Received-SPF: pass (google.com: domain of linux-kernel+bounces-168865-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.199.223 as permitted sender) client-ip=147.75.199.223;
From: Camila Alvarez <cam.alvarez.i@gmail.com>
To: ast@kernel.org,
	daniel@iogearbox.net
Cc: bpf@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Camila Alvarez <cam.alvarez.i@gmail.com>,
	syzbot+d2a2c639d03ac200a4f1@syzkaller.appspotmail.com
Subject: [PATCH] fix array-index-out-of-bounds in bpf_prog_select_runtime
Date: Sat,  4 May 2024 21:46:43 -0400
Message-Id: <20240505014641.203643-1-cam.alvarez.i@gmail.com>
Precedence: bulk
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit

The error indicates that the verifier is letting through a program with
a stack depth bigger than 512.

This is due to the verifier not checking the stack depth after
instruction rewrites are perfomed. For example, the MAY_GOTO instruction
adds 8 bytes to the stack, which means that if the stack at the moment
was already 512 bytes it would overflow after rewriting the instruction.

The fix involves adding a stack depth check after all instruction
rewrites are performed.

Reported-by: syzbot+d2a2c639d03ac200a4f1@syzkaller.appspotmail.com
Signed-off-by: Camila Alvarez <cam.alvarez.i@gmail.com>
---
 kernel/bpf/verifier.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 63749ad5ac6b..a9e23b6b8e8f 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -21285,6 +21285,10 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr, bpfptr_t uattr, __u3
 	if (ret == 0)
 		ret = do_misc_fixups(env);
 
+        /* max stack depth verification must be done after rewrites as well */
+        if (ret == 0)
+                ret = check_max_stack_depth(env);
+
 	/* do 32-bit optimization after insn patching has done so those patched
 	 * insns could be handled correctly.
 	 */
-- 
2.34.1