linux-kernel - Re: CONFIG_UBSAN_TRAP #UD error message on x86 [was: Re: [CRASH][BISECTED] 6.4.1 crash in boot]

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Message-ID: <202307051453.007A5ED7@keescook>
Date:   Wed, 5 Jul 2023 14:54:01 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jann Horn <jannh@...gle.com>, Kees Cook <kees@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux LLVM <llvm@...ts.linux.dev>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: CONFIG_UBSAN_TRAP #UD error message on x86 [was: Re:
 [CRASH][BISECTED] 6.4.1 crash in boot]

On Wed, Jul 05, 2023 at 11:31:13PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 05, 2023 at 02:08:09PM -0700, Kees Cook wrote:
> 
> > > Even just a "WARNING: CONFIG_UBSAN_TRAP active, #UD might be caused by
> > > that" on every #UD that does not come from a known BUG() location or
> > > such might be better than nothing...
> > 
> > I've considered it, but usually CONFIG_UBSAN_TRAP isn't accidentally
> > set. Also, the crash info is something we can get help from on the
> > compiler side, to mark up where the traps are, similar to what we do
> > with KCFI, but it hasn't happened yet for x86. For example, arm64
> > already encodes the details in the trap instruction itself:
> > https://git.kernel.org/linus/25b84002afb9dc9a91a7ea67166879c13ad82422
> 
> Right, so you could easily use a different #UD instruction that has an
> immediate, something like:
> 
>   0f b9 40 ff          ud1    -0x1(%rax),%rax

Ah yeah, that would be easier, probably. It could match what arm64 does.

-- 
Kees Cook