[syzbot] [mm?] [bluetooth?] BUG: unable to handle kernel NULL pointer dereference in __refill_objects_node

[syzbot <syzbot+73e34786d639a8eb1324@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    58c1eb84e977 arm64: sdei: Preserve the interrupted context..
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci
console output: https://syzkaller.appspot.com/x/log.txt?x=12d1d369580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=a58b6b4d667e7a61
dashboard link: https://syzkaller.appspot.com/bug?extid=73e34786d639a8eb1324
compiler:       Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8
userspace arch: arm64
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1255c1a1580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/b1ec47537e12/disk-58c1eb84.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/3bca0c2b1169/vmlinux-58c1eb84.xz
kernel image: https://storage.googleapis.com/syzbot-assets/0a7c49594d5a/Image-58c1eb84.gz.xz

AI-generated root cause summary:
In mm/slub.c, __refill_objects_node() replenishes percpu sheaves by detaching a
freelist from a partial slab via get_freelist_nofreeze() and iterating through
the freelist pointers in a while (count && refilled < max) loop. Because the
loop condition only checks count and refilled rather than validating that object
is non-NULL, a prematurely terminated or corrupted freelist (where fewer objects
are linked in slab->freelist than the expected count = slab->objects -
slab->inuse) causes object to become NULL before count reaches zero. On the
subsequent iteration, get_freepointer() dereferences NULL + s->offset (offset
0x400 for kmalloc-2k), resulting in an immediate kernel NULL pointer dereference
crash at address 0x0000000000000400.

full analysis:  https://syzkaller.appspot.com/x/ai_root_cause?x=73e34786d639a8eb1324

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+73e34786d639a8eb1324@syzkaller.appspotmail.com

Unable to handle kernel NULL pointer dereference at virtual address 00000000000004e8
Mem abort info:
  ESR = 0x0000000096000005
  EC = 0x25: DABT (current EL), IL = 32 bits
  SET = 0, FnV = 0
  EA = 0, S1PTW = 0
  FSC = 0x05: level 1 translation fault
Data abort info:
  ISV = 0, ISS = 0x00000005, ISS2 = 0x00000000
  CM = 0, WnR = 0, TnD = 0, TagAccess = 0
  GCS = 0, Overlay = 0, DirtyBit = 0
user pgtable: 4k pages, 48-bit VAs, pgdp=000000011857f000
[00000000000004e8] pgd=080000011cba0403, p4d=080000011cba0403, pud=0000000000000000
Internal error: Oops: 0000000096000005 [#1]  SMP
Modules linked in:
CPU: 0 UID: 0 PID: 4870 Comm: syz-executor Not tainted syzkaller #0 PREEMPT 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/06/2026
pstate: 83400005 (Nzcv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)
pc : get_freepointer mm/slub.c:596 [inline]
pc : __refill_objects_node+0x308/0x510 mm/slub.c:7413
lr : get_freelist_nofreeze mm/slub.c:4494 [inline]
lr : __refill_objects_node+0x2b8/0x510 mm/slub.c:7409
sp : ffff800094587600
x29: ffff800094587690 x28: 0000000000000000 x27: 0000000000000000
x26: 0000000000000006 x25: fffffdffc3256e00 x24: 0000000000000003
x23: 0000000000000006 x22: 000000000000000c x21: 0000000000000001
x20: ffff0000c0022138 x19: 0000000000000004 x18: ffff800123625000
x17: ffff0001ace953a6 x16: ffff0000db9cd894 x15: 00000000000c8000
x14: 00000000005c615b x13: 0000000000000001 x12: 0000000000000003
x11: 0000000000000000 x10: fffffdffc3256e20 x9 : ffff0000c0022120
x8 : 00000000000004e8 x7 : 0000000000000000 x6 : 0000000000000000
x5 : 0000000000000001 x4 : 0000000000000008 x3 : 00000002000c000c
x2 : 0000000000000000 x1 : 00000002000c0003 x0 : 0000000000000001
Call trace:
 get_freepointer mm/slub.c:595 [inline] (P)
 __refill_objects_node+0x308/0x510 mm/slub.c:7413 (P)
 refill_objects+0x78/0x320 mm/slub.c:7526
 refill_sheaf+0x44/0x6c mm/slub.c:2917
 __pcs_replace_empty_main+0x22c/0x4f4 mm/slub.c:4811
 alloc_from_pcs mm/slub.c:4887 [inline]
 slab_alloc_node mm/slub.c:5021 [inline]
 kmem_cache_alloc_noprof+0x35c/0x5d4 mm/slub.c:5047
 dup_mm kernel/fork.c:1532 [inline]
 copy_mm+0xb8/0x328 kernel/fork.c:1594
 copy_process+0x1698/0x35a0 kernel/fork.c:2320
 kernel_clone+0x234/0x8e8 kernel/fork.c:2790
 __do_sys_clone kernel/fork.c:2932 [inline]
 __se_sys_clone kernel/fork.c:2900 [inline]
 __arm64_sys_clone+0x168/0x1d4 kernel/fork.c:2900
 __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
 invoke_syscall+0x98/0x244 arch/arm64/kernel/syscall.c:49
 el0_svc_common+0xec/0x23c arch/arm64/kernel/syscall.c:121
 do_el0_svc+0x4c/0x5c arch/arm64/kernel/syscall.c:140
 el0_svc+0x60/0x1d8 arch/arm64/kernel/entry-common.c:797
 el0t_64_sync_handler+0x44/0x104 arch/arm64/kernel/entry-common.c:816
 el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:606
Code: f9401fe8 f900029b 2a1a03f7 b9402908 (f87b691c) 
---[ end trace 0000000000000000 ]---
----------------
Code disassembly (best guess):
   0:	f9401fe8 	ldr	x8, [sp, #56]
   4:	f900029b 	str	x27, [x20]
   8:	2a1a03f7 	mov	w23, w26
   c:	b9402908 	ldr	w8, [x8, #40]
* 10:	f87b691c 	ldr	x28, [x8, x27] <-- trapping instruction


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