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

[syzbot ci]

syzbot ci has tested the following series

[v5] Virtual Swap Space (Swap Table Edition)
https://lore.kernel.org/all/20260918180241.3424851-1-nphamcs@xxxxxxxxx
* [PATCH v5 01/11] mm, swap: add virtual swap device infrastructure
* [PATCH v5 02/11] mm, swap: support zswap and zero-filled swap pages as vswap backends
* [PATCH v5 03/11] mm, swap: prepare the swap IO path for vswap
* [PATCH v5 04/11] mm, swap: support physical swap as a vswap backend
* [PATCH v5 05/11] mm, swap: enable THP swapin for vswap entries
* [PATCH v5 06/11] mm, swap: write back vswap zswap entries to physical swap
* [PATCH v5 07/11] mm, swap: reclaim physical slots backing cache-only vswap entries
* [PATCH v5 08/11] mm, swap: only charge physical swap entries
* [PATCH v5 09/11] mm, swap: add debugfs counters for vswap
* [PATCH v5 10/11] mm, swap: defer memcg_table allocation for physical swap clusters
* [RFC PATCH v5 11/11] mm, swap: back vswap clusters with a VM_SPARSE array

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/713f2473-328a-4c16-8bce-63005d15a338

***

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:      27e4e1835109ef599d72abe6c09711e0b1916033
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/26b06329-3b1c-4c73-8ee3-64c53b995186/config
syz repro: https://ci.syzbot.org/findings/f743742b-1934-4b4e-a590-9a36592da902/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:163 [inline]
BUG: KASAN: null-ptr-deref in vswap_to_phys mm/vswap.h:202 [inline]
BUG: KASAN: null-ptr-deref in swap_entry_backend_has_flag+0xea/0x210 mm/vswap.h:409
Read of size 8 at addr 0000000000000e00 by task syz.1.38/5916

CPU: 1 UID: 0 PID: 5916 Comm: syz.1.38 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:163 [inline]
 vswap_to_phys mm/vswap.h:202 [inline]
 swap_entry_backend_has_flag+0xea/0x210 mm/vswap.h:409
 do_swap_page+0x767/0x5a20 mm/memory.c:5042
 __collapse_huge_page_swapin mm/khugepaged.c:1152 [inline]
 collapse_huge_page mm/khugepaged.c:1271 [inline]
 mthp_collapse mm/khugepaged.c:1469 [inline]
 collapse_run_pmd+0xaf1/0x5d90 mm/khugepaged.c:2811
 madvise_collapse mm/madvise.c:1026 [inline]
 madvise_vma_behavior+0x17cf/0x4d80 mm/madvise.c:1548
 madvise_walk_vmas+0x576/0xb00 mm/madvise.c:1896
 madvise_do_behavior+0x385/0x540 mm/madvise.c:2091
 do_madvise+0x26f/0x330 mm/madvise.c:2189
 __do_sys_madvise mm/madvise.c:2198 [inline]
 __se_sys_madvise mm/madvise.c:2196 [inline]
 __x64_sys_madvise+0xa6/0xc0 mm/madvise.c:2196
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f7b5939e159
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:00007f7b5a1ff028 EFLAGS: 00000246 ORIG_RAX: 000000000000001c
RAX: ffffffffffffffda RBX: 00007f7b59625fa0 RCX: 00007f7b5939e159
RDX: 0000000000000019 RSI: 0000000000c00000 RDI: 0000200000000000
RBP: 00007f7b5943506b R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f7b59626038 R14: 00007f7b59625fa0 R15: 00007ffe5f2d8d38
 </TASK>
==================================================================


***

If these findings have caused you to resend the series or submit a
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  Tested-by: syzbot@xxxxxxxxxxxxxxxxxxxxxxxxx

---
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