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 <[email protected]> --- 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) how = LOOKUP_EMPTY; - } if (flags & AT_SYMLINK_FOLLOW) how |= LOOKUP_FOLLOW; -- 1.8.3.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
