Commit d59577b6 authored by Vincent Bernat's avatar Vincent Bernat Committed by David S. Miller
Browse files

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: default avatarVincent Bernat <bernat@luffy.cx>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 5bd30d39
......@@ -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.
Examples
========
Ioctls-
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.
......@@ -77,5 +77,6 @@
/* Instruct lower device to use last 4-bytes of skb data as FCS */
#define SO_NOFCS 43
#define SO_LOCK_FILTER 44
#endif /* _UAPI_ASM_SOCKET_H */
......@@ -70,4 +70,6 @@
/* Instruct lower device to use last 4-bytes of skb data as FCS */
#define SO_NOFCS 43
#define SO_LOCK_FILTER 44
#endif /* __ASM_AVR32_SOCKET_H */
......@@ -72,6 +72,8 @@
/* Instruct lower device to use last 4-bytes of skb data as FCS */
#define SO_NOFCS 43
#define SO_LOCK_FILTER 44
#endif /* _ASM_SOCKET_H */
......@@ -70,5 +70,7 @@
/* Instruct lower device to use last 4-bytes of skb data as FCS */
#define SO_NOFCS 43
#define SO_LOCK_FILTER 44
#endif /* _ASM_SOCKET_H */
......@@ -70,4 +70,6 @@
/* Instruct lower device to use last 4-bytes of skb data as FCS */
#define SO_NOFCS 43
#define SO_LOCK_FILTER 44
#endif /* _ASM_SOCKET_H */
......@@ -79,4 +79,6 @@
/* Instruct lower device to use last 4-bytes of skb data as FCS */
#define SO_NOFCS 43
#define SO_LOCK_FILTER 44
#endif /* _ASM_IA64_SOCKET_H */
......@@ -70,4 +70,6 @@
/* Instruct lower device to use last 4-bytes of skb data as FCS */
#define SO_NOFCS 43
#define SO_LOCK_FILTER 44
#endif /* _ASM_M32R_SOCKET_H */
......@@ -90,5 +90,6 @@ To add: #define SO_REUSEPORT 0x0200 /* Allow local address and port reuse. */
/* Instruct lower device to use last 4-bytes of skb data as FCS */
#define SO_NOFCS 43
#define SO_LOCK_FILTER 44
#endif /* _UAPI_ASM_SOCKET_H */
......@@ -70,4 +70,6 @@
/* Instruct lower device to use last 4-bytes of skb data as FCS */
#define SO_NOFCS 43
#define SO_LOCK_FILTER 44
#endif /* _ASM_SOCKET_H */
......@@ -69,6 +69,7 @@
/* Instruct lower device to use last 4-bytes of skb data as FCS */
#define SO_NOFCS 0x4024
#define SO_LOCK_FILTER 0x4025
/* O_NONBLOCK clashes with the bits used for socket types. Therefore we
* have to define SOCK_NONBLOCK to a different value here.
......
......@@ -77,4 +77,6 @@
/* Instruct lower device to use last 4-bytes of skb data as FCS */
#define SO_NOFCS 43
#define SO_LOCK_FILTER 44
#endif /* _ASM_POWERPC_SOCKET_H */
......@@ -76,4 +76,6 @@
/* Instruct lower device to use last 4-bytes of skb data as FCS */
#define SO_NOFCS 43
#define SO_LOCK_FILTER 44
#endif /* _ASM_SOCKET_H */
......@@ -66,6 +66,7 @@
/* Instruct lower device to use last 4-bytes of skb data as FCS */
#define SO_NOFCS 0x0027
#define SO_LOCK_FILTER 0x0028
/* Security levels - as per NRL IPv6 - don't actually do anything */
#define SO_SECURITY_AUTHENTICATION 0x5001
......
......@@ -81,4 +81,6 @@
/* Instruct lower device to use last 4-bytes of skb data as FCS */
#define SO_NOFCS 43
#define SO_LOCK_FILTER 44
#endif /* _XTENSA_SOCKET_H */
......@@ -664,6 +664,7 @@ enum sock_flags {
* Will use last 4 bytes of packet sent from
* user-space instead.
*/
SOCK_FILTER_LOCKED, /* Filter cannot be changed anymore */
};
static inline void sock_copy_flags(struct sock *nsk, struct sock *osk)
......
......@@ -73,4 +73,6 @@
/* Instruct lower device to use last 4-bytes of skb data as FCS */
#define SO_NOFCS 43
#define SO_LOCK_FILTER 44
#endif /* __ASM_GENERIC_SOCKET_H */
......@@ -721,6 +721,9 @@ int sk_attach_filter(struct sock_fprog *fprog, struct sock *sk)
unsigned int fsize = sizeof(struct sock_filter) * fprog->len;
int err;
if (sock_flag(sk, SOCK_FILTER_LOCKED))
return -EPERM;
/* Make sure new filter is there and in the right amounts. */
if (fprog->filter == NULL)
return -EINVAL;
......@@ -757,6 +760,9 @@ int sk_detach_filter(struct sock *sk)
int ret = -ENOENT;
struct sk_filter *filter;
if (sock_flag(sk, SOCK_FILTER_LOCKED))
return -EPERM;
filter = rcu_dereference_protected(sk->sk_filter,
sock_owned_by_user(sk));
if (filter) {
......
......@@ -861,6 +861,13 @@ set_rcvbuf:
ret = sk_detach_filter(sk);
break;
case SO_LOCK_FILTER:
if (sock_flag(sk, SOCK_FILTER_LOCKED) && !valbool)
ret = -EPERM;
else
sock_valbool_flag(sk, SOCK_FILTER_LOCKED, valbool);
break;
case SO_PASSSEC:
if (valbool)
set_bit(SOCK_PASSSEC, &sock->flags);
......@@ -1140,6 +1147,10 @@ int sock_getsockopt(struct socket *sock, int level, int optname,
goto lenout;
case SO_LOCK_FILTER:
v.val = sock_flag(sk, SOCK_FILTER_LOCKED);
break;
default:
return -ENOPROTOOPT;
}
......
Markdown is supported
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment