Re: [REVIEW][PATCH 2/3] exec: Don't allow ptracing an exec of an unreadable file
From: Eric W. Biederman
Date: Thu Nov 17 2016 - 19:38:42 EST
Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:
> On Thu, Nov 17, 2016 at 3:55 PM, Eric W. Biederman
> <ebiederm@xxxxxxxxxxxx> wrote:
>> Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:
>>> On Thu, Nov 17, 2016 at 9:08 AM, Eric W. Biederman
>>> <ebiederm@xxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx
>>>> Reported-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
>>>> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
>>>> 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.