Re: [PATCH] x86emul: don't call ->read_segment() with x86_seg_none

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.08.2024 20:24, Stefano Stabellini wrote:
> On Mon, 5 Aug 2024, Jan Beulich wrote:
>> 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>
> 
> Looking at oss-fuzz's report and at this patch I think it is correct
> 
> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxx>

S-o-b?

> That said, there are a few other places where read_segment is called
> without any checks for seg being x86_seg_none. The hvm implementation of
> read_segment (hvmemul_read_segment) seems to return error if
> x86_seg_none is passed as an argument, but there is no return error
> checks any time we call ops->read_segment in x86_emulate.c.

There is a pretty limited number of cases where x86_seg_none is used.
For example, state->ea.mem.seg cannot hold this value.
realmode_load_seg() also cannot be passed this value. We could add
assertions to that effect, yet that can get unwieldy as further
x86_seg_* enumerators are added (see "x86: introduce x86_seg_sys"; I
expect we'll need at least one more when adding VMX/SVM insn emulation,
where physical addresses are used as insn operands).

> It seems that there might still be an issue?

In my auditing I didn't spot any.

Jan