[PATCH 1/2] bpf, cgroup: Fix optlen WARN_ON_ONCE toctou

[Tim Gardner]

From: Loris Reiff <loris.reiff at liblor.ch>

CVE-2021-20194

A toctou issue in `__cgroup_bpf_run_filter_getsockopt` can trigger a
WARN_ON_ONCE in a check of `copy_from_user`.

`*optlen` is checked to be non-negative in the individual getsockopt
functions beforehand. Changing `*optlen` in a race to a negative value
will result in a `copy_from_user(ctx.optval, optval, ctx.optlen)` with
`ctx.optlen` being a negative integer.

Fixes: 0d01da6afc54 ("bpf: implement getsockopt and setsockopt hooks")
Signed-off-by: Loris Reiff <loris.reiff at liblor.ch>
Signed-off-by: Daniel Borkmann <daniel at iogearbox.net>
Reviewed-by: Stanislav Fomichev <sdf at google.com>
Link: https://lore.kernel.org/bpf/20210122164232.61770-1-loris.reiff@liblor.ch
(backported from commit bb8b81e396f7afbe7c50d789e2107512274d2a35)
Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
---
 kernel/bpf/cgroup.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index 4f1472409ef8..117a283d278e 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -1100,6 +1100,11 @@ int __cgroup_bpf_run_filter_getsockopt(struct sock *sk, int level,
 			goto out;
 		}
 
+		if (ctx.optlen < 0) {
+			ret = -EFAULT;
+			goto out;
+		}
+
 		if (ctx.optlen > max_optlen)
 			ctx.optlen = max_optlen;
 
-- 
2.17.1