linux-kernel - Re: [PATCH] x86/entry: fix entry_INT80

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Date:   Tue, 16 Aug 2022 11:03:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juergen Gross <jgross@...e.com>
Cc:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        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>, stable@...r.kernel.org
Subject: Re: [PATCH] x86/entry: fix entry_INT80_compat for Xen PV guests

On Tue, Aug 16, 2022 at 09:11:37AM +0200, Juergen Gross wrote:
> Commit c89191ce67ef ("x86/entry: Convert SWAPGS to swapgs and remove
> the definition of SWAPGS") missed one use case of SWAPGS in
> entry_INT80_compat. Removing of the SWAPGS macro led to asm just
> using "swapgs", as it is accepting instructions in capital letters,
> too.
> 
> This in turn leads to splats in Xen PV guests like:
> 
> [   36.145223] general protection fault, maybe for address 0x2d: 0000 [#1] PREEMPT SMP NOPTI
> [   36.145794] CPU: 2 PID: 1847 Comm: ld-linux.so.2 Not tainted 5.19.1-1-default #1 openSUSE Tumbleweed f3b44bfb672cdb9f235aff53b57724eba8b9411b
> [   36.146608] Hardware name: HP ProLiant ML350p Gen8, BIOS P72 11/14/2013
> [   36.148126] RIP: e030:entry_INT80_compat+0x3/0xa3
> 
> Fix that by open coding this single instance of the SWAPGS macro.
> 
> Cc: <stable@...r.kernel.org> # 5.19
> Fixes: c89191ce67ef ("x86/entry: Convert SWAPGS to swapgs and remove the definition of SWAPGS")
> Signed-off-by: Juergen Gross <jgross@...e.com>

It's a little unfortunate int80 is different from the other compat entry
points, but that's life I suppose.

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

> ---
>  arch/x86/entry/entry_64_compat.S | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
> index 682338e7e2a3..4dd19819053a 100644
> --- a/arch/x86/entry/entry_64_compat.S
> +++ b/arch/x86/entry/entry_64_compat.S
> @@ -311,7 +311,7 @@ SYM_CODE_START(entry_INT80_compat)
>  	 * Interrupts are off on entry.
>  	 */
>  	ASM_CLAC			/* Do this early to minimize exposure */
> -	SWAPGS
> +	ALTERNATIVE "swapgs", "", X86_FEATURE_XENPV
>  
>  	/*
>  	 * User tracing code (ptrace or signal handlers) might assume that
> -- 
> 2.35.3
>