syzbot found the following crash on:

HEAD commit:    d9bd94c0bcaa Add linux-next specific files for 20180801
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1001189c400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=cc8964ea4d04518c
dashboard link: https://syzkaller.appspot.com/bug?extid=c966a82db0b14aa37e81
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+c966a82db0b14aa37e81@syzkaller.appspotmail.com

loop7: rw=12288, want=8200, limit=20
netlink: 65342 bytes leftover after parsing attributes in process `syz-executor4'.
openvswitch: netlink: Message has 8 unknown bytes.
kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] SMP KASAN
CPU: 1 PID: 7615 Comm: syz-executor7 Not tainted 4.18.0-rc7-next-20180801+ #29
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:__read_once_size include/linux/compiler.h:188 [inline]
RIP: 0010:compound_head include/linux/page-flags.h:142 [inline]
RIP: 0010:PageLocked include/linux/page-flags.h:272 [inline]
RIP: 0010:f2fs_put_page fs/f2fs/f2fs.h:2011 [inline]
RIP: 0010:validate_checkpoint+0x66d/0xec0 fs/f2fs/checkpoint.c:835
Code: e8 58 05 7f fe 4c 8d 6b 80 4d 8d 74 24 08 48 b8 00 00 00 00 00 fc ff df 4c 89 ea 48 c1 ea 03 c6 04 02 00 4c 89 f2 48 c1 ea 03 <80> 3c 02 00 0f 85 f4 06 00 00 4c 89 ea 4d 8b 7c 24 08 48 b8 00 00
RSP: 0018:ffff8801937cebe8 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffff8801937cef30 RCX: ffffc90006035000
RDX: 0000000000000000 RSI: ffffffff82fd9658 RDI: 0000000000000005
RBP: ffff8801937cef58 R08: ffff8801ab254700 R09: fffff94000d9e026
R10: fffff94000d9e026 R11: ffffea0006cf0137 R12: fffffffffffffffb
R13: ffff8801937ceeb0 R14: 0000000000000003 R15: ffff880193419b40
FS:  00007f36a61d5700(0000) GS:ffff8801db100000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fc04ff93000 CR3: 00000001d0562000 CR4: 00000000001426e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 f2fs_get_valid_checkpoint+0x436/0x1ec0 fs/f2fs/checkpoint.c:860
 f2fs_fill_super+0x2d42/0x8110 fs/f2fs/super.c:2883
 mount_bdev+0x314/0x3e0 fs/super.c:1344
 f2fs_mount+0x3c/0x50 fs/f2fs/super.c:3133
 legacy_get_tree+0x131/0x460 fs/fs_context.c:729
 vfs_get_tree+0x1cb/0x5c0 fs/super.c:1743
 do_new_mount fs/namespace.c:2603 [inline]
 do_mount+0x6f2/0x1e20 fs/namespace.c:2927
 ksys_mount+0x12d/0x140 fs/namespace.c:3143
 __do_sys_mount fs/namespace.c:3157 [inline]
 __se_sys_mount fs/namespace.c:3154 [inline]
 __x64_sys_mount+0xbe/0x150 fs/namespace.c:3154
 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x45943a
Code: b8 a6 00 00 00 0f 05 48 3d 01 f0 ff ff 0f 83 bd 8a fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 49 89 ca b8 a5 00 00 00 0f 05 <48> 3d 01 f0 ff ff 0f 83 9a 8a fb ff c3 66 0f 1f 84 00 00 00 00 00
RSP: 002b:00007f36a61d4a88 EFLAGS: 00000206 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 00007f36a61d4b30 RCX: 000000000045943a
RDX: 00007f36a61d4ad0 RSI: 0000000020000100 RDI: 00007f36a61d4af0
RBP: 0000000020000100 R08: 00007f36a61d4b30 R09: 00007f36a61d4ad0
R10: 0000000000000000 R11: 0000000000000206 R12: 0000000000000013
R13: 0000000000000000 R14: 00000000004c8ea0 R15: 0000000000000000
Modules linked in:
Dumping ftrace buffer:
   (ftrace buffer empty)
---[ end trace bd8550c129352286 ]---
RIP: 0010:__read_once_size include/linux/compiler.h:188 [inline]
RIP: 0010:compound_head include/linux/page-flags.h:142 [inline]
RIP: 0010:PageLocked include/linux/page-flags.h:272 [inline]
RIP: 0010:f2fs_put_page fs/f2fs/f2fs.h:2011 [inline]
RIP: 0010:validate_checkpoint+0x66d/0xec0 fs/f2fs/checkpoint.c:835
Code: e8 58 05 7f fe 4c 8d 6b 80 4d 8d 74 24 08 48 b8 00 00 00 00 00 fc ff df 4c 89 ea 48 c1 ea 03 c6 04 02 00 4c 89 f2 48 c1 ea 03 <80> 3c 02 00 0f 85 f4 06 00 00 4c 89 ea 4d 8b 7c 24 08 48 b8 00 00
RSP: 0018:ffff8801937cebe8 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffff8801937cef30 RCX: ffffc90006035000
RDX: 0000000000000000 RSI: ffffffff82fd9658 RDI: 0000000000000005
netlink: 65342 bytes leftover after parsing attributes in process `syz-executor4'.
RBP: ffff8801937cef58 R08: ffff8801ab254700 R09: fffff94000d9e026
openvswitch: netlink: Message has 8 unknown bytes.
R10: fffff94000d9e026 R11: ffffea0006cf0137 R12: fffffffffffffffb
R13: ffff8801937ceeb0 R14: 0000000000000003 R15: ffff880193419b40
FS:  00007f36a61d5700(0000) GS:ffff8801db100000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fc04ff93000 CR3: 00000001d0562000 CR4: 00000000001426e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400

In validate_checkpoint(), if we failed to call get_checkpoint_version(), we
will pass returned invalid page pointer into f2fs_put_page, cause accessing
invalid memory, this patch tries to handle error path correctly to fix this
issue.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>