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From: Andy Lutomirski <luto@amacapital.net>
Date: Mon, 27 Oct 2014 11:47:27 -0700
Message-ID: <CALCETrWN+bKj_PPRJy2pNDVCRd3xyd-=w7P+9khhd-XuJRXCyA@mail.gmail.com>
Subject: Re: [PATCHv5 1/3] syscalls,x86: implement execveat() system call
To: David Drysdale <drysdale@google.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Meredydd Luff <meredydd@senatehouse.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>, Arnd Bergmann <arnd@arndb.de>,
        X86 ML <x86@kernel.org>, linux-arch <linux-arch@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>
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On Mon, Oct 27, 2014 at 11:03 AM, David Drysdale <drysdale@google.com> wrote:
> On Wed, Oct 22, 2014 at 7:44 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>> On Wed, Oct 22, 2014 at 4:44 AM, David Drysdale <drysdale@google.com> 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.

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.

>
>>> +               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.


-- 
Andy Lutomirski
AMA Capital Management, LLC
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