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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 28 Apr 2020 14:36:43 -0700
Message-ID: <CAHk-=wjTLnMuZmBO2foeHhsLAoUTpUi7oBVJ67F4XKB+tdEDbQ@mail.gmail.com>
Subject: Re: [GIT PULL] Please pull proc and exec work for 5.7-rc1
To:     Jann Horn <jannh@google.com>
Cc:     Oleg Nesterov <oleg@redhat.com>,
        Bernd Edlinger <bernd.edlinger@hotmail.de>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Waiman Long <longman@redhat.com>,
        Ingo Molnar <mingo@kernel.org>, Will Deacon <will@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Alexey Gladkov <gladkov.alexey@gmail.com>
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On Tue, Apr 28, 2020 at 2:06 PM Jann Horn <jannh@google.com> wrote:
>
> In execve:
>
>  - After the point of no return, but before we start waiting for the
>    other threads to go away, finish calculating our post-execve creds
>    and stash them somewhere in the task_struct or so.
>  - Drop the cred_guard_mutex.
>  - Wait for the other threads to die.
>  - Take the cred_guard_mutex again.
>  - Clear out the pointer in the task_struct.
>  - Finish execve and install the new creds.
>  - Drop the cred_guard_mutex again.
>
> Then in ptrace_may_access, after taking the cred_guard_mutex, we'd
> know that the target task is either outside execve or in the middle of
> execve, with old and new credentials known; and then we could say "you
> only get to access that task if you're capable relative to *both* its
> old and new credentials, since the task currently has both state from
> the old executable and from the new one".

That doesn't solve the problem with "check_unsafe_exec()" - you might
miss setting LSM_UNSAFE_PTRACE.

Although maybe that whole function could be moved down (to after you
get the cred_guard_mutex the second time).

               Linus