udp: harden copy_linear_skb()

syzkaller got crashes with CONFIG_HARDENED_USERCOPY=y configs.

Issue here is that recvfrom() can be used with user buffer of Z bytes,
and SO_PEEK_OFF of X bytes, from a skb with Y bytes, and following
condition :

Z < X < Y

kernel BUG at mm/usercopy.c:72!
invalid opcode: 0000 [#1] SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 0 PID: 2917 Comm: syzkaller842281 Not tainted 4.13.0-rc3+ #16
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
task: ffff8801d2fa40c0 task.stack: ffff8801d1fe8000
RIP: 0010:report_usercopy mm/usercopy.c:64 [inline]
RIP: 0010:__check_object_size+0x3ad/0x500 mm/usercopy.c:264
RSP: 0018:ffff8801d1fef8a8 EFLAGS: 00010286
RAX: 0000000000000078 RBX: ffffffff847102c0 RCX: 0000000000000000
RDX: 0000000000000078 RSI: 1ffff1003a3fded5 RDI: ffffed003a3fdf09
RBP: ffff8801d1fef998 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8801d1ea480e
R13: fffffffffffffffa R14: ffffffff84710280 R15: dffffc0000000000
FS:  0000000001360880(0000) GS:ffff8801dc000000(0000)
knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000202ecfe4 CR3: 00000001d1ff8000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 check_object_size include/linux/thread_info.h:108 [inline]
 check_copy_size include/linux/thread_info.h:139 [inline]
 copy_to_iter include/linux/uio.h:105 [inline]
 copy_linear_skb include/net/udp.h:371 [inline]
 udpv6_recvmsg+0x1040/0x1af0 net/ipv6/udp.c:395
 inet_recvmsg+0x14c/0x5f0 net/ipv4/af_inet.c:793
 sock_recvmsg_nosec net/socket.c:792 [inline]
 sock_recvmsg+0xc9/0x110 net/socket.c:799
 SYSC_recvfrom+0x2d6/0x570 net/socket.c:1788
 SyS_recvfrom+0x40/0x50 net/socket.c:1760
 entry_SYSCALL_64_fastpath+0x1f/0xbe

Fixes: b65ac446 ("udp: try to avoid 2 cache miss on dequeue")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/udp.h

@@ -368,6 +368,8 @@ static inline int copy_linear_skb(struct sk_buff *skb, int len, int off,
 {
 	int n, copy = len - off;
 
+	if (copy < 0)
+		return -EINVAL;
 	n = copy_to_iter(skb->data + off, copy, to);
 	if (n == copy)
 		return 0;