KASAN: slab-out-of-bounds Read in skb_ensure_writable

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: b859aa7d ANDROID: squashfs: resolve merge conflict with 4...
git tree: android-4.14
console output: https://syzkaller.appspot.com/x/log.txt?x=156f9cd1400000
kernel config: https://syzkaller.appspot.com/x/.config?x=c0bdd1b757a6ba0b
dashboard link: https://syzkaller.appspot.com/bug?extid=d16e3013eebf32b558d5
compiler: gcc (GCC) 8.0.1 20180413 (experimental)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1537c18e400000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1628ac21400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+d16e30...@syzkaller.appspotmail.com

random: sshd: uninitialized urandom read (32 bytes read)
audit: type=1400 audit(1536313296.376:7): avc: denied { map } for
pid=2009 comm="syz-executor122" path="/root/syz-executor122792749"
dev="sda1" ino=16481 scontext=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023
tcontext=unconfined_u:object_r:user_home_t:s0 tclass=file permissive=1
audit: type=1400 audit(1536313296.406:8): avc: denied { prog_load } for
pid=2009 comm="syz-executor122"
scontext=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023
tcontext=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023 tclass=bpf
permissive=1
audit: type=1400 audit(1536313296.426:9): avc: denied { prog_run } for
pid=2009 comm="syz-executor122"
scontext=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023
tcontext=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023 tclass=bpf
permissive=1
==================================================================
BUG: KASAN: slab-out-of-bounds in skb_headlen include/linux/skbuff.h:1893
[inline]
BUG: KASAN: slab-out-of-bounds in pskb_may_pull include/linux/skbuff.h:2107
[inline]
BUG: KASAN: slab-out-of-bounds in skb_ensure_writable+0x290/0x2e0
net/core/skbuff.c:5001
Read of size 4 at addr ffff8801b83185f8 by task syz-executor122/2009

CPU: 0 PID: 2009 Comm: syz-executor122 Not tainted 4.14.68+ #4
Call Trace:
__dump_stack lib/dump_stack.c:17 [inline]
dump_stack+0xb9/0x11b lib/dump_stack.c:53
print_address_description+0x60/0x22b mm/kasan/report.c:252
kasan_report_error mm/kasan/report.c:351 [inline]
kasan_report.cold.6+0x11b/0x2dd mm/kasan/report.c:409
skb_headlen include/linux/skbuff.h:1893 [inline]
pskb_may_pull include/linux/skbuff.h:2107 [inline]
skb_ensure_writable+0x290/0x2e0 net/core/skbuff.c:5001
__bpf_try_make_writable net/core/filter.c:1403 [inline]
bpf_try_make_writable net/core/filter.c:1409 [inline]
____bpf_l4_csum_replace net/core/filter.c:1578 [inline]
bpf_l4_csum_replace+0x61/0x300 net/core/filter.c:1565
___bpf_prog_run+0x248e/0x5c70 kernel/bpf/core.c:1012

Allocated by task 2009:
save_stack mm/kasan/kasan.c:447 [inline]
set_track mm/kasan/kasan.c:459 [inline]
kasan_kmalloc.part.1+0x4f/0xd0 mm/kasan/kasan.c:551
slab_post_alloc_hook mm/slab.h:442 [inline]
slab_alloc_node mm/slub.c:2723 [inline]
slab_alloc mm/slub.c:2731 [inline]
kmem_cache_alloc+0xe4/0x2b0 mm/slub.c:2736
kmem_cache_alloc_node include/linux/slab.h:361 [inline]
__alloc_skb+0xd8/0x550 net/core/skbuff.c:193
alloc_skb include/linux/skbuff.h:975 [inline]
nlmsg_new include/net/netlink.h:511 [inline]
audit_buffer_alloc kernel/audit.c:1626 [inline]
audit_log_start+0x3dd/0x6f0 kernel/audit.c:1740
common_lsm_audit+0xe8/0x1d00 security/lsm_audit.c:450
slow_avc_audit+0x14a/0x1d0 security/selinux/avc.c:771
avc_audit security/selinux/include/avc.h:141 [inline]
avc_has_perm+0x2f2/0x390 security/selinux/avc.c:1146
selinux_bpf+0xb4/0x100 security/selinux/hooks.c:6285
security_bpf+0x7c/0xb0 security/security.c:1712
SYSC_bpf kernel/bpf/syscall.c:1564 [inline]
SyS_bpf+0x153/0x3640 kernel/bpf/syscall.c:1547
do_syscall_64+0x19b/0x4b0 arch/x86/entry/common.c:289
entry_SYSCALL_64_after_hwframe+0x42/0xb7

Freed by task 32:
save_stack mm/kasan/kasan.c:447 [inline]
set_track mm/kasan/kasan.c:459 [inline]
kasan_slab_free+0xac/0x190 mm/kasan/kasan.c:524
slab_free_hook mm/slub.c:1389 [inline]
slab_free_freelist_hook mm/slub.c:1410 [inline]
slab_free mm/slub.c:2966 [inline]
kmem_cache_free+0x12d/0x350 mm/slub.c:2988
kfree_skbmem+0x9e/0x100 net/core/skbuff.c:582
__kfree_skb net/core/skbuff.c:642 [inline]
kfree_skb+0xd0/0x340 net/core/skbuff.c:659
kauditd_hold_skb+0x115/0x140 kernel/audit.c:543
kauditd_send_queue+0xf9/0x140 kernel/audit.c:702
kauditd_thread+0x4c7/0x660 kernel/audit.c:828
kthread+0x348/0x420 kernel/kthread.c:232
ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:402

The buggy address belongs to the object at ffff8801b8318500
which belongs to the cache skbuff_head_cache of size 224
The buggy address is located 24 bytes to the right of
224-byte region [ffff8801b8318500, ffff8801b83185e0)
The buggy address belongs to the page:
page:ffffea0006e0c600 count:1 mapcount:0 mapping: (null) index:0x0
flags: 0x4000000000000100(slab)
raw: 4000000000000100 0000000000000000 0000000000000000 00000001800c000c
raw: dead000000000100 dead000000000200 ffff8801da312200 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff8801b8318480: fb fb fb fb fc fc fc fc fc fc fc fc fc fc fc fc
ffff8801b8318500: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ffff8801b8318580: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
^
ffff8801b8318600: fc fc fc fc fc fc fc fc 00 00 00 00 00 00 00 00
ffff8801b8318680: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
==================================================================


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