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

[Jan Beulich]

On 05.02.2025 10:10, 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)
>  	pop %ebx
>  	pop %eax
>  #else
> -	lea xen_hypercall_amd(%rip), %rbx
> -	cmp %rax, %rbx

There's no memory access here, but ...

> +	cmp xen_hypercall_amd(%rip), %rax

... you now read from memory here. That can't be right. Afaict the original
use of LEA needs to stay, just with a different scratch register.

Jan