linux-kernel - Re: [PATCH] x86/xen: fix xen_hypercall

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Message-ID: <3eff3975-ace9-4e09-b160-4330b9695e4c@suse.com>
Date: Wed, 5 Feb 2025 11:07:18 +0100
From: Jürgen Groß <jgross@...e.com>
To: Jan Beulich <jbeulich@...e.com>
Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>, xen-devel@...ts.xenproject.org,
 linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH] x86/xen: fix xen_hypercall_hvm() to not clobber %rbx

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@...e.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)
>>   	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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