RE: [PATCH v1 net 1/2] af_unix: Fix fortify_panic() in unix_bind_bsd().

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Kuniyuki Iwashima wrote:
> 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.
> 
> 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 takes 108 as overflow, although 108 still
> fits in struct sockaddr_storage.
> 
> Let's make __fortify_strlen() recognise the actual buffer size.
> 
> [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: df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3")
> Reported-by: syzkaller <syzkaller@googlegroups.com>
> Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>

Reviewed-by: Willem de Bruijn <willemb@google.com>

The extensive comments are really helpful to understand what's
going on.

An alternative would be to just cast sunaddr to a struct
sockaddr_storage *ss and use that both here and in unix_mkname_bsd?
It's not immediately trivial that the caller has always actually
allocated one of those. But the rest becomes self documenting.

> ---
>  net/unix/af_unix.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index 123b35ddfd71..e1b1819b96d1 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -302,6 +302,18 @@ static void unix_mkname_bsd(struct sockaddr_un *sunaddr, int addr_len)
>  	((char *)sunaddr)[addr_len] = 0;
>  }
>  
> +static int unix_strlen_bsd(struct sockaddr_un *sunaddr)
> +{
> +	/* Don't pass sunaddr->sun_path to strlen().  Otherwise, the
> +	 * max valid length UNIX_PATH_MAX (108) will cause panic if
> +	 * CONFIG_FORTIFY_SOURCE=y.  Let __fortify_strlen() know that
> +	 * the actual buffer is struct sockaddr_storage and that 108
> +	 * is within __data[].  See also: unix_mkname_bsd().
> +	 */
> +	return strlen(((struct sockaddr_storage *)sunaddr)->__data) +
> +		offsetof(struct sockaddr_un, sun_path) + 1;
> +}
> +
>  static void __unix_remove_socket(struct sock *sk)
>  {
>  	sk_del_node_init(sk);
> @@ -1209,9 +1221,7 @@ static int unix_bind_bsd(struct sock *sk, struct sockaddr_un *sunaddr,
>  	int err;
>  
>  	unix_mkname_bsd(sunaddr, addr_len);
> -	addr_len = strlen(sunaddr->sun_path) +
> -		offsetof(struct sockaddr_un, sun_path) + 1;
> -
> +	addr_len = unix_strlen_bsd(sunaddr);
>  	addr = unix_create_addr(sunaddr, addr_len);
>  	if (!addr)
>  		return -ENOMEM;
> -- 
> 2.30.2
>