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.
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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