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

From: Andy Lutomirski
Date: Fri Aug 02 2013 - 00:44:42 EST

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@xxxxxxxxxxxxxx>
fs/namei.c | 10 +++-------
1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 8b61d10..89a612e 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)
- }

if (flags & AT_SYMLINK_FOLLOW)

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