Re: [PATCHv5 1/3] syscalls,x86: implement execveat() system call

[David Drysdale]

[Oops, re-send remembering to turn on plaintext mode -- sorry]

On Mon, Oct 27, 2014 at 6:47 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Mon, Oct 27, 2014 at 11:03 AM, David Drysdale <drysdale@xxxxxxxxxx> wrote:
>> On Wed, Oct 22, 2014 at 7:44 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>> On Wed, Oct 22, 2014 at 4:44 AM, David Drysdale <drysdale@xxxxxxxxxx> wrote:
>>>> Add a new system execveat(2) syscall. execveat() is to execve() as
>>>> openat() is to open(): it takes a file descriptor that refers to a
>>>> directory, and resolves the filename relative to that.
>>>>
>>>
>>>>         bprm->file = file;
>>>> -       bprm->filename = bprm->interp = filename->name;
>>>> +       if (fd == AT_FDCWD || filename->name[0] == '/') {
>>>> +               bprm->filename = filename->name;
>>>> +       } else {
>>>> +               /*
>>>> +                * Build a pathname that reflects how we got to the file,
>>>> +                * either "/dev/fd/<fd>" (for an empty filename) or
>>>> +                * "/dev/fd/<fd>/<filename>".
>>>> +                */
>>>> +               pathbuf = kmalloc(PATH_MAX, GFP_TEMPORARY);
>>>> +               if (!pathbuf) {
>>>> +                       retval = -ENOMEM;
>>>> +                       goto out_unmark;
>>>> +               }
>>>> +               bprm->filename = pathbuf;
>>>> +               if (filename->name[0] == '\0')
>>>> +                       sprintf(pathbuf, "/dev/fd/%d", fd);
>>>
>>> If the fd is O_CLOEXEC, then this will result in a confused child
>>> process.  Should we fail exec attempts like that for non-static
>>> programs?  (E.g. set filename to "" or something and fix up the binfmt
>>> drivers to handle that?)
>>
>> Isn't it just scripts that get confused here (as normal executables don't
>> get to see brpm->filename)?
>>
>> Given that we don't know which we have at this point, I'd suggest
>> carrying on regardless.  Or we could fall back to use the previous
>> best-effort d_path() code for O_CLOEXEC fds.  Thoughts?
>
> How hard would it be to mark the bprm as not having a path for the
> binary?  Then we could fail later on if and when we actually need the
> path.

Adding a flag to bprm->interp_flags to indicate that the bprm->filename
will be inaccessible after exec is straightforward.  But I'm not sure who
should/could make use of the flag...

> I don't really have a strong opinion here, though.  I do prefer
> actually failing the execveat call over succeeding but invoking a
> script interpreter than can't possibly work.

Yeah, but that involves the kernel code (e.g. fs/binfmt_script.c) making
an assumption about what the interpreter is going to do -- specifically
that it's going to try to open its argv[1].  Admittedly, that's a very likely
assumption, but I'm not sure it's one the kernel should make -- a script
like "#!/bin/echo" wouldn't be very useful, but fexecve()ing it would still
work OK on a name like "/dev/fd/7" after fd 7 is closed.

(Also, we need some kind of non-empty name in bprm->filename,
even if it's going to be inaccessible later, so that any LSM processing
off of the bprm_set_creds()/bprm_check_security() hooks has something
to work with; those hooks are pre-exec so the "/dev/fd/<fd>" part should
still be OK at that point.)

So I guess I lean towards keeping "/dev/fd/<fd>/<path>" regardless.

>>
>>>> +               else
>>>> +                       snprintf(pathbuf, PATH_MAX,
>>>> +                                "/dev/fd/%d/%s", fd, filename->name);
>>>
>>> Does this need to handle the case where the result exceeds PATH_MAX?
>>
>> I guess we could kmalloc(strlen(filename->name) + 19) to avoid the
>> possibility of failure, but that just defers the inevitable -- the interpreter
>> won't be able to open the script file anyway.  But it would at least then
>> generate the appropriate error (ENAMETOOLONG rather than ENOENT).
>
> Depends whether anyone cares about bprm->filename.  But I think the
> code should either return an error or allocate enough space.

I'll allocate enough space.

>
> --
> Andy Lutomirski
> AMA Capital Management, LLC
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