[bpf,1/5] bpf: sockmap, fix proto update hook to avoid dup calls

Commit Message

John Fastabend Dec. 21, 2023, 11:23 p.m. UTC

When sockets are added to a sockmap or sockhash we allocate and init a
psock. Then update the proto ops with sock_map_init_proto the flow is

  sock_hash_update_common
    sock_map_link
      psock = sock_map_psock_get_checked() <-returns existing psock
      sock_map_init_proto(sk, psock)       <- updates sk_proto

If the socket is already in a map this results in the sock_map_init_proto
being called multiple times on the same socket. We do this because when
a socket is added to multiple maps this might result in a new set of BPF
programs being attached to the socket requiring an updated ops struct.

This creates a rule where it must be safe to call psock_update_sk_prot
multiple times. When we added a fix for UAF through unix sockets in patch
4dd9a38a753fc we broke this rule by adding a sock_hold in that path
to ensure the sock is not released. The result is if a af_unix stream sock
is placed in multiple maps it results in a memory leak because we call
sock_hold multiple times with only a single sock_put on it.

Fixes: 4dd9a38a753fc ("bpf: sockmap, fix proto update hook to avoid dup calls")
Rebported-by: Xingwei Lee <[e-mail]>
Signed-off-by: John Fastabend <[e-mail]>
---
 net/unix/unix_bpf.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

Comments

Jakub Sitnicki Jan. 2, 2024, noon UTC | #1

On Thu, Dec 21, 2023 at 03:23 PM -08, John Fastabend wrote:
> When sockets are added to a sockmap or sockhash we allocate and init a
> psock. Then update the proto ops with sock_map_init_proto the flow is
>
>   sock_hash_update_common
>     sock_map_link
>       psock = sock_map_psock_get_checked() <-returns existing psock
>       sock_map_init_proto(sk, psock)       <- updates sk_proto
>
> If the socket is already in a map this results in the sock_map_init_proto
> being called multiple times on the same socket. We do this because when
> a socket is added to multiple maps this might result in a new set of BPF
> programs being attached to the socket requiring an updated ops struct.
>
> This creates a rule where it must be safe to call psock_update_sk_prot
> multiple times. When we added a fix for UAF through unix sockets in patch
> 4dd9a38a753fc we broke this rule by adding a sock_hold in that path
> to ensure the sock is not released. The result is if a af_unix stream sock
> is placed in multiple maps it results in a memory leak because we call
> sock_hold multiple times with only a single sock_put on it.
>
> Fixes: 4dd9a38a753fc ("bpf: sockmap, fix proto update hook to avoid dup calls")
> Rebported-by: Xingwei Lee <[e-mail]>

Nit: Typo ^

> Signed-off-by: John Fastabend <[e-mail]>
> ---

[...]

Martin KaFai Lau Jan. 4, 2024, 1 a.m. UTC | #2

On 1/2/24 4:00 AM, Jakub Sitnicki wrote:
> On Thu, Dec 21, 2023 at 03:23 PM -08, John Fastabend wrote:
>> When sockets are added to a sockmap or sockhash we allocate and init a
>> psock. Then update the proto ops with sock_map_init_proto the flow is
>>
>>    sock_hash_update_common
>>      sock_map_link
>>        psock = sock_map_psock_get_checked() <-returns existing psock
>>        sock_map_init_proto(sk, psock)       <- updates sk_proto
>>
>> If the socket is already in a map this results in the sock_map_init_proto
>> being called multiple times on the same socket. We do this because when
>> a socket is added to multiple maps this might result in a new set of BPF
>> programs being attached to the socket requiring an updated ops struct.
>>
>> This creates a rule where it must be safe to call psock_update_sk_prot
>> multiple times. When we added a fix for UAF through unix sockets in patch
>> 4dd9a38a753fc we broke this rule by adding a sock_hold in that path
>> to ensure the sock is not released. The result is if a af_unix stream sock
>> is placed in multiple maps it results in a memory leak because we call
>> sock_hold multiple times with only a single sock_put on it.
>>
>> Fixes: 4dd9a38a753fc ("bpf: sockmap, fix proto update hook to avoid dup calls")

The Fixes tag looks wrong ;)

I changed it to

Fixes: 8866730aed51 ("bpf, sockmap: af_unix stream sockets need to hold ref for 
pair sock")

>> Rebported-by: Xingwei Lee <[e-mail]>
> 
> Nit: Typo ^

yep. fixed.

Also added the missing "test_sockmap_pass_prog__destroy(skel)" to the 
sockmap_basic.c selftest.

Applied. Thanks for the fixes and the review.

John Fastabend Jan. 4, 2024, 3:47 a.m. UTC | #3

Martin KaFai Lau wrote:
> On 1/2/24 4:00 AM, Jakub Sitnicki wrote:
> > On Thu, Dec 21, 2023 at 03:23 PM -08, John Fastabend wrote:
> >> When sockets are added to a sockmap or sockhash we allocate and init a
> >> psock. Then update the proto ops with sock_map_init_proto the flow is
> >>
> >>    sock_hash_update_common
> >>      sock_map_link
> >>        psock = sock_map_psock_get_checked() <-returns existing psock
> >>        sock_map_init_proto(sk, psock)       <- updates sk_proto
> >>
> >> If the socket is already in a map this results in the sock_map_init_proto
> >> being called multiple times on the same socket. We do this because when
> >> a socket is added to multiple maps this might result in a new set of BPF
> >> programs being attached to the socket requiring an updated ops struct.
> >>
> >> This creates a rule where it must be safe to call psock_update_sk_prot
> >> multiple times. When we added a fix for UAF through unix sockets in patch
> >> 4dd9a38a753fc we broke this rule by adding a sock_hold in that path
> >> to ensure the sock is not released. The result is if a af_unix stream sock
> >> is placed in multiple maps it results in a memory leak because we call
> >> sock_hold multiple times with only a single sock_put on it.
> >>
> >> Fixes: 4dd9a38a753fc ("bpf: sockmap, fix proto update hook to avoid dup calls")
> 
> The Fixes tag looks wrong ;)
> 
> I changed it to
> 
> Fixes: 8866730aed51 ("bpf, sockmap: af_unix stream sockets need to hold ref for 
> pair sock")
> 
> >> Rebported-by: Xingwei Lee <[e-mail]>
> > 
> > Nit: Typo ^
> 
> yep. fixed.
> 
> Also added the missing "test_sockmap_pass_prog__destroy(skel)" to the 
> sockmap_basic.c selftest.

Thanks! Appreciate it Martin.

> 
> Applied. Thanks for the fixes and the review.
>

Patch

diff --git a/net/unix/unix_bpf.c b/net/unix/unix_bpf.c
index 7ea7c3a0d0d0..bd84785bf8d6 100644
--- a/net/unix/unix_bpf.c
+++ b/net/unix/unix_bpf.c
@@ -161,15 +161,30 @@  int unix_stream_bpf_update_proto(struct sock *sk, struct sk_psock *psock, bool r
 {
 	struct sock *sk_pair;
 
+	/* Restore does not decrement the sk_pair reference yet because we must
+	 * keep the a reference to the socket until after an RCU grace period
+	 * and any pending sends have completed.
+	 */
 	if (restore) {
 		sk->sk_write_space = psock->saved_write_space;
 		sock_replace_proto(sk, psock->sk_proto);
 		return 0;
 	}
 
-	sk_pair = unix_peer(sk);
-	sock_hold(sk_pair);
-	psock->sk_pair = sk_pair;
+	/* psock_update_sk_prot can be called multiple times if psock is
+	 * added to multiple maps and/or slots in the same map. There is
+	 * also an edge case where replacing a psock with itself can trigger
+	 * an extra psock_update_sk_prot during the insert process. So it
+	 * must be safe to do multiple calls. Here we need to ensure we don't
+	 * increment the refcnt through sock_hold many times. There will only
+	 * be a single matching destroy operation.
+	 */
+	if (!psock->sk_pair) {
+		sk_pair = unix_peer(sk);
+		sock_hold(sk_pair);
+		psock->sk_pair = sk_pair;
+	}
+
 	unix_stream_bpf_check_needs_rebuild(psock->sk_proto);
 	sock_replace_proto(sk, &unix_stream_bpf_prot);
 	return 0;