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

["Jürgen Groß" <jgross@suse.com>]

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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@suse.com>
>>>> ---
>>>>    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.


Juergen

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