Re: Re: [BUG REPORT] USE_AFTER_FREE in complete_emulated_mmio found by KASAN/Syzkaller fuzz test (v5.10.0)

[Sean Christopherson]

On Tue, Feb 10, 2026, Sean Christopherson wrote:
> On Tue, Feb 10, 2026, Zhangjiaji wrote:
> > > I think there's a not-completely-awful solution buried in this gigantic cesspool.
> > > The only time KVM uses on-stack variables is for qword or smaller accesses, i.e.
> > > 8 bytes in size or less.  For larger fragments, e.g. AVX to/from MMIO, the target
> > > value will always be an operand in the emulator context.  And so rather than
> > > disallow stack variables, for "small" fragments, we can rework the handling to
> > > copy the value to/from each fragment on-demand instead of stashing a pointer to
> > > the value.
> > 
> > Since we can store the frag->val in struct kvm_mmio_fragment,
> > why not just point frag->data to it? This Way we can save a lot code about
> > (frag->data == NULL).
> 
> It's not quite that simple, because we need to handle reads as well.
> 
> > Though this patch will block any read-into-stack calls, we can add a special path
> > in function emulator_read_write handling feasible read-into-stack calls -- the
> > target is released just after emulator_read_write returns.
> > 
> > ---
> >  arch/x86/kvm/x86.c       | 9 ++++++++-
> >  include/linux/kvm_host.h | 3 ++-
> >  2 files changed, 10 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 72d37c8930ad..12d53d441a39 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -8197,7 +8197,14 @@ static int emulator_read_write_onepage(unsigned long addr, void *val,
> >  	WARN_ON(vcpu->mmio_nr_fragments >= KVM_MAX_MMIO_FRAGMENTS);
> >  	frag = &vcpu->mmio_fragments[vcpu->mmio_nr_fragments++];
> >  	frag->gpa = gpa;
> > -	frag->data = val;
> > +	if (bytes > 8u || ! write) {
> > +		if (WARN_ON_ONCE(object_is_on_stack(val)))
> 
> This is user-triggerable, e.g. em_popa(), em_pop_sreg(), emulate_iret_real(),
> em_ret_near_imm(), em_ret_far(), and em_ret().

*sigh*

And I was wrong.  I finally sat down to write some comments for all of this, and
realized that reads _never_ pass an on-stack @val to emulator_read_write_onepage(),
because read_emulated() always buffers reads through ctxt->mem_read.

So not only is my fancy, complex code unnecessary, it's actively broken.  If a
read splits a page boundary, and the first page is NOT emulated MMIO, trying to
fulfill the read on-demand falls apart because the @val points at the start of
the operand (technically its cache "entry").  I'm sure that's a solvable problem,
but I don't see any point in manufacturing a problem in the first place.

I need to write a changelog, but as Yashu suggested, the fix can more simply be:

--
From: Sean Christopherson <seanjc@xxxxxxxxxx>
Date: Tue, 10 Feb 2026 09:45:37 -0800
Subject: [PATCH 01/14] KVM: x86: Use scratch field in MMIO fragment to hold
 small write values

Fixes: f78146b0f923 ("KVM: Fix page-crossing MMIO")
Suggested-by: Yashu Zhang <zhangjiaji1@xxxxxxxxxx>
Reported-by: Yashu Zhang <zhangjiaji1@xxxxxxxxxx>
Closes: https://lore.kernel.org/all/369eaaa2b3c1425c85e8477066391bc7@xxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/x86.c       | 14 +++++++++++++-
 include/linux/kvm_host.h |  3 ++-
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index db3f393192d9..ff3a6f86973f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8226,7 +8226,13 @@ static int emulator_read_write_onepage(unsigned long addr, void *val,
 	WARN_ON(vcpu->mmio_nr_fragments >= KVM_MAX_MMIO_FRAGMENTS);
 	frag = &vcpu->mmio_fragments[vcpu->mmio_nr_fragments++];
 	frag->gpa = gpa;
-	frag->data = val;
+	if (write && bytes <= 8u) {
+		frag->val = 0;
+		frag->data = &frag->val;
+		memcpy(&frag->val, val, bytes);
+	} else {
+		frag->data = val;
+	}
 	frag->len = bytes;
 	return X86EMUL_CONTINUE;
 }
@@ -8241,6 +8247,9 @@ static int emulator_read_write(struct x86_emulate_ctxt *ctxt,
 	gpa_t gpa;
 	int rc;
 
+	if (WARN_ON_ONCE((bytes > 8u || !ops->write) && object_is_on_stack(val)))
+		return X86EMUL_UNHANDLEABLE;
+
 	if (ops->read_write_prepare &&
 		  ops->read_write_prepare(vcpu, val, bytes))
 		return X86EMUL_CONTINUE;
@@ -11847,6 +11856,9 @@ static int complete_emulated_mmio(struct kvm_vcpu *vcpu)
 		frag++;
 		vcpu->mmio_cur_fragment++;
 	} else {
+		if (WARN_ON_ONCE(frag->data == &frag->val))
+			return -EIO;
+
 		/* Go forward to the next mmio piece. */
 		frag->data += len;
 		frag->gpa += len;
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 2c7d76262898..0bb2a34fb93d 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -320,7 +320,8 @@ static inline bool kvm_vcpu_can_poll(ktime_t cur, ktime_t stop)
 struct kvm_mmio_fragment {
 	gpa_t gpa;
 	void *data;
-	unsigned len;
+	u64 val;
+	unsigned int len;
 };
 
 struct kvm_vcpu {

base-commit: 183bb0ce8c77b0fd1fb25874112bc8751a461e49
--