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_repr...
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.... [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@syzkaller.appspotmail.com
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