|author||Vincent Bernat <email@example.com>||2013-01-16 22:55:49 +0100|
|committer||David S. Miller <firstname.lastname@example.org>||2013-01-17 03:21:25 -0500|
sk-filter: Add ability to lock a socket filter program
While a privileged program can open a raw socket, attach some restrictive filter and drop its privileges (or send the socket to an unprivileged program through some Unix socket), the filter can still be removed or modified by the unprivileged program. This commit adds a socket option to lock the filter (SO_LOCK_FILTER) preventing any modification of a socket filter program. This is similar to OpenBSD BIOCLOCK ioctl on bpf sockets, except even root is not allowed change/drop the filter. The state of the lock can be read with getsockopt(). No error is triggered if the state is not changed. -EPERM is returned when a user tries to remove the lock or to change/remove the filter while the lock is active. The check is done directly in sk_attach_filter() and sk_detach_filter() and does not affect only setsockopt() syscall. Signed-off-by: Vincent Bernat <email@example.com> Signed-off-by: David S. Miller <firstname.lastname@example.org>
Diffstat (limited to 'Documentation/networking')
1 files changed, 9 insertions, 2 deletions
diff --git a/Documentation/networking/filter.txt b/Documentation/networking/filter.txt
index bbf2005270b..cdb3e40b9d1 100644
@@ -17,12 +17,12 @@ creating filters.
LSF is much simpler than BPF. One does not have to worry about
devices or anything like that. You simply create your filter
-code, send it to the kernel via the SO_ATTACH_FILTER ioctl and
+code, send it to the kernel via the SO_ATTACH_FILTER option and
if your filter code passes the kernel check on it, you then
immediately begin filtering data on that socket.
You can also detach filters from your socket via the
-SO_DETACH_FILTER ioctl. This will probably not be used much
+SO_DETACH_FILTER option. This will probably not be used much
since when you close a socket that has a filter on it the
filter is automagically removed. The other less common case
may be adding a different filter on the same socket where you had another
@@ -31,12 +31,19 @@ the old one and placing your new one in its place, assuming your
filter has passed the checks, otherwise if it fails the old filter
will remain on that socket.
+SO_LOCK_FILTER option allows to lock the filter attached to a
+socket. Once set, a filter cannot be removed or changed. This allows
+one process to setup a socket, attach a filter, lock it then drop
+privileges and be assured that the filter will be kept until the
+socket is closed.
setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_FILTER, &Filter, sizeof(Filter));
setsockopt(sockfd, SOL_SOCKET, SO_DETACH_FILTER, &value, sizeof(value));
+setsockopt(sockfd, SOL_SOCKET, SO_LOCK_FILTER, &value, sizeof(value));
See the BSD bpf.4 manpage and the BSD Packet Filter paper written by
Steven McCanne and Van Jacobson of Lawrence Berkeley Laboratory.