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Re: [PATCH] x86/xen: fix xen_hypercall_hvm() to not clobber %rbx
- To: Juergen Gross <jgross@xxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 5 Feb 2025 10:54:37 +0100
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- Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, x86@xxxxxxxxxx
- Delivery-date: Wed, 05 Feb 2025 09:54:46 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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
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