[PATCH bpf v2] bpf: increase {get,set}sockopt optval size limit

[Stanislav Fomichev <sdf@google.com>]

Attaching to these hooks can break iptables because its optval is
usually quite big, or at least bigger than the current PAGE_SIZE limit.

There are two possible ways to fix it:
1. Increase the limit to match iptables max optval.
2. Implement some way to bypass the value if it's too big and trigger
   BPF only with level/optname so BPF can still decide whether
   to allow/deny big sockopts.

I went with #1 which means we are potentially increasing the
amount of data we copy from the userspace from PAGE_SIZE to 512M.

v2:
* proper comments formatting (Jakub Kicinski)

Fixes: 0d01da6afc54 ("bpf: implement getsockopt and setsockopt hooks")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
---
 kernel/bpf/cgroup.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index fdf7836750a3..fb786b0f0f88 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -1276,7 +1276,14 @@ static bool __cgroup_bpf_prog_array_is_empty(struct cgroup *cgrp,
 
 static int sockopt_alloc_buf(struct bpf_sockopt_kern *ctx, int max_optlen)
 {
-	if (unlikely(max_optlen > PAGE_SIZE) || max_optlen < 0)
+	/* The user with the largest known setsockopt optvals is iptables.
+	 * Allocate enough space to accommodate it.
+	 *
+	 * See XT_MAX_TABLE_SIZE and sizeof(struct ipt_replace).
+	 */
+	const int max_supported_optlen = 512 * 1024 * 1024 + 128;
+
+	if (unlikely(max_optlen > max_supported_optlen) || max_optlen < 0)
 		return -EINVAL;
 
 	ctx->optval = kzalloc(max_optlen, GFP_USER);
-- 
2.27.0.278.ge193c7cf3a9-goog