fs: Allow unprivileged linkat(..., AT_EMPTY_PATH) aka flink

Every now and then someone proposes a new flink syscall, and this spawns
a long discussion of whether it would be a security problem.  I think
that this is missing the point: flink is *already* allowed without
privilege as long as /proc is mounted -- it's called AT_SYMLINK_FOLLOW.

Now that O_TMPFILE is here, the ability to create a file with O_TMPFILE,
write it, and link it in is very convenient.  The only problem is that
it requires that /proc be mounted so that you can do:

linkat(AT_FDCWD, "/proc/self/fd/<tmpfd>", dfd, path, AT_SYMLINK_NOFOLLOW)

This sucks -- it's much nicer to do:

linkat(tmpfd, "", dfd, path, AT_EMPTY_PATH)

Let's allow it.

If this turns out to be excessively scary, it we could instead require
that the inode in question be I_LINKABLE, but this seems pointless given
the /proc situation

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 3 insertions, 7 deletions

diff --git a/fs/namei.c b/fs/namei.c
index 8b61d103a8a7..89a612e392eb 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3671,15 +3671,11 @@ SYSCALL_DEFINE5(linkat, int, olddfd, const char __user *, oldname,
         if ((flags & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH)) != 0)
                 return -EINVAL;
         /*
-         * To use null names we require CAP_DAC_READ_SEARCH
-         * This ensures that not everyone will be able to create
-         * handlink using the passed filedescriptor.
+         * Using empty names is equivalent to using AT_SYMLINK_FOLLOW
+         * on /proc/self/fd/<fd>.
          */
-        if (flags & AT_EMPTY_PATH) {
-                if (!capable(CAP_DAC_READ_SEARCH))
-                        return -ENOENT;
+        if (flags & AT_EMPTY_PATH)
                 how = LOOKUP_EMPTY;
-        }
 
         if (flags & AT_SYMLINK_FOLLOW)
                 how |= LOOKUP_FOLLOW;