ptrace versus setuid changes in 4.14?

ptrace versus setuid changes in 4.14?

[Tom Horsley]

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?)

Re: ptrace versus setuid changes in 4.14?

[Laura Abbott]

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@chromium.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@redhat.com>
     Signed-off-by: Kees Cook <keescook@chromium.org>
     Acked-by: Serge Hallyn <serge@hallyn.com>
     Reviewed-by: James Morris <james.l.morris@oracle.com>


Is it time to think about reverting?

Thanks,
Laura

Re: ptrace versus setuid changes in 4.14?

[Tom Horsley]

On Fri, 22 Dec 2017 12:28:25 -0800
Laura Abbott wrote:

> 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@chromium.org>
> Date:   Tue Jul 18 15:25:31 2017 -0700
> 
>      exec: Use secureexec for setting dumpability

You mean there is hope this really is a bug and not a security
enhancement? Amazing :-).

And yes, that is the bugzilla I submitted after I reduced
things to a small test program.

Re: ptrace versus setuid changes in 4.14?

[Kees Cook]

On Fri, Dec 22, 2017 at 12:55 PM, Tom Horsley <horsley1953@gmail.com> wrote:
> On Fri, 22 Dec 2017 12:28:25 -0800
> Laura Abbott wrote:
>
>> 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@chromium.org>
>> Date:   Tue Jul 18 15:25:31 2017 -0700
>>
>>      exec: Use secureexec for setting dumpability
>
> You mean there is hope this really is a bug and not a security
> enhancement? Amazing :-).
>
> And yes, that is the bugzilla I submitted after I reduced
> things to a small test program.

I think the secureexec dumpability logic just needs to be removed --
the logic in commit_creds() _should_ be sufficient, but I want to
double-check it now that I've got some more tests cases.

-Kees

-- 
Kees Cook
Pixel Security