af_unix: Fix fortify_panic() in unix_bind_bsd().

syzkaller found a bug in unix_bind_bsd() [0].  We can reproduce it
by bind()ing a socket on a path with length 108.

108 is the size of sun_addr of struct sockaddr_un and is the maximum
valid length for the pathname socket.  When calling bind(), we use
struct sockaddr_storage as the actual buffer size, so terminating
sun_addr[108] with null is legitimate as done in unix_mkname_bsd().

However, strlen(sunaddr) for such a case causes fortify_panic() if
CONFIG_FORTIFY_SOURCE=y.  __fortify_strlen() has no idea about the
actual buffer size and see the string as unterminated.

Let's use strnlen() to allow sun_addr to be unterminated at 107.

[0]:
detected buffer overflow in __fortify_strlen
kernel BUG at lib/string_helpers.c:1031!
Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
Modules linked in:
CPU: 0 PID: 255 Comm: syz-executor296 Not tainted 6.5.0-rc1-00330-g60cc1f7d0605 #4
Hardware name: linux,dummy-virt (DT)
pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : fortify_panic+0x1c/0x20 lib/string_helpers.c:1030
lr : fortify_panic+0x1c/0x20 lib/string_helpers.c:1030
sp : ffff800089817af0
x29: ffff800089817af0 x28: ffff800089817b40 x27: 1ffff00011302f68
x26: 000000000000006e x25: 0000000000000012 x24: ffff800087e60140
x23: dfff800000000000 x22: ffff800089817c20 x21: ffff800089817c8e
x20: 000000000000006c x19: ffff00000c323900 x18: ffff800086ab1630
x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000001
x14: 1ffff00011302eb8 x13: 0000000000000000 x12: 0000000000000000
x11: 0000000000000000 x10: 0000000000000000 x9 : 64a26b65474d2a00
x8 : 64a26b65474d2a00 x7 : 0000000000000001 x6 : 0000000000000001
x5 : ffff800089817438 x4 : ffff800086ac99e0 x3 : ffff800080f19e8c
x2 : 0000000000000001 x1 : 0000000100000000 x0 : 000000000000002c
Call trace:
 fortify_panic+0x1c/0x20 lib/string_helpers.c:1030
 _Z16__fortify_strlenPKcU25pass_dynamic_object_size1 include/linux/fortify-string.h:217 [inline]
 unix_bind_bsd net/unix/af_unix.c:1212 [inline]
 unix_bind+0xba8/0xc58 net/unix/af_unix.c:1326
 __sys_bind+0x1ac/0x248 net/socket.c:1792
 __do_sys_bind net/socket.c:1803 [inline]
 __se_sys_bind net/socket.c:1801 [inline]
 __arm64_sys_bind+0x7c/0x94 net/socket.c:1801
 __invoke_syscall arch/arm64/kernel/syscall.c:38 [inline]
 invoke_syscall+0x98/0x2c0 arch/arm64/kernel/syscall.c:52
 el0_svc_common+0x134/0x240 arch/arm64/kernel/syscall.c:139
 do_el0_svc+0x64/0x198 arch/arm64/kernel/syscall.c:188
 el0_svc+0x2c/0x7c arch/arm64/kernel/entry-common.c:647
 el0t_64_sync_handler+0x84/0xfc arch/arm64/kernel/entry-common.c:665
 el0t_64_sync+0x190/0x194 arch/arm64/kernel/entry.S:591
Code: aa0003e1 d0000e80 91030000 97ffc91a (d4210000)

Fixes: df8fc4e ("kbuild: Enable -fstrict-flex-arrays=3")
Reported-by: syzkaller <syzkaller@googlegroups.com>
Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20230724213425.22920-2-kuniyu@amazon.com
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/unix/af_unix.c

@@ -1208,10 +1208,8 @@ static int unix_bind_bsd(struct sock *sk, struct sockaddr_un *sunaddr,
 	struct path parent;
 	int err;
 
-	unix_mkname_bsd(sunaddr, addr_len);
-	addr_len = strlen(sunaddr->sun_path) +
-		offsetof(struct sockaddr_un, sun_path) + 1;
-
+	addr_len = strnlen(sunaddr->sun_path, sizeof(sunaddr->sun_path))
+		+ offsetof(struct sockaddr_un, sun_path) + 1;
 	addr = unix_create_addr(sunaddr, addr_len);
 	if (!addr)
 		return -ENOMEM;