netdev - Re: [PATCH v1 net 2/2] af_packet: Fix warning of fortified memcpy() in packet

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Date: Wed, 19 Jul 2023 14:08:41 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <kuniyu@...zon.com>
CC: <davem@...emloft.net>, <edumazet@...gle.com>, <gustavoars@...nel.org>,
	<keescook@...omium.org>, <kuba@...nel.org>, <kuni1840@...il.com>,
	<leitao@...ian.org>, <netdev@...r.kernel.org>, <pabeni@...hat.com>,
	<syzkaller@...glegroups.com>, <willemdebruijn.kernel@...il.com>
Subject: Re: [PATCH v1 net 2/2] af_packet: Fix warning of fortified memcpy() in packet_getname().

From: Kuniyuki Iwashima <kuniyu@...zon.com>
Date: Wed, 19 Jul 2023 11:53:22 -0700
> syzkaller found a warning in packet_getname() [0], where we try to
> copy 16 bytes to sockaddr_ll.sll_addr[8].
> 
> Some devices (ip6gre, vti6, ip6tnl) have 16 bytes address expressed
> by struct in6_addr.
> 
> The write seems to overflow, but actually not since we use struct
> sockaddr_storage defined in __sys_getsockname().
> 
> To avoid the warning, we need to let __fortify_memcpy_chk() know the
> actual buffer size.
> 
> Another option would be to use strncpy() and limit the copied length
> to sizeof(sll_addr), but it will return the partial address and might
> break an application that passes sockaddr_storage to getsockname().
> 
> [0]:
> memcpy: detected field-spanning write (size 16) of single field "sll->sll_addr" at net/packet/af_packet.c:3604 (size 8)
> WARNING: CPU: 0 PID: 255 at net/packet/af_packet.c:3604 packet_getname+0x25c/0x3a0 net/packet/af_packet.c:3604
> Modules linked in:
> CPU: 0 PID: 255 Comm: syz-executor750 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 : packet_getname+0x25c/0x3a0 net/packet/af_packet.c:3604
> lr : packet_getname+0x25c/0x3a0 net/packet/af_packet.c:3604
> sp : ffff800089887bc0
> x29: ffff800089887bc0 x28: ffff000010f80f80 x27: 0000000000000003
> x26: dfff800000000000 x25: ffff700011310f80 x24: ffff800087d55000
> x23: dfff800000000000 x22: ffff800089887c2c x21: 0000000000000010
> x20: ffff00000de08310 x19: ffff800089887c20 x18: ffff800086ab1630
> x17: 20646c6569662065 x16: 6c676e697320666f x15: 0000000000000001
> x14: 1fffe0000d56d7ca x13: 0000000000000000 x12: 0000000000000000
> x11: 0000000000000000 x10: 0000000000000000 x9 : 3e60944c3da92b00
> x8 : 3e60944c3da92b00 x7 : 0000000000000001 x6 : 0000000000000001
> x5 : ffff8000898874f8 x4 : ffff800086ac99e0 x3 : ffff8000803f8808
> x2 : 0000000000000001 x1 : 0000000100000000 x0 : 0000000000000000
> Call trace:
>  packet_getname+0x25c/0x3a0 net/packet/af_packet.c:3604
>  __sys_getsockname+0x168/0x24c net/socket.c:2042
>  __do_sys_getsockname net/socket.c:2057 [inline]
>  __se_sys_getsockname net/socket.c:2054 [inline]
>  __arm64_sys_getsockname+0x7c/0x94 net/socket.c:2054
>  __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
> 
> Fixes: df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3")
> Reported-by: syzkaller <syzkaller@...glegroups.com>
> Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> ---
>  net/packet/af_packet.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index 85ff90a03b0c..5eef94a32a4f 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -3601,7 +3601,10 @@ static int packet_getname(struct socket *sock, struct sockaddr *uaddr,
>  	if (dev) {
>  		sll->sll_hatype = dev->type;
>  		sll->sll_halen = dev->addr_len;
> -		memcpy(sll->sll_addr, dev->dev_addr, dev->addr_len);
> +
> +		/* Let __fortify_memcpy_chk() know the actual buffer size. */
> +		memcpy(((struct sockaddr_storage *)sll)->__data +
> +		       offsetof(struct sockaddr_ll, sll_addr), dev->dev_addr, dev->addr_len);

Sorry, this offset was wrong and needs minus
offsetof(struct sockaddr_ll, sll_family).

Will fix in v2.

pw-bot: cr


>  	} else {
>  		sll->sll_hatype = 0;	/* Bad: we have no ARPHRD_UNSPEC */
>  		sll->sll_halen = 0;
> -- 
> 2.30.2