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

[Jürgen Groß]

On 05.02.25 10:54, Jan Beulich wrote:
> 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.

Oh, right. Thanks for noticing.


Juergen

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