEs
- * aren't in place.
+ * Force the guest to retry if installing an upper level SPTE
+ * failed, e.g. because a different task modified the SPTE.
*/
if (r) {
tdp_mmu_free_sp(sp);
@@ -1208,6 +1208,14 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
}
}

+ /*
+ * The walk aborted before reaching the target level, e.g. because the
+ * iterator detected an upper level SPTE was frozen during traversal.
+ */
+ WARN_ON_ONCE(iter.level == fault->goal_level);
+ goto retry;
+
+map_target_level:
ret = tdp_mmu_map_handle_target_level(vcpu, fault, &iter);

retry:
--
2.39.0.rc1.256.g54fd8350bd-goog
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