Re: [syzbot] [mm?] general protection fault in zap_huge_pmd

[Lance Yang <lance.yang@linux.dev>]

On 2026/3/18 23:03, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    b84a0ebe421c Add linux-next specific files for 20260313
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=119ddd52580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=e7280ad1f68b2dce
> dashboard link: https://syzkaller.appspot.com/bug?extid=de14f7701c22477db718
> compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=173b44da580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1537b8da580000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/09145161a8a9/disk-b84a0ebe.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/b64c254e474c/vmlinux-b84a0ebe.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/a7c33f5f7f45/bzImage-b84a0ebe.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+de14f7701c22477db718@syzkaller.appspotmail.com
> 
> Oops: general protection fault, probably for non-canonical address 0xdffffc0000000003: 0000 [#1] SMP KASAN PTI

Looks like it hits a general protection fault in zap_huge_pmd() while
dereferencing folio->mapping via folio_test_anon() ...

zap_huge_pmd() fails to handle non-present, non-none PMD entries that
are not valid PMD softleaf entries, leaving folio as NULL and
dereferencing it ...

For PMD-sized hugetlb mappings like the reproducer above,
hugetlb/userfaultfd would make such PMD entries that can be
non-present and non-none without being valid PMD softleaf entries?

I'll look into it :)

Thanks,
Lance

> KASAN: null-ptr-deref in range [0x0000000000000018-0x000000000000001f]
> CPU: 1 UID: 0 PID: 5994 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
> RIP: 0010:folio_test_anon include/linux/page-flags.h:718 [inline]
> RIP: 0010:zap_huge_pmd+0x7b1/0x1030 mm/huge_memory.c:2463
> Code: 08 00 00 e8 11 e0 92 ff 48 c7 44 24 10 00 00 00 00 4c 8b 3c 24 4c 8d 75 18 4c 89 f0 48 c1 e8 03 48 b9 00 00 00 00 00 fc ff df <80> 3c 08 00 74 08 4c 89 f7 e8 f1 43 fc ff 49 8b 1e 48 89 de 48 83
> RSP: 0018:ffffc90003bb7550 EFLAGS: 00010206
> RAX: 0000000000000003 RBX: f000000000000000 RCX: dffffc0000000000
> RDX: 0000000000000000 RSI: 0000000000000006 RDI: 0000000000000003
> RBP: 0000000000000000 R08: ffff88807cc9802f R09: 1ffff1100f993005
> R10: dffffc0000000000 R11: ffffed100f993006 R12: ffff88807cc98028
> R13: fffffffffffffa00 R14: 0000000000000018 R15: ffffc90003bb7ac0
> FS:  0000000000000000(0000) GS:ffff888124ee0000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00002000000000c0 CR3: 000000000e94a000 CR4: 00000000003526f0
> Call Trace:
>   <TASK>
>   zap_pmd_range mm/memory.c:1990 [inline]
>   zap_pud_range mm/memory.c:2032 [inline]
>   zap_p4d_range mm/memory.c:2053 [inline]
>   __zap_vma_range+0xa82/0x4bd0 mm/memory.c:2093
>   unmap_vmas+0x379/0x530 mm/memory.c:2162
>   exit_mmap+0x280/0xa10 mm/mmap.c:1302
>   __mmput+0x118/0x430 kernel/fork.c:1180
>   exit_mm+0x18e/0x250 kernel/exit.c:581
>   do_exit+0x8b9/0x2490 kernel/exit.c:962
>   do_group_exit+0x21b/0x2d0 kernel/exit.c:1116
>   __do_sys_exit_group kernel/exit.c:1127 [inline]
>   __se_sys_exit_group kernel/exit.c:1125 [inline]
>   __x64_sys_exit_group+0x3f/0x40 kernel/exit.c:1125
>   x64_sys_call+0x221a/0x2240 arch/x86/include/generated/asm/syscalls_64.h:232
>   do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>   do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
>   entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7fdd0b39c799
> Code: Unable to access opcode bytes at 0x7fdd0b39c76f.
> RSP: 002b:00007fff630f1d78 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fdd0b39c799
> RDX: 0000000000000064 RSI: 0000000000000000 RDI: 0000000000000000
> RBP: 0000000000000003 R08: 0000000000000000 R09: 00007fdd0b5e63e0
> R10: 0000000000000001 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007fdd0b5e63e0 R14: 0000000000000003 R15: 00007fff630f1e30
>   </TASK>
> Modules linked in:
> ---[ end trace 0000000000000000 ]---
> RIP: 0010:folio_test_anon include/linux/page-flags.h:718 [inline]
> RIP: 0010:zap_huge_pmd+0x7b1/0x1030 mm/huge_memory.c:2463
> Code: 08 00 00 e8 11 e0 92 ff 48 c7 44 24 10 00 00 00 00 4c 8b 3c 24 4c 8d 75 18 4c 89 f0 48 c1 e8 03 48 b9 00 00 00 00 00 fc ff df <80> 3c 08 00 74 08 4c 89 f7 e8 f1 43 fc ff 49 8b 1e 48 89 de 48 83
> RSP: 0018:ffffc90003bb7550 EFLAGS: 00010206
> RAX: 0000000000000003 RBX: f000000000000000 RCX: dffffc0000000000
> RDX: 0000000000000000 RSI: 0000000000000006 RDI: 0000000000000003
> RBP: 0000000000000000 R08: ffff88807cc9802f R09: 1ffff1100f993005
> R10: dffffc0000000000 R11: ffffed100f993006 R12: ffff88807cc98028
> R13: fffffffffffffa00 R14: 0000000000000018 R15: ffffc90003bb7ac0
> FS:  0000000000000000(0000) GS:ffff888124ee0000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00002000000000c0 CR3: 000000000e94a000 CR4: 00000000003526f0
> ----------------
> Code disassembly (best guess):
>     0:	08 00                	or     %al,(%rax)
>     2:	00 e8                	add    %ch,%al
>     4:	11 e0                	adc    %esp,%eax
>     6:	92                   	xchg   %eax,%edx
>     7:	ff 48 c7             	decl   -0x39(%rax)
>     a:	44 24 10             	rex.R and $0x10,%al
>     d:	00 00                	add    %al,(%rax)
>     f:	00 00                	add    %al,(%rax)
>    11:	4c 8b 3c 24          	mov    (%rsp),%r15
>    15:	4c 8d 75 18          	lea    0x18(%rbp),%r14
>    19:	4c 89 f0             	mov    %r14,%rax
>    1c:	48 c1 e8 03          	shr    $0x3,%rax
>    20:	48 b9 00 00 00 00 00 	movabs $0xdffffc0000000000,%rcx
>    27:	fc ff df
> * 2a:	80 3c 08 00          	cmpb   $0x0,(%rax,%rcx,1) <-- trapping instruction
>    2e:	74 08                	je     0x38
>    30:	4c 89 f7             	mov    %r14,%rdi
>    33:	e8 f1 43 fc ff       	call   0xfffc4429
>    38:	49 8b 1e             	mov    (%r14),%rbx
>    3b:	48 89 de             	mov    %rbx,%rsi
>    3e:	48                   	rex.W
>    3f:	83                   	.byte 0x83
> 
> 
> ---
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