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 16:10:47 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Linux Containers <containers@...ts.linux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.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

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.

>
> 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
>
>



-- 
Andy Lutomirski
AMA Capital Management, LLC