Subject: Re: [PATCH v2 net 1/2] af_unix: Fix fortify_panic() in unix_bind_bsd().

From: Kuniyuki Iwashima <[email-masked]>
Date: Thu, 20 Jul 2023 10:09:25 -0700

From: Kees Cook <[email-masked]>
Date: Thu, 20 Jul 2023 07:39:48 -0700
> On Wed, Jul 19, 2023 at 05:44:09PM -0700, 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.
> 
> Oh, the max size is 108, but it's allowed to be unterminated? This seems
> to contradict the comment for unix_validate_addr() (which then doesn't
> validate this ...)

Because we call it for the abstract socket too which starts with \0 and
does not handle it specially.

> Reading "max 7 unix" seems to clear this up and
> confirm that it doesn't need to be terminated. Bleh.
> 
> Regardless, see below for a simpler solution, since this doesn't need to
> be arbitrarily long, just potentially unterminated.

unix_mkname_bsd() terminates it.  Technically, we need not call strlen()
if we don't optimise the allocated string length.  connect() does not need
strlen() and just calls unix_mkname_bsd() in unix_find_bsd().

> 
> > 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 <[email-masked]>
> > Signed-off-by: Kuniyuki Iwashima <[email-masked]>
> > Reviewed-by: Willem de Bruijn <[email-masked]>
> > ---
> >  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) +
> 
> Instead of a whole new function, I think this can just be:
> 
> 		strnlen(sunaddr->sun_path, sizeof(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
> > 
> 
> -- 
> Kees Cook