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

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    b85051e7 Merge tag 'fixes-for-5.7-rc6' of git://git.kernel..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=165d2b81100000
kernel config:  https://syzkaller.appspot.com/x/.config?x=b3368ce0cc5f5ace
dashboard link: https://syzkaller.appspot.com/bug?extid=61958888b1c60361a791
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=168e6272100000

The bug was bisected to:

commit cbe7fba8edfc8cb8e621599e376f8ac5c224fa72
Author: Amir Goldstein <amir73il@xxxxxxxxx>
Date:   Fri Nov 15 11:33:03 2019 +0000

    ovl: make sure that real fid is 32bit aligned in memory

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=11f95922100000
final crash:    https://syzkaller.appspot.com/x/report.txt?x=13f95922100000
console output: https://syzkaller.appspot.com/x/log.txt?x=15f95922100000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+61958888b1c60361a791@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: cbe7fba8edfc ("ovl: make sure that real fid is 32bit aligned in memory")

==================================================================
BUG: KASAN: slab-out-of-bounds in ovl_check_fb_len+0x171/0x1a0 fs/overlayfs/namei.c:89
Read of size 1 at addr ffff88809727834d by task syz-executor.4/8488

CPU: 0 PID: 8488 Comm: syz-executor.4 Not tainted 5.7.0-rc6-syzkaller #0
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+0x188/0x20d lib/dump_stack.c:118
 print_address_description.constprop.0.cold+0xd3/0x413 mm/kasan/report.c:382
 __kasan_report.cold+0x20/0x38 mm/kasan/report.c:511
 kasan_report+0x33/0x50 mm/kasan/common.c:625
 ovl_check_fb_len+0x171/0x1a0 fs/overlayfs/namei.c:89
 ovl_check_fh_len fs/overlayfs/overlayfs.h:358 [inline]
 ovl_fh_to_dentry+0x1ab/0x814 fs/overlayfs/export.c:812
 exportfs_decode_fh+0x11f/0x717 fs/exportfs/expfs.c:434


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For information about bisection process see: https://goo.gl/tpsmEJ#bisection
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