[syzbot] [mm?] BUG: sleeping function called from invalid context in shmem_undo_range

[syzbot] [mm?] BUG: sleeping function called from invalid context in shmem_undo_range

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    2a4c0c11c019 Merge tag 's390-7.1-1' of git://git.kernel.or..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=109c12d2580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=24c8da4692f901cb
dashboard link: https://syzkaller.appspot.com/bug?extid=9722a25de70a85ff48a1
compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
userspace arch: i386

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

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-2a4c0c11.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/acfb852f3eb2/vmlinux-2a4c0c11.xz
kernel image: https://storage.googleapis.com/syzbot-assets/d8166ea5d3da/bzImage-2a4c0c11.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+9722a25de70a85ff48a1@syzkaller.appspotmail.com

BUG: sleeping function called from invalid context at mm/shmem.c:1150
in_atomic(): 0, irqs_disabled(): 0, non_block: 0, pid: 5904, name: rm
preempt_count: 0, expected: 0
RCU nest depth: 1, expected: 0
2 locks held by rm/5904:
 #0: ffff88802b0d0410 (sb_writers#5){.+.+}-{0:0}, at: filename_unlinkat+0x1ad/0x730 fs/namei.c:5545
 #1: ffffffff8e7e5260 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire.constprop.0+0x7/0x30 include/linux/rcupdate.h:300
CPU: 1 UID: 0 PID: 5904 Comm: rm 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
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
 __might_resched.cold+0x1ec/0x232 kernel/sched/core.c:9162
 shmem_undo_range+0x447/0x1570 mm/shmem.c:1150
 shmem_truncate_range mm/shmem.c:1277 [inline]
 shmem_evict_inode+0x3f3/0xc40 mm/shmem.c:1407
 evict+0x3c2/0xad0 fs/inode.c:841
 iput_final fs/inode.c:1960 [inline]
 iput.part.0+0x605/0xf50 fs/inode.c:2009
 iput+0x35/0x40 fs/inode.c:1975
 filename_unlinkat+0x466/0x730 fs/namei.c:5572
 __do_sys_unlink fs/namei.c:5603 [inline]
 __se_sys_unlink fs/namei.c:5600 [inline]
 __x64_sys_unlink+0x46/0x70 fs/namei.c:5600
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x10b/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7eff00dbe937
Code: 00 00 e9 a9 fd ff ff 66 2e 0f 1f 84 00 00 00 00 00 66 90 b8 5f 00 00 00 0f 05 c3 0f 1f 84 00 00 00 00 00 b8 57 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 01 c3 48 8b 15 91 b4 0d 00 f7 d8 64 89 02 b8
RSP: 002b:00007ffef2c377b8 EFLAGS: 00000246 ORIG_RAX: 0000000000000057
RAX: ffffffffffffffda RBX: 00007ffef2c37dd7 RCX: 00007eff00dbe937
RDX: 00007ffef2c377c8 RSI: 00007ffef2c37dd7 RDI: 00007ffef2c37dd7
RBP: 0000000000000008 R08: 00007ffef2c37dc0 R09: 0000000000000000
R10: 0000000000000100 R11: 0000000000000246 R12: 0000000000000000
R13: 00007eff00f4e9cc R14: 00007eff00faa000 R15: 000055fe0234bd98
 </TASK>

================================================
WARNING: lock held when returning to user space!
syzkaller #0 Tainted: G        W          
------------------------------------------------
rm/5904 is leaving the kernel with locks still held!
1 lock held by rm/5904:
 #0: ffffffff8e7e5260 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire.constprop.0+0x7/0x30 include/linux/rcupdate.h:300
------------[ cut here ]------------
Voluntary context switch within RCU read-side critical section!
WARNING: kernel/rcu/tree_plugin.h:332 at rcu_note_context_switch+0x859/0x19c0 kernel/rcu/tree_plugin.h:332, CPU#1: rm/5904
Modules linked in:
CPU: 1 UID: 0 PID: 5904 Comm: rm Tainted: G        W           syzkaller #0 PREEMPT(full) 
Tainted: [W]=WARN
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:rcu_note_context_switch+0x859/0x19c0 kernel/rcu/tree_plugin.h:332
Code: c1 ea 03 80 3c 02 00 0f 85 9b 0b 00 00 48 8b 53 28 b9 01 00 00 00 4c 89 ef e8 a3 cf fe ff e9 1d f9 ff ff 48 8d 3d b7 5e f1 0e <67> 48 0f b9 3a e9 99 f8 ff ff 48 b8 00 00 00 00 00 fc ff df 48 8d
RSP: 0000:ffffc9000407fcf0 EFLAGS: 00010002
RAX: 0000000000000001 RBX: ffff88802b33c3c0 RCX: ffffffff81ee0fc1
RDX: 0000000000000000 RSI: ffffffff8c1c1000 RDI: ffffffff90dfe2a0
RBP: ffff8880267b2500 R08: 0000000000000000 R09: fffffbfff21b89ca
R10: ffffffff90dc4e57 R11: ffffffff94142590 R12: 0000000000000000
R13: ffff8880267b29c4 R14: ffff88802b33b380 R15: ffffffff90dc81c4
FS:  00007eff00c00c80(0000) GS:ffff8880971e2000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007eff00da0850 CR3: 000000004cf7d000 CR4: 0000000000352ef0
Call Trace:
 <TASK>
 __schedule+0x276/0x6820 kernel/sched/core.c:7043
 __schedule_loop kernel/sched/core.c:7267 [inline]
 schedule+0xdd/0x390 kernel/sched/core.c:7282
 __exit_to_user_mode_loop kernel/entry/common.c:54 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:98 [inline]
 __exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline]
 irqentry_exit_to_user_mode_prepare include/linux/irq-entry-common.h:252 [inline]
 irqentry_exit_to_user_mode include/linux/irq-entry-common.h:323 [inline]
 irqentry_exit+0x359/0x790 kernel/entry/common.c:162
 asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:697
RIP: 0033:0x7eff00dbe937
Code: 00 00 e9 a9 fd ff ff 66 2e 0f 1f 84 00 00 00 00 00 66 90 b8 5f 00 00 00 0f 05 c3 0f 1f 84 00 00 00 00 00 b8 57 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 01 c3 48 8b 15 91 b4 0d 00 f7 d8 64 89 02 b8
RSP: 002b:00007ffef2c377b8 EFLAGS: 00000246
RAX: 0000000000000000 RBX: 00007ffef2c37dd7 RCX: 00007eff00dbe937
RDX: 00007ffef2c377c8 RSI: 00007ffef2c37dd7 RDI: 00007ffef2c37dd7
RBP: 0000000000000008 R08: 00007ffef2c37dc0 R09: 0000000000000000
R10: 0000000000000100 R11: 0000000000000246 R12: 0000000000000000
R13: 00007eff00f4e9cc R14: 00007eff00faa000 R15: 000055fe0234bd98
 </TASK>
----------------
Code disassembly (best guess):
   0:	c1 ea 03             	shr    $0x3,%edx
   3:	80 3c 02 00          	cmpb   $0x0,(%rdx,%rax,1)
   7:	0f 85 9b 0b 00 00    	jne    0xba8
   d:	48 8b 53 28          	mov    0x28(%rbx),%rdx
  11:	b9 01 00 00 00       	mov    $0x1,%ecx
  16:	4c 89 ef             	mov    %r13,%rdi
  19:	e8 a3 cf fe ff       	call   0xfffecfc1
  1e:	e9 1d f9 ff ff       	jmp    0xfffff940
  23:	48 8d 3d b7 5e f1 0e 	lea    0xef15eb7(%rip),%rdi        # 0xef15ee1
* 2a:	67 48 0f b9 3a       	ud1    (%edx),%rdi <-- trapping instruction
  2f:	e9 99 f8 ff ff       	jmp    0xfffff8cd
  34:	48 b8 00 00 00 00 00 	movabs $0xdffffc0000000000,%rax
  3b:	fc ff df
  3e:	48                   	rex.W
  3f:	8d                   	.byte 0x8d


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Re: [syzbot] [mm?] BUG: sleeping function called from invalid context in shmem_undo_range

[Arjan van de Ven]

This email is created by automation to help kernel developers
deal with a large volume of AI generated bug reports by decoding
oopses into more actionable information.


Decoded Backtrace

shmem_undo_range (mm/shmem.c:1108-1151) -- crash site
------------------------------------------------------
1108 static void shmem_undo_range(struct inode *inode, loff_t lstart,
1109                               uoff_t lend, bool unfalloc)
1110 {
1111     struct address_space *mapping = inode->i_mapping;
1112     struct shmem_inode_info *info = SHMEM_I(inode);
1113     pgoff_t start = (lstart + PAGE_SIZE - 1) >> PAGE_SHIFT;
1114     pgoff_t end = (lend + 1) >> PAGE_SHIFT;
1115     struct folio_batch fbatch;
1116     pgoff_t indices[FOLIO_BATCH_SIZE];
1117     struct folio *folio;
1118     bool same_folio;
1119     long nr_swaps_freed = 0;
1120     pgoff_t index;
1121     int i;
...
1129     folio_batch_init(&fbatch);
1130     index = start;
1131     while (index < end && find_lock_entries(mapping, &index,
1132             end - 1, &fbatch, indices)) {
1133         for (i = 0; i < folio_batch_count(&fbatch); i++) {
1134             folio = fbatch.folios[i];
1135             if (xa_is_value(folio)) {
...
1143             }
1144             if (!unfalloc || !folio_test_uptodate(folio))
1145                 truncate_inode_folio(mapping, folio);
1146             folio_unlock(folio);
1147         }
1148         folio_batch_remove_exceptionals(&fbatch);
1149         folio_batch_release(&fbatch);
1150         cond_resched();   /* <-- BUG fires here; RCU nest depth=1 */
1151     }

filename_unlinkat (fs/namei.c:5526-5587) -- top-level caller
-------------------------------------------------------------
5526 int filename_unlinkat(int dfd, struct filename *name)
5527 {
...
5545     error = mnt_want_write(path.mnt);  /* lock #0 (sb_writers) */
5546     if (error)
5547         goto exit_path_put;
5548 retry_deleg:
5549     dentry = start_dirop(path.dentry, &last, lookup_flags);
...
5563     inode = dentry->d_inode;
5564     ihold(inode);
5565     error = security_path_unlink(&path, dentry);
5566     if (error)
5567         goto exit_end_dirop;
5568     error = vfs_unlink(mnt_idmap(path.mnt), path.dentry->d_inode,
5569                        dentry, &delegated_inode);
5570 exit_end_dirop:
5571     end_dirop(dentry);
5572     iput(inode);   /* triggers shmem_evict_inode -> shmem_undo_range */
...
5579     mnt_drop_write(path.mnt);
...
5587 }


Tentative Analysis

The crash is triggered by running rm(1) on a tmpfs/shmem file. The call
chain is:

  sys_unlink -> filename_unlinkat -> iput -> iput_final -> evict
    -> shmem_evict_inode -> shmem_truncate_range -> shmem_undo_range
    -> cond_resched()   <-- BUG fires here

At crash time, two locks are held (in acquisition order):

  #0 sb_writers#5  --  acquired at filename_unlinkat+0x1ad
                       (fs/namei.c:5545 -- mnt_want_write)
  #1 rcu_read_lock --  acquired at rcu_lock_acquire.constprop.0
                       (include/linux/rcupdate.h:300)

Lock #1 (rcu_read_lock) was acquired AFTER mnt_want_write (lock #0 is
always recorded first in the lockdep list) and before the iput() call
at line 5572.  That window contains start_dirop(), security_path_unlink()
and vfs_unlink() (which calls fsnotify_unlink() and d_delete_notify() on
the success path).

On a PREEMPT(full) kernel, rcu_read_lock() increments
current->rcu_read_lock_nesting instead of disabling preemption.
cond_resched() -> __cond_resched() -> __might_resched() checks that
counter, finds it is 1 (not 0), and fires the BUG.

The same leaked rcu_read_lock persists all the way back to userspace
("lock held when returning to user space!"), and a timer interrupt
firing during the syscall produces a third warning:
"Voluntary context switch within RCU read-side critical section!"

All three messages are symptoms of a single rcu_read_lock() call that
is never matched by an rcu_read_unlock() somewhere between
mnt_want_write and iput in filename_unlinkat.


Potential Solution

Find the rcu_read_lock() call that is missing its matching
rcu_read_unlock() in the vfs_unlink() / fsnotify_unlink() /
d_delete_notify() code path.  Adding a WARN_ON(rcu_read_lock_held())
to iput() (alongside the existing might_sleep() check) should produce
a backtrace pointing at the exact acquisition site on the next
reproduction.


More information

Oops-Analysis: http://oops.fenrus.org/reports/lkml/69eab803.a00a0220.17a17.004b.GAE_google.com/report.html
Assisted-by: GitHub Copilot:claude-sonnet-4.6 linux-kernel-oops-x86.