KASAN: use-after-free Read in skb_ensure_writable

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    bcece5dc40b9 bpf: Change bpf_fib_lookup to return -EAFNOSU..
git tree:       bpf-next
console output: https://syzkaller.appspot.com/x/log.txt?x=10ee76b7800000
kernel config:  https://syzkaller.appspot.com/x/.config?x=e4078980b886800c
dashboard link: https://syzkaller.appspot.com/bug?extid=c8504affd4fdd0c1b626
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=10d926df800000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1778c26f800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+c8504affd4fdd0c1b626@xxxxxxxxxxxxxxxxxxxxxxxxx

random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
==================================================================
BUG: KASAN: use-after-free in pskb_may_pull include/linux/skbuff.h:2108  
[inline]
BUG: KASAN: use-after-free in skb_ensure_writable+0x554/0x620  
net/core/skbuff.c:5118
Read of size 4 at addr ffff8801b0b40fc0 by task syz-executor258/4479

CPU: 0 PID: 4479 Comm: syz-executor258 Not tainted 4.17.0-rc6+ #29
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+0x1b9/0x294 lib/dump_stack.c:113
 print_address_description+0x6c/0x20b mm/kasan/report.c:256
 kasan_report_error mm/kasan/report.c:354 [inline]
 kasan_report.cold.7+0x242/0x2fe mm/kasan/report.c:412
 __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:432
 pskb_may_pull include/linux/skbuff.h:2108 [inline]
 skb_ensure_writable+0x554/0x620 net/core/skbuff.c:5118
 __bpf_try_make_writable net/core/filter.c:1606 [inline]
 bpf_try_make_writable net/core/filter.c:1612 [inline]
 ____bpf_l3_csum_replace net/core/filter.c:1774 [inline]
 bpf_l3_csum_replace+0x8c/0x4d0 net/core/filter.c:1765

The buggy address belongs to the page:
page:ffffea0006c2d000 count:0 mapcount:0 mapping:0000000000000000 index:0x0
flags: 0x2fffc0000000000()
raw: 02fffc0000000000 0000000000000000 0000000000000000 00000000ffffffff
raw: ffffea00075ea760 ffffea0006c39660 ffff8801b5848738 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801b0b40e80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff8801b0b40f00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> ffff8801b0b40f80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                                           ^
 ffff8801b0b41000: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff8801b0b41080: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================


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