Subject: net: use after free in ip6_make_skb

From: Sasha Levin <[email-masked]>
Date: Sat, 28 Nov 2015 11:33:27 -0500

Hi,

Fuzzing with syzkaller on the latest -next kernel produced this error:

[  891.389013] ==================================================================

[  891.390006] BUG: KASAN: use-after-free in ip6_make_skb+0x106/0x3d0 at addr ffff8806e9773a34

[  891.393459] Read of size 2 by task syzkaller_execu/8350

[  891.394128] =============================================================================

[  891.395121] BUG kmalloc-64 (Not tainted): kasan: bad access detected

[  891.395886] -----------------------------------------------------------------------------

[  891.395886]

[  891.398479] Disabling lock debugging due to kernel taint

[  891.399156] INFO: Allocated in p9pdu_vreadf+0x7d4/0x1da0 age=2 cpu=12 pid=8331

[  891.400255] 	___slab_alloc+0x434/0x5b0

[  891.400917] 	__slab_alloc.isra.37+0x79/0xd0

[  891.401642] 	__kmalloc+0x12f/0x390

[  891.402172] 	pipe_fcntl+0x195/0x4c0

[  891.402743] 	SyS_fcntl+0xd70/0xe50

[  891.403162] 	entry_SYSCALL_64_fastpath+0x35/0x9e

[  891.403715] INFO: Freed in kfree_put_link+0x1a/0x20 age=6 cpu=12 pid=8331

[  891.404349] 	__slab_free+0x5c/0x2b0

[  891.404665] 	kfree+0x281/0x2f0

[  891.404915] 	kfree_put_link+0x1a/0x20

[  891.405201] 	path_openat+0x391f/0x5040

[  891.405508] 	do_filp_open+0x1b8/0x250

[  891.405814] 	do_open_execat+0x105/0x4d0

[  891.406295] 	open_exec+0x3b/0x60

[  891.406802] INFO: Slab 0xffffea001ba5dc00 objects=41 used=20 fp=0xffff8806e9773a30 flags=0x2fffff80004080

[  891.407978] INFO: Object 0xffff8806e9773a30 @offset=14896 fp=0xffff8806e9772028

[  891.407978]

[  891.408888] Bytes b4 ffff8806e9773a20: e3 07 05 00 01 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a  ........ZZZZZZZZ

[  891.410121] Object ffff8806e9773a30: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk

[  891.411425] Object ffff8806e9773a40: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk

[  891.412603] Object ffff8806e9773a50: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk

[  891.413775] Object ffff8806e9773a60: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5  kkkkkkkkkkkkkkk.

[  891.414977] Redzone ffff8806e9773a70: bb bb bb bb bb bb bb bb                          ........

[  891.416092] Padding ffff8806e9773bb0: 5a 5a 5a 5a 5a 5a 5a 5a                          ZZZZZZZZ

[  891.417221] CPU: 7 PID: 8350 Comm: syzkaller_execu Tainted: G    B           4.4.0-rc2-next-20151126-sasha-00007-g7083bec-dirty #2655

[  891.424771] Call Trace:

[  891.425454] dump_stack (lib/dump_stack.c:52)
[  891.426433] print_trailer (mm/slub.c:655)
[  891.427482] object_err (mm/slub.c:662)
[  891.428495] kasan_report_error (mm/kasan/report.c:138 mm/kasan/report.c:236)
[  891.430825] __asan_report_load2_noabort (mm/kasan/report.c:278)
[  891.432908] ip6_make_skb (net/ipv6/ip6_output.c:1757)
[  891.446294] udpv6_sendmsg (include/linux/err.h:40 net/ipv6/udp.c:1319)
[  891.461731] inet_sendmsg (net/ipv4/af_inet.c:733)
[  891.465359] sock_sendmsg (net/socket.c:611 net/socket.c:620)
[  891.466211] sock_write_iter (net/socket.c:820)
[  891.470869] __vfs_write (fs/read_write.c:480 fs/read_write.c:492)
[  891.473525] vfs_write (fs/read_write.c:540)
[  891.474341] SyS_write (fs/read_write.c:587 fs/read_write.c:578)
[  891.477995] entry_SYSCALL_64_fastpath (arch/x86/entry/entry_64.S:188)
[  891.478975] Memory state around the buggy address:

[  891.479722]  ffff8806e9773900: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc

[  891.480830]  ffff8806e9773980: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc

[  891.481954] >ffff8806e9773a00: fc fc fc fc fc fc 00 00 00 00 00 00 00 fc fc fc

[  891.483025]                                      ^

[  891.483622]  ffff8806e9773a80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc

[  891.484469]  ffff8806e9773b00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc

[  891.485307] ==================================================================

Thanks,
Sasha