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

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 0512e0134582 Merge tag 'xfs-4.17-fixes-3' of git://git.ker..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=17eb2d7b800000
kernel config: https://syzkaller.appspot.com/x/.config?x=968b0b23c7854c0b
dashboard link: https://syzkaller.appspot.com/bug?extid=efae31b384d5badbd620
compiler: gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=162c6def800000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=14fe3db7800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+efae31...@syzkaller.appspotmail.com

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: slab-out-of-bounds in ____bpf_csum_update
net/core/filter.c:1679 [inline]
BUG: KASAN: slab-out-of-bounds in bpf_csum_update+0xb4/0xc0
net/core/filter.c:1673
Read of size 1 at addr ffff8801d9235b50 by task syz-executor507/4513

CPU: 0 PID: 4513 Comm: syz-executor507 Not tainted 4.17.0-rc7+ #78
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_load1_noabort+0x14/0x20 mm/kasan/report.c:430
____bpf_csum_update net/core/filter.c:1679 [inline]
bpf_csum_update+0xb4/0xc0 net/core/filter.c:1673

Allocated by task 0:
(stack is not available)

Freed by task 0:
(stack is not available)

The buggy address belongs to the object at ffff8801d9235a40
which belongs to the cache skbuff_head_cache of size 232
The buggy address is located 40 bytes to the right of
232-byte region [ffff8801d9235a40, ffff8801d9235b28)
The buggy address belongs to the page:
page:ffffea0007648d40 count:1 mapcount:0 mapping:ffff8801d9235040 index:0x0
flags: 0x2fffc0000000100(slab)
raw: 02fffc0000000100 ffff8801d9235040 0000000000000000 000000010000000c
raw: ffffea00074360a0 ffff8801d944d848 ffff8801d9bdd6c0 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff8801d9235a00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff8801d9235a80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff8801d9235b00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
^
ffff8801d9235b80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff8801d9235c00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================


---
This bug is generated by a bot. It may contain errors.
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[Dmitry Vyukov]

/\/\/\/\/\

Are there any known bugs with unwind through bpf functions?

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[Daniel Borkmann]

Looks like you don't have kallsyms export enabled, here's a syzkaller diff
to get jit images exposed, then it should work:

diff --git a/tools/create-image.sh b/tools/create-image.sh
index 9f82482..395a2a0 100755
--- a/tools/create-image.sh
+++ b/tools/create-image.sh
@@ -23,6 +23,7 @@ echo 'SELINUX=disabled' | sudo tee $DIR/etc/selinux/config
echo "kernel.printk = 7 4 1 3" | sudo tee -a $DIR/etc/sysctl.conf
echo 'debug.exception-trace = 0' | sudo tee -a $DIR/etc/sysctl.conf
echo "net.core.bpf_jit_enable = 1" | sudo tee -a $DIR/etc/sysctl.conf
+echo "net.core.bpf_jit_kallsyms = 1" | sudo tee -a $DIR/etc/sysctl.conf
echo "kernel.softlockup_all_cpu_backtrace = 1" | sudo tee -a $DIR/etc/sysctl.conf
echo "kernel.kptr_restrict = 0" | sudo tee -a $DIR/etc/sysctl.conf
echo "kernel.watchdog_thresh = 60" | sudo tee -a $DIR/etc/sysctl.conf

Cheers,
Daniel

[Daniel Borkmann]

On 06/04/2018 01:36 AM, syzbot wrote:
> Hello,
>
> syzbot found the following crash on:
>
> HEAD commit:    0512e0134582 Merge tag 'xfs-4.17-fixes-3' of git://git.ker..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=17eb2d7b800000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=968b0b23c7854c0b
> dashboard link: https://syzkaller.appspot.com/bug?extid=efae31b384d5badbd620
> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=162c6def800000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14fe3db7800000

#syz fix: bpf: reject passing modified ctx to helper functions