From: KP Singh <kpsingh@google.com>
Date: Mon, 30 Mar 2020 16:42:46 +0200
Subject: bpf: btf: Fix arg verification in btf_ctx_access()
Patch-mainline: v5.7-rc1
Git-commit: f50b49a0bfcaf53e6394a873b588bc4cca2aab78
References: bsc#1177028

The bounds checking for the arguments accessed in the BPF program breaks
when the expected_attach_type is not BPF_TRACE_FEXIT, BPF_LSM_MAC or
BPF_MODIFY_RETURN resulting in no check being done for the default case
(the programs which do not receive the return value of the attached
function in its arguments) when the index of the argument being accessed
is equal to the number of arguments (nr_args).

This was a result of a misplaced "else if" block  introduced by the
Commit 6ba43b761c41 ("bpf: Attachment verification for
BPF_MODIFY_RETURN")

Fixes: 6ba43b761c41 ("bpf: Attachment verification for BPF_MODIFY_RETURN")
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200330144246.338-1-kpsingh@chromium.org
Acked-by: Gary Lin <glin@suse.com>
---
 kernel/bpf/btf.c |   24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)

--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -3710,9 +3710,16 @@ bool btf_ctx_access(int off, int size, e
 		nr_args--;
 	}
 
+	if (arg > nr_args) {
+		bpf_log(log, "func '%s' doesn't have %d-th argument\n",
+			tname, arg + 1);
+		return false;
+	}
+
 	if (arg == nr_args) {
-		if (prog->expected_attach_type == BPF_TRACE_FEXIT ||
-		    prog->expected_attach_type == BPF_LSM_MAC) {
+		switch (prog->expected_attach_type) {
+		case BPF_LSM_MAC:
+		case BPF_TRACE_FEXIT:
 			/* When LSM programs are attached to void LSM hooks
 			 * they use FEXIT trampolines and when attached to
 			 * int LSM hooks, they use MODIFY_RETURN trampolines.
@@ -3729,7 +3736,8 @@ bool btf_ctx_access(int off, int size, e
 			if (!t)
 				return true;
 			t = btf_type_by_id(btf, t->type);
-		} else if (prog->expected_attach_type == BPF_MODIFY_RETURN) {
+			break;
+		case BPF_MODIFY_RETURN:
 			/* For now the BPF_MODIFY_RETURN can only be attached to
 			 * functions that return an int.
 			 */
@@ -3743,17 +3751,19 @@ bool btf_ctx_access(int off, int size, e
 					btf_kind_str[BTF_INFO_KIND(t->info)]);
 				return false;
 			}
+			break;
+		default:
+			bpf_log(log, "func '%s' doesn't have %d-th argument\n",
+				tname, arg + 1);
+			return false;
 		}
-	} else if (arg >= nr_args) {
-		bpf_log(log, "func '%s' doesn't have %d-th argument\n",
-			tname, arg + 1);
-		return false;
 	} else {
 		if (!t)
 			/* Default prog with 5 args */
 			return true;
 		t = btf_type_by_id(btf, args[arg].type);
 	}
+
 	/* skip modifiers */
 	while (btf_type_is_modifier(t))
 		t = btf_type_by_id(btf, t->type);