[syzbot ci] Re: Virtual Swap Space (Swap Table Edition)

syzbot ci has tested the following series

[v3] Virtual Swap Space (Swap Table Edition)
https://lore.kernel.org/all/[email protected]
* [PATCH v3 01/11] mm, swap: add virtual swap device infrastructure
* [PATCH v3 02/11] mm, swap: support zswap and zeroswap as vswap backends
* [PATCH v3 03/11] mm, swap: prepare the swap IO path for vswap
* [PATCH v3 04/11] mm, swap: support physical swap as a vswap backend
* [PATCH v3 05/11] mm, swap: enable THP swapin for vswap entries
* [PATCH v3 06/11] mm, swap: write back vswap zswap entries to physical swap
* [PATCH v3 07/11] mm, swap: reclaim physical slots backing cache-only vswap entries
* [PATCH v3 08/11] mm, swap: only charge physical swap entries
* [PATCH v3 09/11] mm, swap: add debugfs counters for vswap
* [PATCH v3 10/11] mm, swap: defer memcg_table allocation for physical swap clusters
* [PATCH v3 11/11] mm, swap: widen swap_info_struct max/pages to unsigned long

and found the following issue:
KASAN: null-ptr-deref Read in swap_entry_backend_has_flag

Full report is available here:
https://ci.syzbot.org/series/e7da1097-3230-4e50-80cb-1dfafeebea40

***

KASAN: null-ptr-deref Read in swap_entry_backend_has_flag

tree:      linux-next
URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/next/linux-next
base:      bacc32cc7de65ffff70080a48eb294f89e434d5e
arch:      amd64
compiler:  Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
config:    https://ci.syzbot.org/builds/4f943657-9481-4637-9b1f-9be1bff18f92/config
syz repro: https://ci.syzbot.org/findings/f6c4f1ab-951c-4759-b397-e22c23c3bf32/syz_repro

==================================================================
BUG: KASAN: null-ptr-deref in instrument_atomic_read include/linux/instrumented.h:82 [inline]
BUG: KASAN: null-ptr-deref in atomic_long_read include/linux/atomic/atomic-instrumented.h:3188 [inline]
BUG: KASAN: null-ptr-deref in __vtable_get mm/vswap.h:147 [inline]
BUG: KASAN: null-ptr-deref in vswap_to_phys mm/vswap.h:197 [inline]
BUG: KASAN: null-ptr-deref in swap_entry_backend_has_flag+0xfe/0x220 mm/vswap.h:441
Read of size 8 at addr 0000000000000000 by task syz.2.20/5818

CPU: 1 UID: 0 PID: 5818 Comm: syz.2.20 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 kasan_report+0x117/0x150 mm/kasan/report.c:595
 check_region_inline mm/kasan/generic.c:-1 [inline]
 kasan_check_range+0x264/0x2c0 mm/kasan/generic.c:200
 instrument_atomic_read include/linux/instrumented.h:82 [inline]
 atomic_long_read include/linux/atomic/atomic-instrumented.h:3188 [inline]
 __vtable_get mm/vswap.h:147 [inline]
 vswap_to_phys mm/vswap.h:197 [inline]
 swap_entry_backend_has_flag+0xfe/0x220 mm/vswap.h:441
 do_swap_page+0x3c0/0x5620 mm/memory.c:4975
 __collapse_huge_page_swapin mm/khugepaged.c:1202 [inline]
 collapse_huge_page mm/khugepaged.c:1325 [inline]
 mthp_collapse mm/khugepaged.c:1524 [inline]
 collapse_scan_pmd mm/khugepaged.c:1786 [inline]
 collapse_single_pmd+0x24c6/0x3da0 mm/khugepaged.c:2803
 madvise_collapse+0x2cf/0x790 mm/khugepaged.c:3237
 madvise_vma_behavior+0x115f/0x4170 mm/madvise.c:1363
 madvise_walk_vmas+0x576/0xb00 mm/madvise.c:1712
 madvise_do_behavior+0x385/0x540 mm/madvise.c:1907
 do_madvise+0x327/0x3a0 mm/madvise.c:2005
 __do_sys_madvise mm/madvise.c:2014 [inline]
 __se_sys_madvise mm/madvise.c:2012 [inline]
 __x64_sys_madvise+0xa6/0xc0 mm/madvise.c:2012
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fbcad79e019
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:00007fbcae5c4028 EFLAGS: 00000246 ORIG_RAX: 000000000000001c
RAX: ffffffffffffffda RBX: 00007fbcada25fa0 RCX: 00007fbcad79e019
RDX: 0000000000000019 RSI: 0000000000c00000 RDI: 0000200000000000
RBP: 00007fbcad83500c R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fbcada26038 R14: 00007fbcada25fa0 R15: 00007fff095fa0b8
 </TASK>
==================================================================


***

If these findings have caused you to resend the series or submit a
separate fix, please add the following tag to your commit message:
  Tested-by: [email protected]

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