linux-kernel - Re: [PATCH 1/2] x86/unwind/orc: Disable KASAN checking in the ORC unwinder, part 2

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Date:   Fri, 5 Feb 2021 10:27:22 -0800
From:   Ivan Babrou <ivan@...udflare.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>, stable@...r.kernel.org,
        kernel-team <kernel-team@...udflare.com>
Subject: Re: [PATCH 1/2] x86/unwind/orc: Disable KASAN checking in the ORC
 unwinder, part 2

On Fri, Feb 5, 2021 at 6:24 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> KASAN reserves "redzone" areas between stack frames in order to detect
> stack overruns.  A read or write to such an area triggers a KASAN
> "stack-out-of-bounds" BUG.
>
> Normally, the ORC unwinder stays in-bounds and doesn't access the
> redzone.  But sometimes it can't find ORC metadata for a given
> instruction.  This can happen for code which is missing ORC metadata, or
> for generated code.  In such cases, the unwinder attempts to fall back
> to frame pointers, as a best-effort type thing.
>
> This fallback often works, but when it doesn't, the unwinder can get
> confused and go off into the weeds into the KASAN redzone, triggering
> the aforementioned KASAN BUG.
>
> But in this case, the unwinder's confusion is actually harmless and
> working as designed.  It already has checks in place to prevent
> off-stack accesses, but those checks get short-circuited by the KASAN
> BUG.  And a BUG is a lot more disruptive than a harmless unwinder
> warning.
>
> Disable the KASAN checks by using READ_ONCE_NOCHECK() for all stack
> accesses.  This finishes the job started by commit 881125bfe65b
> ("x86/unwind: Disable KASAN checking in the ORC unwinder"), which only
> partially fixed the issue.
>
> Fixes: ee9f8fce9964 ("x86/unwind: Add the ORC unwinder")
> Reported-by: Ivan Babrou <ivan@...udflare.com>
> Cc: stable@...r.kernel.org
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>

I haven't seen any previously observed issues with this after a day of uptime.

Tested-by: Ivan Babrou <ivan@...udflare.com>