bpf: Don't return EINVAL from {get,set}sockopt when optlen > PAGE_SIZE

Attaching to these hooks can break iptables because its optval is
usually quite big, or at least bigger than the current PAGE_SIZE limit.
David also mentioned some SCTP options can be big (around 256k).

For such optvals we expose only the first PAGE_SIZE bytes to
the BPF program. BPF program has two options:
1. Set ctx->optlen to 0 to indicate that the BPF's optval
should be ignored and the kernel should use original userspace
value.
2. Set ctx->optlen to something that's smaller than the PAGE_SIZE.

v5:
* use ctx->optlen == 0 with trimmed buffer (Alexei Starovoitov)
* update the docs accordingly

v4:
* use temporary buffer to avoid optval == optval_end == NULL;
this removes the corner case in the verifier that might assume
non-zero PTR_TO_PACKET/PTR_TO_PACKET_END.

v3:
* don't increase the limit, bypass the argument

v2:
* proper comments formatting (Jakub Kicinski)

Fixes: 0d01da6afc54 ("bpf: implement getsockopt and setsockopt hooks")
Signed-off-by: Stanislav Fomichev <[e-mail]>
Signed-off-by: Alexei Starovoitov <[e-mail]>
Cc: David Laight <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

kernel/bpf/cgroup.c

···
 
 static int sockopt_alloc_buf(struct bpf_sockopt_kern *ctx, int max_optlen)
 {
-	if (unlikely(max_optlen > PAGE_SIZE) || max_optlen < 0)
+	if (unlikely(max_optlen < 0))
 		return -EINVAL;
+
+	if (unlikely(max_optlen > PAGE_SIZE)) {
+		/* We don't expose optvals that are greater than PAGE_SIZE
+		 * to the BPF program.
+		 */
+		max_optlen = PAGE_SIZE;
+	}
 
 	ctx->optval = kzalloc(max_optlen, GFP_USER);
 	if (!ctx->optval)
···
 
 	ctx->optval_end = ctx->optval + max_optlen;
 
-	return 0;
+	return max_optlen;
 }
 
 static void sockopt_free_buf(struct bpf_sockopt_kern *ctx)
···
 	 */
 	max_optlen = max_t(int, 16, *optlen);
 
-	ret = sockopt_alloc_buf(&ctx, max_optlen);
-	if (ret)
-		return ret;
+	max_optlen = sockopt_alloc_buf(&ctx, max_optlen);
+	if (max_optlen < 0)
+		return max_optlen;
 
 	ctx.optlen = *optlen;
 
-	if (copy_from_user(ctx.optval, optval, *optlen) != 0) {
+	if (copy_from_user(ctx.optval, optval, min(*optlen, max_optlen)) != 0) {
 		ret = -EFAULT;
 		goto out;
 	}
···
 		/* export any potential modifications */
 		*level = ctx.level;
 		*optname = ctx.optname;
-		*optlen = ctx.optlen;
-		*kernel_optval = ctx.optval;
+
+		/* optlen == 0 from BPF indicates that we should
+		 * use original userspace data.
+		 */
+		if (ctx.optlen != 0) {
+			*optlen = ctx.optlen;
+			*kernel_optval = ctx.optval;
+		}
 	}
 
 out:
···
 	    __cgroup_bpf_prog_array_is_empty(cgrp, BPF_CGROUP_GETSOCKOPT))
 		return retval;
 
-	ret = sockopt_alloc_buf(&ctx, max_optlen);
-	if (ret)
-		return ret;
-
 	ctx.optlen = max_optlen;
+
+	max_optlen = sockopt_alloc_buf(&ctx, max_optlen);
+	if (max_optlen < 0)
+		return max_optlen;
 
 	if (!retval) {
 		/* If kernel getsockopt finished successfully,
···
 			goto out;
 		}
 
-		if (ctx.optlen > max_optlen)
-			ctx.optlen = max_optlen;
-
-		if (copy_from_user(ctx.optval, optval, ctx.optlen) != 0) {
+		if (copy_from_user(ctx.optval, optval,
+				   min(ctx.optlen, max_optlen)) != 0) {
 			ret = -EFAULT;
 			goto out;
 		}
···
 		goto out;
 	}
 
-	if (copy_to_user(optval, ctx.optval, ctx.optlen) ||
-	    put_user(ctx.optlen, optlen)) {
-		ret = -EFAULT;
-		goto out;
+	if (ctx.optlen != 0) {
+		if (copy_to_user(optval, ctx.optval, ctx.optlen) ||
+		    put_user(ctx.optlen, optlen)) {
+			ret = -EFAULT;
+			goto out;
+		}
 	}
 
 	ret = ctx.retval;