Re: Should unprivileged linkat(..., AT_EMPTY_PATH) work on O_TMPFILE files?

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

Andy Lutomirski <luto@amacapital.net> writes:

> The change:
>
> commit f4e0c30c191f87851c4a53454abb55ee276f4a7e
> Author: Al Viro <viro@zeniv.linux.org.uk>
> Date:   Tue Jun 11 08:34:36 2013 +0400
>
>     allow the temp files created by open() to be linked to
>
>     O_TMPFILE | O_CREAT => linkat() with AT_SYMLINK_FOLLOW and /proc/self/fd/<n>
>     as oldpath (i.e. flink()) will create a link
>     O_TMPFILE | O_CREAT | O_EXCL => ENOENT on attempt to link those guys
>
>     Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
>
> makes it possible to hardlink an O_TMPFILE file using procfs.  Should
> linkat(fd, "", newdirfd, newpath, AT_EMPTY_PATH) also work?
>
> AFAICS it currently requires CAP_DAC_READ_SEARCH, but I'm a bit
> confused as to why linkat(..., AT_EMPTY_PATH) should have a stricter
> check than linkat(AT_FDCWD, "/proc/self/fd/n", ...,
> AT_SYMLINK_FOLLOW),  (The relevant change is
> 11a7b371b64ef39fc5fb1b6f2218eef7c4d035e3.)
>
> FWIW, this program works on Linux 3.9, which makes me doubt that the
> security restriction on linkat is doing any good:
>
> #include <stdio.h>
> #include <err.h>
> #include <fcntl.h>
> #include <unistd.h>
>
> int main(int argc, char **argv)
> {
>   char buf[128];
>
>   if (argc != 3)
>     errx(1, "Usage: flink FD PATH");
>
>   sprintf(buf, "/proc/self/fd/%d", atoi(argv[1]));
>   if (linkat(AT_FDCWD, buf, AT_FDCWD, argv[2], AT_SYMLINK_FOLLOW) != 0)
>     err(1, "linkat");
>   return 0;
> }
>
>
> Removing the check from the AT_EMPTY_PATH case would simplify code
> that wants to write a file, fsync it, and then flink it in.

I understand that this got merged upstream. But in case of the above
test we would walk the path pointed by /proc/self/fd/<x> right ?

ie,

20 -> /home/no-access/k

will the above test work ? Now if i pass the '20' to another application
I can affectively create a hardlink to that outside no-access and if k
happens to have 'r' for others, then everybody will be able to read
right ?. I understand that limitting the read access based on directory
permission is not a good idea. But aren't we expected to keep that ?

For O_TMPFILE we don't have a path name hence the above may not be a
real issue ? Can you help me understand what i am missing ?

-aneesh