linux-kernel - Re: [PATCH] x86/fred: Correct speculative safety in fred

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Message-ID: <3f9765e1-8f02-409f-813c-24ff378f7a90@citrix.com>
Date: Tue, 6 Jan 2026 16:46:41 +0000
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Andrew Cooper <andrew.cooper3@...rix.com>,
 LKML <linux-kernel@...r.kernel.org>, Xin Li <xin@...or.com>,
 "H. Peter Anvin" <hpa@...or.com>, 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>,
 x86@...nel.org
Subject: Re: [PATCH] x86/fred: Correct speculative safety in fred_extint()

On 06/01/2026 3:20 pm, Peter Zijlstra wrote:
> On Tue, Jan 06, 2026 at 01:15:04PM +0000, Andrew Cooper wrote:
>> This is why we have array_access_nospec() in Xen, so you can't separate the
>> safety calculation from the array access.
>>
>> The observant reader might notice that the result of reading sysvec_table[] is
>> also subject to memory predictions.  Aren't CPUs wonderful...
>>
>> In practice, even having array_index_nospec() part of the array access
>> expression is no guarantee of avoiding spilling to the stack.  KASAN is liable
>> to hide a function call behind the scenes, while UBSAN is very good at
>> inserting it's own unsafe range checks around objects it knows the size of.
>> Aren't compilers wonderful...
> Yeah, then again nobody should be running *SAN kernels in production,
> right ;-)

Aren't distros wonderful...

~Andrew