[PATCH] apparmor: fix out-of-bounds read in unix socket address audit

[Henry Martin <bsdhenrymartin@gmail.com>]

audit_unix_addr() logs pathname unix addresses with
audit_log_untrustedstring(), which sizes the string with strlen().
The sockaddr passed to the LSM hook is not NUL-terminated:
__sys_bind() copies only addrlen bytes from userspace into an
on-stack buffer and leaves the tail uninitialized. A pathname
address without a NUL byte within addrlen makes strlen() run past
the stack object, and adjacent kernel stack contents end up in the
audit record. E.g. on mainline (05441d697f7f, KASAN build):

    BUG: KASAN: stack-out-of-bounds in strlen+0x93/0xa0
    Read of size 1 at addr ffffc9000287fe98 by task poc/1004
    CPU: 3 UID: 1000 PID: 1004 Comm: poc
    Call Trace:
     strlen+0x93/0xa0
     audit_log_untrustedstring+0x19/0x30
     audit_net_cb+0x3cb/0x760
     aa_check_perms+0x19d/0x340
     aa_unix_bind_perm+0x73a/0xb60
     security_socket_bind+0x2c/0x60
     __sys_bind+0x103/0x200

Bound the string by the address length with strnlen(), matching
the abstract-address branch.

This issue was discovered by Tencent CodeBuddy Security.

Cc: stable@vger.kernel.org
Fixes: c05e705812d1 ("apparmor: add fine grained af_unix mediation")
Signed-off-by: Henry Martin <bsdhenrymartin@gmail.com>
---
 security/apparmor/net.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/security/apparmor/net.c b/security/apparmor/net.c
index 136d14226b97d..cc8dcc5bd619c 100644
--- a/security/apparmor/net.c
+++ b/security/apparmor/net.c
@@ -86,7 +86,8 @@ static void audit_unix_addr(struct audit_buffer *ab, const char *str,
 		audit_log_format(ab, " %s=none", str);
 	} else if (addr->sun_path[0]) {
 		audit_log_format(ab, " %s=", str);
-		audit_log_untrustedstring(ab, addr->sun_path);
+		audit_log_n_untrustedstring(ab, addr->sun_path,
+					    strnlen(addr->sun_path, len));
 	} else {
 		audit_log_format(ab, " %s=\"@", str);
 		if (audit_string_contains_control(&addr->sun_path[1], len - 1))
-- 
2.43.7