Re: [PATCH] x86/xen: fix xen_hypercall_hvm() to not clobber %rbx

[Jan Beulich]

On 05.02.2025 11:04, Jürgen Groß wrote:
> On 05.02.25 10:38, Andrew Cooper wrote:
>> On 05/02/2025 9:17 am, Jürgen Groß wrote:
>>> On 05.02.25 10:16, Andrew Cooper wrote:
>>>> On 05/02/2025 9:10 am, Juergen Gross wrote:
>>>>> xen_hypercall_hvm(), which is used when running as a Xen PVH guest at
>>>>> most only once during early boot, is clobbering %rbx. Depending on
>>>>> whether the caller relies on %rbx to be preserved across the call or
>>>>> not, this clobbering might result in an early crash of the system.
>>>>>
>>>>> This can be avoided by not modifying %rbx in xen_hypercall_hvm().
>>>>>
>>>>> Fixes: b4845bb63838 ("x86/xen: add central hypercall functions")
>>>>> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
>>>>> ---
>>>>>    arch/x86/xen/xen-head.S | 3 +--
>>>>>    1 file changed, 1 insertion(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
>>>>> index 9252652afe59..4378b817ed32 100644
>>>>> --- a/arch/x86/xen/xen-head.S
>>>>> +++ b/arch/x86/xen/xen-head.S
>>>>> @@ -117,8 +117,7 @@ SYM_FUNC_START(xen_hypercall_hvm)
>>>>
>>>> The 32bit case, out of context up here, also clobbers %ebx.
>>>>
>>>> ~Andrew
>>>>
>>>>>        pop %ebx
>>>
>>> It does not, as this part of the context is showing.
>>
>> Hmm, so it is, and worse, it can't be changed to match the 64bit side.
>> That's nasty.
>>
>> But while I'm here looking at the code, what's up with
>>
>> #ifdef CONFIG_FRAME_POINTER
>>          pushq $0        /* Dummy push for stack alignment. */
>> #endif
>>
>> ?
>>
>> That's covered by FRAME_{START,END} normally, and Linux's preferred
>> stack alignment is 8 not 16.
> 
> I've added this due to a review comment by Jan. As he is more into ABI
> matters, I believed him.
> 
> Google is telling me you are right, so I'll remove those extra hunks in
> V2 of the patch adding FRAME_END.

Oh, I'm sorry for misleading you. Clearly I must have been mis-remembering
(or things may have changed, and I should have re-checked).

Jan