[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Message-ID: <f6697acb-c7a7-4126-b0d0-e5add35bafc7@linux.dev>
Date: Tue, 30 Sep 2025 19:17:58 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: syzbot ci <syzbot+ci80449aea3ab57787@...kaller.appspotmail.com>
Cc: syzbot@...ts.linux.dev, syzkaller-bugs@...glegroups.com,
yuzhao@...gle.com, ziy@...dia.com, ying.huang@...ux.alibaba.com,
jannh@...gle.com, vbabka@...e.cz, riel@...riel.com, usamaarif642@...il.com,
david@...hat.com, dev.jain@....com, akpm@...ux-foundation.org,
ryan.roberts@....com, stable@...r.kernel.org, apopple@...dia.com,
rakie.kim@...com, gourry@...rry.net, matthew.brost@...el.com,
lorenzo.stoakes@...cle.com, baohua@...nel.org, liam.howlett@...cle.com,
ioworker0@...il.com, harry.yoo@...cle.com, peterx@...hat.com,
npache@...hat.com, baolin.wang@...ux.alibaba.com, linux-mm@...ck.org,
joshua.hahnjy@...il.com, linux-kernel@...r.kernel.org, byungchul@...com
Subject: Re: [syzbot ci] Re: mm/rmap: fix soft-dirty and uffd-wp bit loss when
remapping zero-filled mTHP subpage to shared zeropage
Thanks for the report!
On 2025/9/30 18:56, syzbot ci wrote:
> syzbot ci has tested the following series
>
> [v3] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
> https://lore.kernel.org/all/20250930060557.85133-1-lance.yang@linux.dev
> * [PATCH v3 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage
>
> and found the following issue:
> general protection fault in remove_migration_pte
>
> Full report is available here:
> https://ci.syzbot.org/series/a2021abd-c238-431c-a92e-cc29beb53cbf
>
> ***
>
> general protection fault in remove_migration_pte
>
> tree: torvalds
> URL: https://kernel.googlesource.com/pub/scm/linux/kernel/git/torvalds/linux
> base: 1896ce8eb6c61824f6c1125d69d8fda1f44a22f8
> arch: amd64
> compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
> config: https://ci.syzbot.org/builds/84a2085e-d609-43ea-8b19-f9af8ea3d54a/config
> C repro: https://ci.syzbot.org/findings/3e211477-5a8d-4d4d-935b-15076499b001/c_repro
> syz repro: https://ci.syzbot.org/findings/3e211477-5a8d-4d4d-935b-15076499b001/syz_repro
>
> Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN PTI
> KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
This is a known issue that I introduced in the v3 patch. I spotted
this exact NULL pointer dereference bug[1] myself and have already
sent out a v5 version[2] with the fix.
The root cause is that ptep_get() is called before the !pwmw.pte
check, which handles PMD-mapped THP migration entries.
[1]
https://lore.kernel.org/linux-mm/2d21c9bc-e299-4ca6-85ba-b01a1f346d9d@linux.dev
[2]
https://lore.kernel.org/linux-mm/20250930081040.80926-1-lance.yang@linux.dev
Thanks,
Lance
> CPU: 0 UID: 0 PID: 5985 Comm: syz.0.27 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
> RIP: 0010:ptep_get include/linux/pgtable.h:340 [inline]
> RIP: 0010:remove_migration_pte+0x37f/0x2340 mm/migrate.c:361
> Code: 43 20 48 89 84 24 08 01 00 00 49 8d 47 40 48 89 84 24 00 01 00 00 4c 89 64 24 50 4c 8b b4 24 70 01 00 00 4c 89 f0 48 c1 e8 03 <42> 80 3c 28 00 74 08 4c 89 f7 e8 22 3e ff ff 49 8b 06 48 89 44 24
> RSP: 0018:ffffc90002c2f3c0 EFLAGS: 00010246
> RAX: 0000000000000000 RBX: ffff888027799080 RCX: 1ffffd40008d1006
> RDX: 0000000000000000 RSI: 00000000000387ff RDI: 0000000000038600
> RBP: ffffc90002c2f5d0 R08: 0000000000000003 R09: 0000000000000004
> R10: dffffc0000000000 R11: fffff52000585e30 R12: ffffea0004688008
> R13: dffffc0000000000 R14: 0000000000000000 R15: ffffea0004688000
> FS: 0000555589124500(0000) GS:ffff8880b8d7e000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000200000000300 CR3: 0000000026118000 CR4: 00000000000006f0
> Call Trace:
> <TASK>
> rmap_walk_anon+0x553/0x730 mm/rmap.c:2842
> remove_migration_ptes mm/migrate.c:478 [inline]
> migrate_folio_move mm/migrate.c:1394 [inline]
> migrate_folios_move mm/migrate.c:1725 [inline]
> migrate_pages_batch+0x200a/0x35c0 mm/migrate.c:1972
> migrate_pages_sync mm/migrate.c:2002 [inline]
> migrate_pages+0x1bcc/0x2930 mm/migrate.c:2111
> migrate_to_node mm/mempolicy.c:1244 [inline]
> do_migrate_pages+0x5ee/0x800 mm/mempolicy.c:1343
> kernel_migrate_pages mm/mempolicy.c:1858 [inline]
> __do_sys_migrate_pages mm/mempolicy.c:1876 [inline]
> __se_sys_migrate_pages+0x544/0x650 mm/mempolicy.c:1872
> do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f922b98ec29
> 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:00007ffccaf966f8 EFLAGS: 00000246 ORIG_RAX: 0000000000000100
> RAX: ffffffffffffffda RBX: 00007f922bbd5fa0 RCX: 00007f922b98ec29
> RDX: 0000200000000300 RSI: 0000000000000003 RDI: 0000000000000000
> RBP: 00007f922ba11e41 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000200000000040 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007f922bbd5fa0 R14: 00007f922bbd5fa0 R15: 0000000000000004
> </TASK>
> Modules linked in:
> ---[ end trace 0000000000000000 ]---
> RIP: 0010:ptep_get include/linux/pgtable.h:340 [inline]
> RIP: 0010:remove_migration_pte+0x37f/0x2340 mm/migrate.c:361
> Code: 43 20 48 89 84 24 08 01 00 00 49 8d 47 40 48 89 84 24 00 01 00 00 4c 89 64 24 50 4c 8b b4 24 70 01 00 00 4c 89 f0 48 c1 e8 03 <42> 80 3c 28 00 74 08 4c 89 f7 e8 22 3e ff ff 49 8b 06 48 89 44 24
> RSP: 0018:ffffc90002c2f3c0 EFLAGS: 00010246
> RAX: 0000000000000000 RBX: ffff888027799080 RCX: 1ffffd40008d1006
> RDX: 0000000000000000 RSI: 00000000000387ff RDI: 0000000000038600
> RBP: ffffc90002c2f5d0 R08: 0000000000000003 R09: 0000000000000004
> R10: dffffc0000000000 R11: fffff52000585e30 R12: ffffea0004688008
> R13: dffffc0000000000 R14: 0000000000000000 R15: ffffea0004688000
> FS: 0000555589124500(0000) GS:ffff8880b8d7e000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000200000000300 CR3: 0000000026118000 CR4: 00000000000006f0
> ----------------
> Code disassembly (best guess):
> 0: 43 20 48 89 rex.XB and %cl,-0x77(%r8)
> 4: 84 24 08 test %ah,(%rax,%rcx,1)
> 7: 01 00 add %eax,(%rax)
> 9: 00 49 8d add %cl,-0x73(%rcx)
> c: 47 rex.RXB
> d: 40 rex
> e: 48 89 84 24 00 01 00 mov %rax,0x100(%rsp)
> 15: 00
> 16: 4c 89 64 24 50 mov %r12,0x50(%rsp)
> 1b: 4c 8b b4 24 70 01 00 mov 0x170(%rsp),%r14
> 22: 00
> 23: 4c 89 f0 mov %r14,%rax
> 26: 48 c1 e8 03 shr $0x3,%rax
> * 2a: 42 80 3c 28 00 cmpb $0x0,(%rax,%r13,1) <-- trapping instruction
> 2f: 74 08 je 0x39
> 31: 4c 89 f7 mov %r14,%rdi
> 34: e8 22 3e ff ff call 0xffff3e5b
> 39: 49 8b 06 mov (%r14),%rax
> 3c: 48 rex.W
> 3d: 89 .byte 0x89
> 3e: 44 rex.R
> 3f: 24 .byte 0x24
>
>
> ***
>
> 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: syzbot@...kaller.appspotmail.com
>
> ---
> This report is generated by a bot. It may contain errors.
> syzbot ci engineers can be reached at syzkaller@...glegroups.com.
Powered by blists - more mailing lists