KVM: x86: Don't (re)check L1 intercepts when completing userspace I/O

[ Upstream commit e750f85391286a4c8100275516973324b621a269 ]

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>
[ is_guest_mode() was open coded ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 98b25a7af8ce8ca7dab33eeae66420e9b44d1b1e..dae68691f42484e66cb5efd683c2fc8ab0fb46d6 100644 (file)
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -5452,12 +5452,11 @@ 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;
-       unsigned emul_flags;
 
        ctxt->mem_read.pos = 0;
 
@@ -5471,8 +5470,6 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
                rc = emulate_ud(ctxt);
                goto done;
        }
-
-       emul_flags = ctxt->ops->get_hflags(ctxt);
        if (unlikely(ctxt->d &
                     (No64|Undefined|Sse|Mmx|Intercept|CheckPerm|Priv|Prot|String))) {
                if ((ctxt->mode == X86EMUL_MODE_PROT64 && (ctxt->d & No64)) ||
@@ -5506,7 +5503,7 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
                                fetch_possible_mmx_operand(&ctxt->dst);
                }
 
-               if (unlikely(emul_flags & X86EMUL_GUEST_MASK) && ctxt->intercept) {
+               if (unlikely(check_intercepts) && ctxt->intercept) {
                        rc = emulator_check_intercept(ctxt, ctxt->intercept,
                                                      X86_ICPT_PRE_EXCEPT);
                        if (rc != X86EMUL_CONTINUE)
@@ -5535,7 +5532,7 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
                                goto done;
                }
 
-               if (unlikely(emul_flags & X86EMUL_GUEST_MASK) && (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)
@@ -5589,7 +5586,7 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
 
 special_insn:
 
-       if (unlikely(emul_flags & X86EMUL_GUEST_MASK) && (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 fb09cd22cb7f5be1ad9f5533083f852ec7e60308..6507340513cdde1e050f891e7d1b097d3a7b8115 100644 (file)
--- a/arch/x86/kvm/kvm_emulate.h
+++ b/arch/x86/kvm/kvm_emulate.h
@@ -496,7 +496,7 @@ 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 2ba297da7bdec774dbd92b8afd7435cad622d1cf..648f80f73e66c975843cfa133197f31813085bd9 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8209,7 +8209,14 @@ restart:
                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;