Re: [PATCH] mm: mmap: Fix general protection fault in unlink_file_vma()

[Andrew Morton]

On Wed, 16 Sep 2020 05:07:33 -0400 Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:

> The syzbot reported the below general protection fault:
> 
> general protection fault, probably for non-canonical address
> 0xe00eeaee0000003b: 0000 [#1] PREEMPT SMP KASAN
> KASAN: maybe wild-memory-access in range
> [0x00777770000001d8-0x00777770000001df]
> CPU: 1 PID: 10488 Comm: syz-executor721 Not tainted 5.9.0-rc3-syzkaller #0
> RIP: 0010:unlink_file_vma+0x57/0xb0 mm/mmap.c:164
> Code: 4c 8b a5 a0 00 00 00 4d 85 e4 74 4e e8 92 d7 cd ff 49 8d bc 24
> d8 01 00 00 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c
> 02 00 75 3d 4d 8b b4 24 d8 01 00 00 4d 8d 6e 78 4c 89 ef e8
> RSP: 0018:ffffc9000ac0f9b0 EFLAGS: 00010202
> RAX: dffffc0000000000 RBX: ffff88800010ceb0 RCX: ffffffff81592421
> RDX: 000eeeee0000003b RSI: ffffffff81a6736e RDI: 00777770000001d8
> RBP: ffff88800010ceb0 R08: 0000000000000001 R09: ffff88801291a50f
> R10: ffffed10025234a1 R11: 0000000000000001 R12: 0077777000000000
> R13: 00007f1eea0da000 R14: 00007f1eea0d9000 R15: 0000000000000000
> FS:  0000000000000000(0000) GS:ffff8880ae700000(0000)
> knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f1eea11a9d0 CR3: 000000000007e000 CR4: 00000000001506e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  free_pgtables+0x1b3/0x2f0 mm/memory.c:415
>  exit_mmap+0x2c0/0x530 mm/mmap.c:3184
>  __mmput+0x122/0x470 kernel/fork.c:1076
>  mmput+0x53/0x60 kernel/fork.c:1097
>  exit_mm kernel/exit.c:483 [inline]
>  do_exit+0xa8b/0x29f0 kernel/exit.c:793
>  do_group_exit+0x125/0x310 kernel/exit.c:903
>  get_signal+0x428/0x1f00 kernel/signal.c:2757
>  arch_do_signal+0x82/0x2520 arch/x86/kernel/signal.c:811
>  exit_to_user_mode_loop kernel/entry/common.c:136 [inline]
>  exit_to_user_mode_prepare+0x1ae/0x200 kernel/entry/common.c:167
>  syscall_exit_to_user_mode+0x7e/0x2e0 kernel/entry/common.c:242
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> It's because the ->mmap() callback can change vma->vm_file and fput the
> original file. But the commit d70cec898324 ("mm: mmap: merge vma after
> call_mmap() if possible") failed to catch this case and always fput() the
> original file, hence add an extra fput().
> 
> ...
>
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1815,7 +1815,11 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  			merge = vma_merge(mm, prev, vma->vm_start, vma->vm_end, vma->vm_flags,
>  				NULL, vma->vm_file, vma->vm_pgoff, NULL, NULL_VM_UFFD_CTX);
>  			if (merge) {
> -				fput(file);
> +				/* ->mmap() can change vma->vm_file and fput the original file. So
> +				 * fput the vma->vm_file here or we would add an extra fput for file
> +				 * and cause general protection fault ultimately.
> +				 */
> +				fput(vma->vm_file);
>  				vm_area_free(vma);
>  				vma = merge;
>  				/* Update vm_flags and possible addr to pick up the change. We don't

What about the case where this code block does its `goto unmap_writable'?

	/* Once vma denies write, undo our temporary denial count */
	if (file) {
unmap_writable:
		if (vm_flags & VM_SHARED)
			mapping_unmap_writable(file->f_mapping);
		if (vm_flags & VM_DENYWRITE)
			allow_write_access(file);
	}
	file = vma->vm_file;

is this using the correct file?  I think it is, but please do check.