rose_write_internal() uses a temp buffer of 100 bytes, but a manual
inspection showed that given arbitrary input, rose_create_facilities()
can fill up to 110 bytes.

Lets use a tailroom of 256 bytes for peace of mind, and remove
the bounce buffer : we can simply allocate a big enough skb
and adjust its length as needed.

syzbot report :

BUG: KASAN: stack-out-of-bounds in memcpy include/linux/string.h:352 [inline]
BUG: KASAN: stack-out-of-bounds in rose_create_facilities net/rose/rose_subr.c:521 [inline]
BUG: KASAN: stack-out-of-bounds in rose_write_internal+0x597/0x15d0 net/rose/rose_subr.c:116
Write of size 7 at addr ffff88808b1ffbef by task syz-executor.0/24854

CPU: 0 PID: 24854 Comm: syz-executor.0 Not tainted 5.0.0+ #97
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x172/0x1f0 lib/dump_stack.c:113
 print_address_description.cold+0x7c/0x20d mm/kasan/report.c:187
 kasan_report.cold+0x1b/0x40 mm/kasan/report.c:317
 check_memory_region_inline mm/kasan/generic.c:185 [inline]
 check_memory_region+0x123/0x190 mm/kasan/generic.c:191
 memcpy+0x38/0x50 mm/kasan/common.c:131
 memcpy include/linux/string.h:352 [inline]
 rose_create_facilities net/rose/rose_subr.c:521 [inline]
 rose_write_internal+0x597/0x15d0 net/rose/rose_subr.c:116
 rose_connect+0x7cb/0x1510 net/rose/af_rose.c:826
 __sys_connect+0x266/0x330 net/socket.c:1685
 __do_sys_connect net/socket.c:1696 [inline]
 __se_sys_connect net/socket.c:1693 [inline]
 __x64_sys_connect+0x73/0xb0 net/socket.c:1693
 do_syscall_64+0x103/0x610 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x458079
Code: ad b8 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 7b b8 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f47b8d9dc78 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000458079
RDX: 000000000000001c RSI: 0000000020000040 RDI: 0000000000000004
RBP: 000000000073bf00 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f47b8d9e6d4
R13: 00000000004be4a4 R14: 00000000004ceca8 R15: 00000000ffffffff

The buggy address belongs to the page:
page:ffffea00022c7fc0 count:0 mapcount:0 mapping:0000000000000000 index:0x0
flags: 0x1fffc0000000000()
raw: 01fffc0000000000 0000000000000000 ffffffff022c0101 0000000000000000
raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff88808b1ffa80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff88808b1ffb00: 00 00 00 00 00 00 00 00 f1 f1 f1 f1 00 00 00 03
>ffff88808b1ffb80: f2 f2 00 00 00 00 00 00 00 00 00 00 00 00 04 f3
                                                             ^
 ffff88808b1ffc00: f3 f3 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
 ffff88808b1ffc80: 00 00 00 00 00 00 00 f1 f1 f1 f1 f1 f1 01 f2 01
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>