Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> 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
>> 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.
