Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <cover.1612534649.git.jpoimboe@redhat.com> <9583327904ebbbeda399eca9c56d6c7085ac20fe.1612534649.git.jpoimboe@redhat.com>
In-Reply-To: <9583327904ebbbeda399eca9c56d6c7085ac20fe.1612534649.git.jpoimboe@redhat.com>
From:   Ivan Babrou <ivan@cloudflare.com>
Date:   Fri, 5 Feb 2021 10:27:22 -0800
Message-ID: <CABWYdi0Dh7DJZGHX+1P-Huu=dBCwCxaf_WuyXmMLueVg9eomcA@mail.gmail.com>
Subject: Re: [PATCH 1/2] x86/unwind/orc: Disable KASAN checking in the ORC
 unwinder, part 2
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     x86@kernel.org, linux-kernel <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>, stable@vger.kernel.org,
        kernel-team <kernel-team@cloudflare.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Fri, Feb 5, 2021 at 6:24 AM Josh Poimboeuf <jpoimboe@redhat.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@cloudflare.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>

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

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