Re: [syzbot] [mm?] [fuse?] WARNING in shmem_initxattrs

[syzbot <syzbot+4324f771847cecb27c54@syzkaller.appspotmail.com>]

syzbot has found a reproducer for the following issue on:

HEAD commit:    f66469d9905b Merge tag 'net-7.4-rc2' of git://git.kernel.o..
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
console output: https://syzkaller.appspot.com/x/log.txt?x=110ad735580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=1cf189394a856f11
dashboard link: https://syzkaller.appspot.com/bug?extid=4324f771847cecb27c54
compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13bb58d9580000

AI-generated root cause summary:
In mm/shmem.c:shmem_initxattrs(), the initial reservation of inode xattr space
(ispace) from sbinfo->free_ispace is gated by 'if (sbinfo->max_inodes)', and
subsequent per-xattr subtraction from ispace is also gated by 'if
(sbinfo->max_inodes)'. Because sbinfo->max_inodes is accessed locklessly and can
be concurrently modified to 0 during a remount (shmem_reconfigure() with
nr_inodes=0), ispace can be reserved when max_inodes is non-zero, but fail to be
decremented when max_inodes becomes 0. Consequently, after all security xattrs
are successfully allocated and added, ispace remains non-zero, tripping the
WARN_ON(ispace) assertion.

full analysis:  https://syzkaller.appspot.com/x/ai_root_cause?x=4324f771847cecb27c54

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+4324f771847cecb27c54@syzkaller.appspotmail.com

------------[ cut here ]------------
ispace
WARNING: mm/shmem.c:4499 at shmem_initxattrs+0x3f5/0x650 mm/shmem.c:4499, CPU#3: syz.3.20/6999
Modules linked in:
CPU: 3 UID: 0 PID: 6999 Comm: syz.3.20 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:shmem_initxattrs+0x3f5/0x650 mm/shmem.c:4499
Code: 03 80 3c 02 00 0f 85 5d 02 00 00 48 83 3b 00 0f 85 fd fc ff ff e8 eb 3b ae ff 4c 89 ee 31 ff e8 81 36 ae ff e8 dc 3b ae ff 90 <0f> 0b 90 e9 dd fe ff ff e8 ce 3b ae ff 4d 8d 7e 78 4c 89 ff e8 12
RSP: 0018:ffffc90004fa7bb0 EFLAGS: 00010293
RAX: 0000000000000000 RBX: ffff888031a28818 RCX: 0000000000000000
RDX: ffff88803397a580 RSI: ffffffff82603cc4 RDI: 0000000000000007
RBP: ffff888037912b00 R08: 0000000000000000 R09: 0000000000000053
R10: ffff88805aa5a973 R11: 0000000000000000 R12: dffffc0000000000
R13: 0000000000000053 R14: ffff88805aa58000 R15: ffff888037912b30
FS:  00007fd3ae9026c0(0000) GS:ffff8880d4775000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fcb0d7556c8 CR3: 000000001d7a3000 CR4: 0000000000352ef0
Call Trace:
 <TASK>
 security_inode_init_security+0x242/0x370 security/security.c:1384
 shmem_mknod+0x2ce/0x480 mm/shmem.c:4087
 shmem_mkdir+0x31/0x80 mm/shmem.c:4142
 vfs_mkdir+0x36d/0x8a0 fs/namei.c:5428
 filename_mkdirat+0x4c4/0x630 fs/namei.c:5461
 __do_sys_mkdirat fs/namei.c:5482 [inline]
 __se_sys_mkdirat fs/namei.c:5479 [inline]
 __x64_sys_mkdirat+0x89/0xc0 fs/namei.c:5479
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x123/0x790 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fd3ad99fb19
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 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 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fd3ae902028 EFLAGS: 00000246 ORIG_RAX: 0000000000000102
RAX: ffffffffffffffda RBX: 00007fd3adc26090 RCX: 00007fd3ad99fb19
RDX: 00000000000001ff RSI: 0000200000000800 RDI: ffffffffffffff9c
RBP: 00007fd3ada37b70 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fd3adc26128 R14: 00007fd3adc26090 R15: 00007ffcae78f808
 </TASK>


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