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 17:55:16 -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 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.

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.

Eric