netdev - [PATCH net] bpf: fix panic in SO_GET_FILTER with native ebpf programs

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Message-Id: <98ab6d68ec6db39bfa3b1655d228692457861d60.1443779830.git.daniel@iogearbox.net>
Date:	Fri,  2 Oct 2015 12:06:03 +0200
From:	Daniel Borkmann <daniel@...earbox.net>
To:	davem@...emloft.net
Cc:	ast@...mgrid.com, netdev@...r.kernel.org,
	Daniel Borkmann <daniel@...earbox.net>
Subject: [PATCH net] bpf: fix panic in SO_GET_FILTER with native ebpf programs

When sockets have a native eBPF program attached through
setsockopt(sk, SOL_SOCKET, SO_ATTACH_BPF, ...), and then try to
dump these over getsockopt(sk, SOL_SOCKET, SO_GET_FILTER, ...),
the following panic appears:

  [49904.178642] BUG: unable to handle kernel NULL pointer dereference at (null)
  [49904.178762] IP: [<ffffffff81610fd9>] sk_get_filter+0x39/0x90
  [49904.182000] PGD 86fc9067 PUD 531a1067 PMD 0
  [49904.185196] Oops: 0000 [#1] SMP
  [...]
  [49904.224677] Call Trace:
  [49904.226090]  [<ffffffff815e3d49>] sock_getsockopt+0x319/0x740
  [49904.227535]  [<ffffffff812f59e3>] ? sock_has_perm+0x63/0x70
  [49904.228953]  [<ffffffff815e2fc8>] ? release_sock+0x108/0x150
  [49904.230380]  [<ffffffff812f5a43>] ? selinux_socket_getsockopt+0x23/0x30
  [49904.231788]  [<ffffffff815dff36>] SyS_getsockopt+0xa6/0xc0
  [49904.233267]  [<ffffffff8171b9ae>] entry_SYSCALL_64_fastpath+0x12/0x71

The underlying issue is the very same as in commit b382c0865600
("sock, diag: fix panic in sock_diag_put_filterinfo"), that is,
native eBPF programs don't store an original program since this
is only needed in cBPF ones.

However, sk_get_filter() wasn't updated to test for this at the
time when eBPF could be attached. Just throw an error to the user
to indicate that eBPF cannot be dumped over this interface.
That way, it can also be known that a program _is_ attached (as
opposed to just return 0), and a different (future) method needs
to be consulted for a dump.

Fixes: 89aa075832b0 ("net: sock: allow eBPF programs to be attached to sockets")
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
---
 net/core/filter.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 05a04ea..87b78ef 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -1854,9 +1854,13 @@ int sk_get_filter(struct sock *sk, struct sock_filter __user *ubuf,
 		goto out;
 
 	/* We're copying the filter that has been originally attached,
-	 * so no conversion/decode needed anymore.
+	 * so no conversion/decode needed anymore. eBPF programs that
+	 * have no original program cannot be dumped through this.
 	 */
+	ret = -EACCES;
 	fprog = filter->prog->orig_prog;
+	if (!fprog)
+		goto out;
 
 	ret = fprog->len;
 	if (!len)
-- 
1.9.3

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