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

[linmiaohe <linmiaohe@huawei.com>]

Andrew Morton <akpm@linux-foundation.org> wrote:
> On Wed, 16 Sep 2020 05:07:33 -0400 Miaohe Lin <linmiaohe@huawei.com> 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
>> Trace:
>>  free_pgtables+0x1b3/0x2f0 mm/memory.c:415
>>  exit_mmap+0x2c0/0x530 mm/mmap.c:3184
>>  __mmput+0x122/0x470 kernel/fork.c:1076
>> 
>> 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.
>

Many thanks for your reply.

Yes, I think so too. We do deny_write_access and mapping_map_writable on @file, so we should undo all of this on @file.
Since @file is unchanged over the second vma_merge() time, we'are using the correct @file to undo our temporary denial count.

But how should I check this explicitly ? I can't find out a way to do this. Could you please figure it out for me?
Thanks again.