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Message-ID: <20250715190638.1899116-1-seanjc@google.com>
Date: Tue, 15 Jul 2025 12:06:38 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
Jim Mattson <jmattson@...gle.com>
Subject: [PATCH] KVM: x86: Don't (re)check L1 intercepts when completing
userspace I/O
When completing emulation of instruction that generated a userspace exit
for I/O, don't recheck L1 intercepts as KVM has already finished that
phase of instruction execution, i.e. has already committed to allowing L2
to perform I/O. If L1 (or host userspace) modifies the I/O permission
bitmaps during the exit to userspace, KVM will treat the access as being
intercepted despite already having emulated the I/O access.
Pivot on EMULTYPE_NO_DECODE to detect that KVM is completing emulation.
Of the three users of EMULTYPE_NO_DECODE, only complete_emulated_io() (the
intended "recipient") can reach the code in question. gp_interception()'s
use is mutually exclusive with is_guest_mode(), and
complete_emulated_insn_gp() unconditionally pairs EMULTYPE_NO_DECODE with
EMULTYPE_SKIP.
The bad behavior was detected by a syzkaller program that toggles port I/O
interception during the userspace I/O exit, ultimately resulting in a WARN
on vcpu->arch.pio.count being non-zero due to KVM no completing emulation
of the I/O instruction.
WARNING: CPU: 23 PID: 1083 at arch/x86/kvm/x86.c:8039 emulator_pio_in_out+0x154/0x170 [kvm]
Modules linked in: kvm_intel kvm irqbypass
CPU: 23 UID: 1000 PID: 1083 Comm: repro Not tainted 6.16.0-rc5-c1610d2d66b1-next-vm #74 NONE
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
RIP: 0010:emulator_pio_in_out+0x154/0x170 [kvm]
PKRU: 55555554
Call Trace:
<TASK>
kvm_fast_pio+0xd6/0x1d0 [kvm]
vmx_handle_exit+0x149/0x610 [kvm_intel]
kvm_arch_vcpu_ioctl_run+0xda8/0x1ac0 [kvm]
kvm_vcpu_ioctl+0x244/0x8c0 [kvm]
__x64_sys_ioctl+0x8a/0xd0
do_syscall_64+0x5d/0xc60
entry_SYSCALL_64_after_hwframe+0x4b/0x53
</TASK>
Fixes: 8a76d7f25f8f ("KVM: x86: Add x86 callback for intercept check")
Cc: stable@...r.kernel.org
Cc: Jim Mattson <jmattson@...gle.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
arch/x86/kvm/emulate.c | 9 ++++-----
arch/x86/kvm/kvm_emulate.h | 3 +--
arch/x86/kvm/x86.c | 15 ++++++++-------
3 files changed, 13 insertions(+), 14 deletions(-)
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 1349e278cd2a..542d3664afa3 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -5107,12 +5107,11 @@ void init_decode_cache(struct x86_emulate_ctxt *ctxt)
ctxt->mem_read.end = 0;
}
-int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
+int x86_emulate_insn(struct x86_emulate_ctxt *ctxt, bool check_intercepts)
{
const struct x86_emulate_ops *ops = ctxt->ops;
int rc = X86EMUL_CONTINUE;
int saved_dst_type = ctxt->dst.type;
- bool is_guest_mode = ctxt->ops->is_guest_mode(ctxt);
ctxt->mem_read.pos = 0;
@@ -5160,7 +5159,7 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
fetch_possible_mmx_operand(&ctxt->dst);
}
- if (unlikely(is_guest_mode) && ctxt->intercept) {
+ if (unlikely(check_intercepts) && ctxt->intercept) {
rc = emulator_check_intercept(ctxt, ctxt->intercept,
X86_ICPT_PRE_EXCEPT);
if (rc != X86EMUL_CONTINUE)
@@ -5189,7 +5188,7 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
goto done;
}
- if (unlikely(is_guest_mode) && (ctxt->d & Intercept)) {
+ if (unlikely(check_intercepts) && (ctxt->d & Intercept)) {
rc = emulator_check_intercept(ctxt, ctxt->intercept,
X86_ICPT_POST_EXCEPT);
if (rc != X86EMUL_CONTINUE)
@@ -5243,7 +5242,7 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
special_insn:
- if (unlikely(is_guest_mode) && (ctxt->d & Intercept)) {
+ if (unlikely(check_intercepts) && (ctxt->d & Intercept)) {
rc = emulator_check_intercept(ctxt, ctxt->intercept,
X86_ICPT_POST_MEMACCESS);
if (rc != X86EMUL_CONTINUE)
diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h
index c1df5acfacaf..7b5ddb787a25 100644
--- a/arch/x86/kvm/kvm_emulate.h
+++ b/arch/x86/kvm/kvm_emulate.h
@@ -235,7 +235,6 @@ struct x86_emulate_ops {
void (*set_nmi_mask)(struct x86_emulate_ctxt *ctxt, bool masked);
bool (*is_smm)(struct x86_emulate_ctxt *ctxt);
- bool (*is_guest_mode)(struct x86_emulate_ctxt *ctxt);
int (*leave_smm)(struct x86_emulate_ctxt *ctxt);
void (*triple_fault)(struct x86_emulate_ctxt *ctxt);
int (*set_xcr)(struct x86_emulate_ctxt *ctxt, u32 index, u64 xcr);
@@ -521,7 +520,7 @@ bool x86_page_table_writing_insn(struct x86_emulate_ctxt *ctxt);
#define EMULATION_RESTART 1
#define EMULATION_INTERCEPTED 2
void init_decode_cache(struct x86_emulate_ctxt *ctxt);
-int x86_emulate_insn(struct x86_emulate_ctxt *ctxt);
+int x86_emulate_insn(struct x86_emulate_ctxt *ctxt, bool check_intercepts);
int emulator_task_switch(struct x86_emulate_ctxt *ctxt,
u16 tss_selector, int idt_index, int reason,
bool has_error_code, u32 error_code);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index de51dbd85a58..44ef3492bfd2 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8609,11 +8609,6 @@ static bool emulator_is_smm(struct x86_emulate_ctxt *ctxt)
return is_smm(emul_to_vcpu(ctxt));
}
-static bool emulator_is_guest_mode(struct x86_emulate_ctxt *ctxt)
-{
- return is_guest_mode(emul_to_vcpu(ctxt));
-}
-
#ifndef CONFIG_KVM_SMM
static int emulator_leave_smm(struct x86_emulate_ctxt *ctxt)
{
@@ -8697,7 +8692,6 @@ static const struct x86_emulate_ops emulate_ops = {
.guest_cpuid_is_intel_compatible = emulator_guest_cpuid_is_intel_compatible,
.set_nmi_mask = emulator_set_nmi_mask,
.is_smm = emulator_is_smm,
- .is_guest_mode = emulator_is_guest_mode,
.leave_smm = emulator_leave_smm,
.triple_fault = emulator_triple_fault,
.set_xcr = emulator_set_xcr,
@@ -9282,7 +9276,14 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
ctxt->exception.address = 0;
}
- r = x86_emulate_insn(ctxt);
+ /*
+ * Check L1's instruction intercepts when emulating instructions for
+ * L2, unless KVM is re-emulating a previously decoded instruction,
+ * e.g. to complete userspace I/O, in which case KVM has already
+ * checked the intercepts.
+ */
+ r = x86_emulate_insn(ctxt, is_guest_mode(vcpu) &&
+ !(emulation_type & EMULTYPE_NO_DECODE));
if (r == EMULATION_INTERCEPTED)
return 1;
base-commit: 4578a747f3c7950be3feb93c2db32eb597a3e55b
--
2.50.0.727.gbf7dc18ff4-goog
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