Re: [RFC] Possible new execveat(2) Linux syscall

[Andy Lutomirski <luto at amacapital dot net>]

On Sun, Nov 16, 2014 at 2:08 PM, Rich Felker <dalias@aerifal.cx> wrote:
> On Sun, Nov 16, 2014 at 01:20:39PM -0800, Andy Lutomirski wrote:
>> On Nov 16, 2014 11:53 AM, "Rich Felker" <dalias@aerifal.cx> wrote:
>> >
>> > On Fri, Nov 14, 2014 at 02:54:19PM +0000, David Drysdale wrote:
>> > > Hi,
>> > >
>> > > Over at the LKML[1] we've been discussing a possible new syscall, execveat(2),
>> > > and it would be good to hear a glibc perspective about it (and whether there
>> > > are any interface changes that would make it easier to use from userspace).
>> > >
>> > > The syscall prototype is:
>> > >   int execveat(int fd, const char *pathname,
>> > >                       char *const argv[],  char *const envp[],
>> > >                       int flags); /* AT_EMPTY_PATH, AT_SYMLINK_NOFOLLOW */
>> > > and it works similarly to execve(2) except:
>> > >  - the executable to run is identified by the combination of fd+pathname, like
>> > >    other *at(2) syscalls
>> > >  - there's an extra flags field to control behaviour.
>> > > (I've attached a text version of the suggested man page below)
>> > >
>> > > One particular benefit of this is that it allows an fexecve(3) implementation
>> > > that doesn't rely on /proc being accessible, which is useful for sandboxed
>> > > applications.  (However, that does only work for non-interpreted programs:
>> > > the name passed to a script interpreter is of the form "/dev/fd/<fd>/<path>"
>> > > or "/dev/fd/<fd>", so the executed interpreter will normally still need /proc
>> > > access to load the script file).
>> > >
>> > > How does this sound from a glibc perspective?
>> >
>> > I've been following the discussions so far and everything looks mostly
>> > okay. There are still issues to be resolved with the different
>> > semantics between Linux O_PATH and what POSIX requires for O_EXEC (and
>> > O_SEARCH) but as long as the intent is that, once O_EXEC is defined to
>> > save the permissions at the time of open and cause them to be used in
>> > place of the current file permissions at the time of execveat
>>
>> Is something missing here?
>>
>> FWIW, I don't understand O_PATH or O_EXEC very well, so from my POV,
>> help would be appreciated.
>
> Yes. POSIX requires that permission checks for execution (fexecve with
> O_EXEC file descriptors) and directory-search (*at functions with
> O_SEARCH file descriptors) succeed if the open operation succeeded --
> the permissions check is required to take place at open time rather
> than at exec/search time. There's a separate discussion about how to
> make this work on the kernel side.

It may be worth making this work as part of adding execveat to the
kernel.  Does the kernel even have O_EXEC right now?

>
>> > One major issue however is FD_CLOEXEC with scripts. Last I checked,
>> > this didn't work because the file is already closed by the time the
>> > interpreted runs. The intended usage of fexecve is almost certainly to
>> > call it with the file descriptor set close-on-exec; otherwise, there
>> > would be no clean way to close it, since the program being executed
>> > doesn't know that it's being executed via fexecve. So this is a
>> > serious problem that needs to be solved if it hasn't already. I have
>> > some ideas I could offer, but I'm not an expert on the kernel side
>> > things so I'm not sure they'd be correct.
>>
>> Bring on the ideas.
>
> My thought is that when the kernel opens the binary and sees that it's
> a script that needs an interpreter, the kernel should not pass
> /proc/self/fd/%d to the interpreter, but instead should pass the name
> of a new magic symlink in /proc/self that's connected to the inode for
> the script to be executed but that ceases to exist as soon as it's
> opened. In theory this could also be used for suid scripts to make
> them secure.

This doesn't help if /proc is not mounted, which is an important use case.

>
>> FWIW, I've often thought that interpreter binaries should mark
>> themselves as such to enable better interactions with the kernel.
>
> That's hard since users expect to be able to use arbitrary
> interpreters (and sometimes even pass through multiple ones, e.g.
> #!/usr/bin/env perl).
>

Hmm.  I'd be okay with old interpreters having a somewhat degraded experience.

I guess that #!/some/interpreted/script isn't allowed, but maybe
#!/usr/bin/env some-interpreted-script should work.

It could be that all that's really needed is some convention to tell
an interpreter that it should use fd N as a script *and close it*.
Something like /dev/fd_and_close/N could work, but that has all kinds
of problems.

Alternatively, if we could have a way to mark an fd so that it's
close-on-exec after exec, that would solve the nesting problem, as
long as every interpreter in the chain does it.  And the kernel could
certainly implement execve on a close-on-exec fd by passing /dev/fd/N
where N is a close-on-exec fd, at least in the non-nested case.

--Andy

> Rich



-- 
Andy Lutomirski
AMA Capital Management, LLC