The patch below does not apply to the 5.15-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y git checkout FETCH_HEAD git cherry-pick -x e750f85391286a4c8100275516973324b621a269 # <resolve conflicts, build, test, etc.> git commit -s git send-email --to 'stable@vger.kernel.org' --in-reply-to '2025101006-moonwalk-smilingly-3725@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From e750f85391286a4c8100275516973324b621a269 Mon Sep 17 00:00:00 2001 From: Sean Christopherson seanjc@google.com Date: Tue, 15 Jul 2025 12:06:38 -0700 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>

Reported-by: syzbot+cc2032ba16cc2018ca25@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/68790db4.a00a0220.3af5df.0020.GAE@google.com Fixes: 8a76d7f25f8f ("KVM: x86: Add x86 callback for intercept check") Cc: stable@vger.kernel.org Cc: Jim Mattson jmattson@google.com Link: https://lore.kernel.org/r/20250715190638.1899116-1-seanjc@google.com Signed-off-by: Sean Christopherson seanjc@google.com

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 a1c49bc681c4..79057622fa76 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -8470,11 +8470,6 @@ 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) { @@ -8558,7 +8553,6 @@ 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, @@ -9143,7 +9137,14 @@ 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;