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| author | Andy Lutomirski <luto@amacapital.net> | 2013-08-01 21:44:31 -0700 |
|---|---|---|
| committer | Al Viro <viro@zeniv.linux.org.uk> | 2013-08-05 18:24:11 +0400 |
| commit | bb2314b47996491bbc5add73633905c3120b6268 (patch) | |
| tree | a7ce9012a84c22c5a08f89db6fb88dee093a0f88 | |
| parent | e305f48bc453da773a3601135a2cce40b8e62856 (diff) | |
| download | linux-bb2314b47996491bbc5add73633905c3120b6268.tar.bz2 | |
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>
| -rw-r--r-- | fs/namei.c | 10 |
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; |