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

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    cf7cd542 Add linux-next specific files for 20201104
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=14649314500000
kernel config:  https://syzkaller.appspot.com/x/.config?x=e8dc0c5ac73afb92
dashboard link: https://syzkaller.appspot.com/bug?extid=46061b9b42fecc6e7d6d
compiler:       gcc (GCC) 10.1.0-syz 20200507

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

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+46061b9b42fecc6e7d6d@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KASAN: slab-out-of-bounds in io_uring_show_cred+0x598/0x5f0 fs/io_uring.c:9225
Read of size 4 at addr ffff8880253d13c8 by task syz-executor.3/10456

CPU: 1 PID: 10456 Comm: syz-executor.3 Not tainted 5.10.0-rc2-next-20201104-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+0x107/0x163 lib/dump_stack.c:118
 print_address_description.constprop.0.cold+0xae/0x4c8 mm/kasan/report.c:385
 __kasan_report mm/kasan/report.c:545 [inline]
 kasan_report.cold+0x1f/0x37 mm/kasan/report.c:562
 io_uring_show_cred+0x598/0x5f0 fs/io_uring.c:9225
 idr_for_each+0x113/0x220 lib/idr.c:208
 __io_uring_show_fdinfo fs/io_uring.c:9272 [inline]
 io_uring_show_fdinfo+0x923/0xda0 fs/io_uring.c:9294
 seq_show+0x4a8/0x700 fs/proc/fd.c:65
 seq_read+0x432/0x1070 fs/seq_file.c:208
 do_loop_readv_writev fs/read_write.c:761 [inline]
 do_loop_readv_writev fs/read_write.c:748 [inline]
 do_iter_read+0x48e/0x6e0 fs/read_write.c:803
 vfs_readv+0xe5/0x150 fs/read_write.c:921
 do_preadv fs/read_write.c:1013 [inline]
 __do_sys_preadv fs/read_write.c:1063 [inline]
 __se_sys_preadv fs/read_write.c:1058 [inline]
 __x64_sys_preadv+0x231/0x310 fs/read_write.c:1058
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x45deb9
Code: 0d b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 db b3 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f691c7eac78 EFLAGS: 00000246 ORIG_RAX: 0000000000000127
RAX: ffffffffffffffda RBX: 0000000000025e40 RCX: 000000000045deb9
RDX: 0000000000000333 RSI: 00000000200017c0 RDI: 0000000000000005
RBP: 000000000118c018 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 000000000118bfd4
R13: 00007ffef15d676f R14: 00007f691c7eb9c0 R15: 000000000118bfd4

Allocated by task 10448:
 kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
 kasan_set_track mm/kasan/common.c:56 [inline]
 __kasan_kmalloc.constprop.0+0xc2/0xd0 mm/kasan/common.c:461
 kmalloc include/linux/slab.h:552 [inline]
 io_register_personality fs/io_uring.c:9647 [inline]
 __io_uring_register fs/io_uring.c:9883 [inline]
 __do_sys_io_uring_register+0x10f0/0x40a0 fs/io_uring.c:9933
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

The buggy address belongs to the object at ffff8880253d1380
 which belongs to the cache kmalloc-96 of size 96
The buggy address is located 72 bytes inside of
 96-byte region [ffff8880253d1380, ffff8880253d13e0)
The buggy address belongs to the page:
page:000000003c207268 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x253d1
flags: 0xfff00000000200(slab)
raw: 00fff00000000200 ffffea0000a900c0 0000000500000005 ffff888010041780
raw: 0000000000000000 0000000080200020 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8880253d1280: 00 00 00 00 00 00 00 00 00 00 fc fc fc fc fc fc
 ffff8880253d1300: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
>ffff8880253d1380: 00 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc
                                              ^
 ffff8880253d1400: 00 00 00 00 00 00 00 00 00 00 fc fc fc fc fc fc
 ffff8880253d1480: 00 00 00 00 00 00 00 00 00 00 fc fc fc fc fc fc
==================================================================


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