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From:   Jann Horn <jannh@google.com>
Date:   Tue, 28 Apr 2020 23:53:08 +0200
Message-ID: <CAG48ez3EQOvdbzu9aO-cEAJwF_=fJzn1Cg0LMs3ruc=5r1ie5w@mail.gmail.com>
Subject: Re: [GIT PULL] Please pull proc and exec work for 5.7-rc1
To:     Linus Torvalds <torvalds@linux-foundation.org>
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 11:37 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> 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.

You don't need LSM_UNSAFE_PTRACE if the tracer has already passed a
ptrace_may_access() check against the post-execve creds of the target
- that's no different from having done PTRACE_ATTACH after execve is
over.