linux-kernel - Re: [syzbot] [mm?] [cgroups?] kernel BUG in swap_cgroup

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Message-ID: <696135f2.050a0220.1c677c.03d0.GAE@google.com>
Date: Fri, 09 Jan 2026 09:08:02 -0800
From: syzbot <syzbot+d97580a8cceb9b03c13e@...kaller.appspotmail.com>
To: kartikey406@...il.com, linux-kernel@...r.kernel.org, 
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [mm?] [cgroups?] kernel BUG in swap_cgroup_record (2)

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
KASAN: vmalloc-out-of-bounds Read in swap_cgroup_lookup

==================================================================
BUG: KASAN: vmalloc-out-of-bounds in instrument_atomic_read include/linux/instrumented.h:68 [inline]
BUG: KASAN: vmalloc-out-of-bounds in atomic_read include/linux/atomic/atomic-instrumented.h:32 [inline]
BUG: KASAN: vmalloc-out-of-bounds in swap_cgroup_lookup+0x89/0xd0 mm/swap_cgroup.c:67
Read of size 4 at addr ffffc900034e0800 by task syz.0.17/6463

CPU: 0 UID: 0 PID: 6463 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 print_address_description mm/kasan/report.c:378 [inline]
 print_report+0xca/0x240 mm/kasan/report.c:482
 kasan_report+0x118/0x150 mm/kasan/report.c:595
 check_region_inline mm/kasan/generic.c:-1 [inline]
 kasan_check_range+0x2b0/0x2c0 mm/kasan/generic.c:200
 instrument_atomic_read include/linux/instrumented.h:68 [inline]
 atomic_read include/linux/atomic/atomic-instrumented.h:32 [inline]
 swap_cgroup_lookup+0x89/0xd0 mm/swap_cgroup.c:67
 memcg1_swapout+0x235/0x8c0 mm/memcontrol-v1.c:634
 __remove_mapping+0xac5/0xe30 mm/vmscan.c:764
 shrink_folio_list+0x28a8/0x5200 mm/vmscan.c:1523
 reclaim_folio_list+0xeb/0x4e0 mm/vmscan.c:2203
 reclaim_pages+0x454/0x520 mm/vmscan.c:2240
 madvise_cold_or_pageout_pte_range+0x19a0/0x1ce0 mm/madvise.c:563
 walk_pmd_range mm/pagewalk.c:130 [inline]
 walk_pud_range mm/pagewalk.c:224 [inline]
 walk_p4d_range mm/pagewalk.c:262 [inline]
 walk_pgd_range+0x1037/0x1d30 mm/pagewalk.c:303
 __walk_page_range+0x14c/0x710 mm/pagewalk.c:410
 walk_page_range_vma_unsafe+0x34c/0x400 mm/pagewalk.c:714
 madvise_pageout_page_range mm/madvise.c:622 [inline]
 madvise_pageout mm/madvise.c:647 [inline]
 madvise_vma_behavior+0x3132/0x4170 mm/madvise.c:1366
 madvise_walk_vmas+0x575/0xaf0 mm/madvise.c:1721
 madvise_do_behavior+0x38e/0x550 mm/madvise.c:1937
 do_madvise+0x1bc/0x270 mm/madvise.c:2030
 __do_sys_madvise mm/madvise.c:2039 [inline]
 __se_sys_madvise mm/madvise.c:2037 [inline]
 __x64_sys_madvise+0xa7/0xc0 mm/madvise.c:2037
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xec/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f6b0fb8f749
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 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 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f6b10aca038 EFLAGS: 00000246 ORIG_RAX: 000000000000001c
RAX: ffffffffffffffda RBX: 00007f6b0fde6090 RCX: 00007f6b0fb8f749
RDX: 0000000000000015 RSI: 0000000000800000 RDI: 0000200000000000
RBP: 00007f6b0fc13f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f6b0fde6128 R14: 00007f6b0fde6090 R15: 00007ffcdb080178
 </TASK>

The buggy address belongs to a vmalloc virtual mapping
Memory state around the buggy address:
 ffffc900034e0700: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
 ffffc900034e0780: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
>ffffc900034e0800: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
                   ^
 ffffc900034e0880: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
 ffffc900034e0900: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
==================================================================


Tested on:

commit:         f417b7ff Add linux-next specific files for 20260109
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=138f919a580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=9abcd50d16f3c8b5
dashboard link: https://syzkaller.appspot.com/bug?extid=d97580a8cceb9b03c13e
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
patch:          https://syzkaller.appspot.com/x/patch.diff?x=15dab19a580000