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From:   Jann Horn <jannh@google.com>
Date:   Sun, 1 Mar 2020 19:21:03 +0100
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Subject: Re: [PATCH] exec: Fix a deadlock in ptrace
To:     Bernd Edlinger <bernd.edlinger@hotmail.de>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Ingo Molnar <mingo@kernel.org>,
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        Yuyang Du <duyuyang@gmail.com>,
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On Sun, Mar 1, 2020 at 12:27 PM Bernd Edlinger
<bernd.edlinger@hotmail.de> wrote:
> The proposed solution is to have a second mutex that is
> used in mm_access, so it is allowed to continue while the
> dying threads are not yet terminated.

Just for context: When I proposed something similar back in 2016,
https://lore.kernel.org/linux-fsdevel/20161102181806.GB1112@redhat.com/
was the resulting discussion thread. At least back then, I looked
through the various existing users of cred_guard_mutex, and the only
places that couldn't be converted to the new second mutex were
PTRACE_ATTACH and SECCOMP_FILTER_FLAG_TSYNC.


The ideal solution would IMO be something like this: Decide what the
new task's credentials should be *before* reaching de_thread(),
install them into a second cred* on the task (together with the new
dumpability), drop the cred_guard_mutex, and let ptrace_may_access()
check against both. After that, some further restructuring might even
allow the cred_guard_mutex to not be held across all of the VFS
operations that happen early on in execve, which may block
indefinitely. But that would be pretty complicated, so I think your
proposed solution makes sense for now, given that nobody has managed
to implement anything better in the last few years.