linux-kernel - Re: ptrace versus setuid changes in 4.14?

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Message-ID: <ddbb8e2e-895e-8377-ed02-8950404e9708@redhat.com>
Date:   Fri, 22 Dec 2017 12:28:25 -0800
From:   Laura Abbott <labbott@...hat.com>
To:     Tom Horsley <horsley1953@...il.com>, linux-kernel@...r.kernel.org,
        Kees Cook <keescook@...omium.org>
Cc:     David Howells <dhowells@...hat.com>,
        Serge Hallyn <serge@...lyn.com>,
        James Morris <james.l.morris@...cle.com>
Subject: Re: ptrace versus setuid changes in 4.14?

On 12/21/2017 06:18 AM, Tom Horsley wrote:
> On my fedora 26 box with a 4.13 kernel, when a process
> under ptrace control did an exec of a setuid program,
> the program lost all of its setuid privileges and
> ptrace could operate on it like a normal program.
> 
> Experimental evidence seems to indicate that on
> fedora 27 with a 4.14 kernel, ptrace cannot
> do a PEEKDATA to read anything from the just
> execed setuid program. (I get errno 5 - I/O error).
> 
> Am I confused somehow, or did something really change
> in this vicinity?
> 
> It puts a real crimp in my fancy debug feature to
> patch code into a setuid program to make it re-exec itself,
> then detach from it. (I don't suppose we could get
> a setoptions feature to tell the kernel to detach
> from setuid programs automagically and let the debugger
> know it is no longer in control of the process?)
> 

Assuming this is https://bugzilla.redhat.com/show_bug.cgi?id=1528633
This is yet another victim of

commit e37fdb785a5f95ecadf43b773c97f676500ac7b8 (refs/bisect/bad)
Author: Kees Cook <keescook@...omium.org>
Date:   Tue Jul 18 15:25:31 2017 -0700

     exec: Use secureexec for setting dumpability
     
     The examination of "current" to decide dumpability is wrong. This was a
     check of and euid/uid (or egid/gid) mismatch in the existing process,
     not the newly created one. This appears to stretch back into even the
     "history.git" tree. Luckily, dumpability is later set in commit_creds().
     In earlier kernel versions before creds existed, similar checks also
     existed late in the exec flow, covering up the mistake as far back as I
     could find.
     
     Note that because the commit_creds() check examines differences of euid,
     uid, egid, gid, and capabilities between the old and new creds, it would
     look like the setup_new_exec() dumpability test could be entirely removed.
     However, the secureexec test may cover a different set of tests (specific
     to the LSMs) than what commit_creds() checks for. So, fix this test to
     use secureexec (the removed euid tests are redundant to the commoncap
     secureexec checks now).
     
     Cc: David Howells <dhowells@...hat.com>
     Signed-off-by: Kees Cook <keescook@...omium.org>
     Acked-by: Serge Hallyn <serge@...lyn.com>
     Reviewed-by: James Morris <james.l.morris@...cle.com>


Is it time to think about reverting?

Thanks,
Laura