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[PATCH] x86emul: don't call ->read_segment() with x86_seg_none
- To: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 5 Aug 2024 15:26:26 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Mon, 05 Aug 2024 13:26:45 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
LAR, LSL, VERR, and VERW emulation involve calling protmode_load_seg()
with x86_seg_none. The fuzzer's read_segment() hook function has an
assertion which triggers in this case. Calling the hook function,
however, makes little sense for those insns, as there's no data to
retrieve. Instead zero-filling the output structure is what properly
corresponds to those insns being invoked with a NUL selector.
Fixes: 06a3b8cd7ad2 ("x86emul: support LAR/LSL/VERR/VERW")
Oss-fuzz: 70918
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
It is pure guesswork that one of those insns did trigger the assertion.
The report from oss-fuzz sadly lacks details on the insn under
emulation. I'm further surprised that AFL never found this.
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -839,7 +839,8 @@ protmode_load_seg(
case x86_seg_tr:
goto raise_exn;
}
- if ( !_amd_like(cp) || vcpu_has_nscb() || !ops->read_segment ||
+ if ( seg == x86_seg_none || !_amd_like(cp) || vcpu_has_nscb() ||
+ !ops->read_segment ||
ops->read_segment(seg, sreg, ctxt) != X86EMUL_OKAY )
memset(sreg, 0, sizeof(*sreg));
else
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