linux-kernel - Re: [REVIEW][PATCH 2/3] exec: Don't allow ptracing an exec of an unreadable file

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Date:   Thu, 17 Nov 2016 18:35:57 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Linux Containers <containers@...ts.linux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm\@kvack.org" <linux-mm@...ck.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Michal Hocko <mhocko@...nel.org>, Jann Horn <jann@...jh.net>,
        Willy Tarreau <w@....eu>, Kees Cook <keescook@...omium.org>
Subject: Re: [REVIEW][PATCH 2/3] exec: Don't allow ptracing an exec of an unreadable file

Andy Lutomirski <luto@...capital.net> writes:

> On Thu, Nov 17, 2016 at 3:55 PM, Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>> Andy Lutomirski <luto@...capital.net> writes:
>>
>>> On Thu, Nov 17, 2016 at 9:08 AM, Eric W. Biederman
>>> <ebiederm@...ssion.com> wrote:
>>>>
>>>> It is the reasonable expectation that if an executable file is not
>>>> readable there will be no way for a user without special privileges to
>>>> read the file.  This is enforced in ptrace_attach but if we are
>>>> already attached there is no enforcement if a readonly executable
>>>> is exec'd.
>>>>
>>>> Therefore do the simple thing and if there is a non-dumpable
>>>> executable that we are tracing without privilege fail to exec it.
>>>>
>>>> Fixes: v1.0
>>>> Cc: stable@...r.kernel.org
>>>> Reported-by: Andy Lutomirski <luto@...capital.net>
>>>> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
>>>> ---
>>>>  fs/exec.c | 8 +++++++-
>>>>  1 file changed, 7 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/fs/exec.c b/fs/exec.c
>>>> index fdec760bfac3..de107f74e055 100644
>>>> --- a/fs/exec.c
>>>> +++ b/fs/exec.c
>>>> @@ -1230,6 +1230,11 @@ int flush_old_exec(struct linux_binprm * bprm)
>>>>  {
>>>>         int retval;
>>>>
>>>> +       /* Fail if the tracer can't read the executable */
>>>> +       if ((bprm->interp_flags & BINPRM_FLAGS_ENFORCE_NONDUMP) &&
>>>> +           !ptracer_capable(current, bprm->mm->user_ns))
>>>> +               return -EPERM;
>>>> +
>>>
>>> At the very least, I think that BINPRM_FLAGS_ENFORCE_NONDUMP needs to
>>> check capable_wrt_inode_uidgid too.  Otherwise we risk breaking:
>>>
>>> $ gcc whatever.c
>>> $ chmod 400 a.out
>>> $ strace a.out
>>
>> It is an invariant that if you have caps in mm->user_ns you will
>> also be capable_write_inode_uidgid of all files that a process exec's.
>
> I meant to check whether you *are* the owner, too.

I don't follow.  BINPRM_FLAGS_ENFORCE_NONDUMP is only set if
the caller of exec does not have inode_permission(inode, MAY_READ).

Which in your example would have guaranteed that
BINPRM_FLAGS_ENFORCE_NONDUMP would have be unset.

The ptracer_capable thing is only asking in this instance if we can
ignore the nondumpable status because we have CAP_SYS_PTRACE over
a user namespace that includes all of the files that would_dump
was called on (mm->user_ns).

ptrace_access_vm in the replacement patch has essentially the same
permission check.  It is just at PTRACE_PEEKTEXT, PTRACE_PEEKDATA,
PTRACE_POKETEXT, or PTRACE_POKEDATA time.

So I am curious if you are seeing something that is worth fixing.

>> My third patch winds up changing mm->user_ns to maintain this invariant.
>>
>> It is also true that Willy convinced me while this check is trivial it
>> will break historic uses so I have replaced this patch with:
>> "ptrace: Don't allow accessing an undumpable mm.
>
> I think that's better.

Eric