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Message-ID: <20230720170925.4798-1-kuniyu@amazon.com>
Date: Thu, 20 Jul 2023 10:09:25 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <keescook@...omium.org>
CC: <davem@...emloft.net>, <edumazet@...gle.com>, <gustavoars@...nel.org>,
<kuba@...nel.org>, <kuni1840@...il.com>, <kuniyu@...zon.com>,
<leitao@...ian.org>, <netdev@...r.kernel.org>, <pabeni@...hat.com>,
<syzkaller@...glegroups.com>, <willemb@...gle.com>,
<willemdebruijn.kernel@...il.com>
Subject: Re: [PATCH v2 net 1/2] af_unix: Fix fortify_panic() in unix_bind_bsd().
From: Kees Cook <keescook@...omium.org>
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 <syzkaller@...glegroups.com>
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> > Reviewed-by: Willem de Bruijn <willemb@...gle.com>
> > ---
> > 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
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