On Fri, Jun 26, 2020 at 12:35:58PM +0100, Steve McIntyre wrote:
> Hi folks,
> I'm the maintainer in Debian for strace. Trying to reproduce
> https://bugs.debian.org/963462 on my machine (Thinkpad T470), I've
> found a repeatable hard lockup running the strace testsuite. Each time
> it seems to have failed in a slightly different place in the testsuite
> (suggesting it's not one particular syscall test that's triggering the
> failure). I initially found this using Debian's current Buster kernel
> (4.19.118+2+deb10u1), then backtracking I found that 4.19.98+1+deb10u1
> worked fine.
> I've bisected to find the failure point along the linux-4.19.y stable
> branch and what I've got to is the following commit:
> e58f543fc7c0926f31a49619c1a3648e49e8d233 is the first bad commit
> commit e58f543fc7c0926f31a49619c1a3648e49e8d233
> Author: Jann Horn <ja...@google.com>
> Date:   Thu Sep 13 18:12:09 2018 +0200
>     apparmor: don't try to replace stale label in ptrace access check
>     [ Upstream commit 1f8266ff58840d698a1e96d2274189de1bdf7969 ]
>     As a comment above begin_current_label_crit_section() explains,
>     begin_current_label_crit_section() must run in sleepable context because
>     when label_is_stale() is true, aa_replace_current_label() runs, which uses
>     prepare_creds(), which can sleep.
>     Until now, the ptrace access check (which runs with a task lock held)
>     violated this rule.
>     Also add a might_sleep() assertion to begin_current_label_crit_section(),
>     because asserts are less likely to be ignored than comments.
>     Fixes: b2d09ae449ced ("apparmor: move ptrace checks to using labels")
>     Signed-off-by: Jann Horn <ja...@google.com>
>     Signed-off-by: John Johansen <john.johan...@canonical.com>
>     Signed-off-by: Sasha Levin <sas...@kernel.org>
> :040000 040000 ca92f885a38c1747b812116f19de6967084a647e 
> 865a227665e460e159502f21e8a16e6fa590bf50 M security
> Considering I'm running strace build tests to provoke this bug,
> finding the failure in a commit talking about ptrace changes does look
> very suspicious...!
> Annoyingly, I can't reproduce this on my disparate other machines
> here, suggesting it's maybe(?) timing related.
> Hope this helps - happy to give more information, test things, etc.

So if you just revert this one patch, all works well?

I've added the authors of the patch to the cc: list...

Also, does this problem happen on Linus's tree?


greg k-h

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