Subject: Re: [PATCH v2 net 2/2] af_packet: Fix warning of fortified memcpy() in packet_getname().

From: Kees Cook <[email-masked]>
Date: Thu, 20 Jul 2023 08:04:53 -0700

On Wed, Jul 19, 2023 at 05:44:10PM -0700, Kuniyuki Iwashima wrote:
> 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.  Also, Infiniband has 32 bytes as MAX_ADDR_LEN.
> 
> The write seems to overflow, but actually not since we use struct
> sockaddr_storage defined in __sys_getsockname() and its size is 128
> (_K_SS_MAXSIZE) bytes.  Thus, we have sufficient room after sll_addr[]
> as __data[].

Ah, so the issue here is that the UAPI for sll_addr is lying about its
size. I think a better fix here is to fix the structure (without
breaking UAPI sizes or names):

diff --git a/include/uapi/linux/if_packet.h b/include/uapi/linux/if_packet.h
index 9efc42382fdb..4d0ad22f83b5 100644
--- a/include/uapi/linux/if_packet.h
+++ b/include/uapi/linux/if_packet.h
@@ -18,7 +18,11 @@ struct sockaddr_ll {
 	unsigned short	sll_hatype;
 	unsigned char	sll_pkttype;
 	unsigned char	sll_halen;
-	unsigned char	sll_addr[8];
+	union {
+		unsigned char	sll_addr[8];
+		/* Actual length is in sll_halen. */
+		__DECLARE_FLEX_ARRAY(unsigned char, sll_addr_flex);
+	};
 };
 
 /* Packet types */
diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 85ff90a03b0c..8e3ddec4c3d5 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -3601,7 +3601,7 @@ 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);
+		memcpy(sll->sll_addr_flex, dev->dev_addr, dev->addr_len);
 	} else {
 		sll->sll_hatype = 0;	/* Bad: we have no ARPHRD_UNSPEC */
 		sll->sll_halen = 0;

We can't rename sll_data nor change it's size, as userspace uses it
pretty extensively:
https://codesearch.debian.net/search?q=sizeof.*sll_addr&literal=0

-- 
Kees Cook