Re: KMSAN: uninit-value in fat_evict_inode

[syzbot]

syzbot has found a reproducer for the following crash on:

HEAD commit:    8bbbc5cf kmsan: don't compile memmove
git tree:       https://github.com/google/kmsan.git master
console output: https://syzkaller.appspot.com/x/log.txt?x=11c3774ee00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=cd0e9a6b0e555cc3
dashboard link: https://syzkaller.appspot.com/bug?extid=9d82b8de2992579da5d0
compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
userspace arch: i386
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=171ce265e00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15bb1109e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+9d82b8de2992579da5d0@xxxxxxxxxxxxxxxxxxxxxxxxx

FAT-fs (loop0): error, invalid access to FAT (entry 0x00006500)
FAT-fs (loop0): Filesystem has been set read-only
=====================================================
BUG: KMSAN: uninit-value in fat_free_eofblocks fs/fat/inode.c:628 [inline]
BUG: KMSAN: uninit-value in fat_evict_inode+0x2f4/0x920 fs/fat/inode.c:658
CPU: 0 PID: 11199 Comm: syz-executor375 Not tainted 5.6.0-rc2-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+0x1c9/0x220 lib/dump_stack.c:118
 kmsan_report+0xf7/0x1e0 mm/kmsan/kmsan_report.c:118
 __msan_warning+0x58/0xa0 mm/kmsan/kmsan_instr.c:215
 fat_free_eofblocks fs/fat/inode.c:628 [inline]
 fat_evict_inode+0x2f4/0x920 fs/fat/inode.c:658
 evict+0x4ab/0xe10 fs/inode.c:575
 iput_final fs/inode.c:1571 [inline]
 iput+0xa70/0xe10 fs/inode.c:1597
 fat_build_inode+0x6a3/0x840 fs/fat/inode.c:610
 vfat_mkdir+0x547/0x7d0 fs/fat/namei_vfat.c:871
 vfs_mkdir+0x691/0x920 fs/namei.c:3889
 do_mkdirat+0x39f/0x680 fs/namei.c:3912
 __do_sys_mkdir fs/namei.c:3928 [inline]
 __se_sys_mkdir fs/namei.c:3926 [inline]
 __ia32_sys_mkdir+0x9f/0xd0 fs/namei.c:3926
 do_syscall_32_irqs_on arch/x86/entry/common.c:339 [inline]
 do_fast_syscall_32+0x3c7/0x6e0 arch/x86/entry/common.c:410
 entry_SYSENTER_compat+0x68/0x77 arch/x86/entry/entry_64_compat.S:139
RIP: 0023:0xf7f1bd99
Code: 90 e8 0b 00 00 00 f3 90 0f ae e8 eb f9 8d 74 26 00 89 3c 24 c3 90 90 90 90 90 90 90 90 90 90 90 90 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90 90 90 90 eb 0d 90 90 90 90 90 90 90 90 90 90 90 90
RSP: 002b:00000000ffb4ff4c EFLAGS: 00000292 ORIG_RAX: 0000000000000027
RAX: ffffffffffffffda RBX: 0000000020000740 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 00000000ffb4ffbe RDI: 0000000000000001
RBP: 00000000000000c2 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000

Uninit was created at:
 kmsan_save_stack_with_flags+0x3c/0x90 mm/kmsan/kmsan.c:144
 kmsan_internal_alloc_meta_for_pages mm/kmsan/kmsan_shadow.c:307 [inline]
 kmsan_alloc_page+0x12a/0x310 mm/kmsan/kmsan_shadow.c:336
 __alloc_pages_nodemask+0x5712/0x5e80 mm/page_alloc.c:4775
 alloc_pages_current+0x67d/0x990 mm/mempolicy.c:2211
 alloc_pages include/linux/gfp.h:534 [inline]
 alloc_slab_page+0x111/0x12f0 mm/slub.c:1530
 allocate_slab mm/slub.c:1675 [inline]
 new_slab+0x2bc/0x1130 mm/slub.c:1741
 new_slab_objects mm/slub.c:2492 [inline]
 ___slab_alloc+0x1533/0x1f30 mm/slub.c:2643
 __slab_alloc mm/slub.c:2683 [inline]
 slab_alloc_node mm/slub.c:2757 [inline]
 slab_alloc mm/slub.c:2802 [inline]
 kmem_cache_alloc+0xb23/0xd70 mm/slub.c:2807
 fat_alloc_inode+0x58/0x120 fs/fat/inode.c:748
 alloc_inode fs/inode.c:231 [inline]
 new_inode_pseudo+0xb1/0x590 fs/inode.c:927
 new_inode+0x5a/0x3d0 fs/inode.c:956
 fat_fill_super+0x634b/0x89b0 fs/fat/inode.c:1844
 vfat_fill_super+0xa6/0xc0 fs/fat/namei_vfat.c:1050
 mount_bdev+0x654/0x880 fs/super.c:1417
 vfat_mount+0xc9/0xe0 fs/fat/namei_vfat.c:1057
 legacy_get_tree+0x169/0x2e0 fs/fs_context.c:622
 vfs_get_tree+0xdd/0x580 fs/super.c:1547
 do_new_mount fs/namespace.c:2822 [inline]
 do_mount+0x365c/0x4ac0 fs/namespace.c:3107
 __do_compat_sys_mount fs/compat.c:122 [inline]
 __se_compat_sys_mount+0x3a8/0xa10 fs/compat.c:89
 __ia32_compat_sys_mount+0x157/0x1b0 fs/compat.c:89
 do_syscall_32_irqs_on arch/x86/entry/common.c:339 [inline]
 do_fast_syscall_32+0x3c7/0x6e0 arch/x86/entry/common.c:410
 entry_SYSENTER_compat+0x68/0x77 arch/x86/entry/entry_64_compat.S:139
=====================================================