On 5/12/20 11:11 AM, Mike Kravetz wrote:
> On 5/12/20 8:04 AM, Miklos Szeredi wrote:
>> On Tue, Apr 7, 2020 at 12:06 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>>> On 4/5/20 8:06 PM, syzbot wrote:
>>>
>>> The routine is_file_hugepages() is just comparing the file ops to huegtlbfs:
>>>
>>> if (file->f_op == &hugetlbfs_file_operations)
>>> return true;
>>>
>>> Since the file is in an overlayfs, file->f_op == ovl_file_operations.
>>> Therefore, length will not be rounded up to huge page size and we create a
>>> mapping with incorrect size which leads to the BUG.
>>>
>>> Because of the code in mmap, the hugetlbfs mmap() routine assumes length is
>>> rounded to a huge page size. I can easily add a check to hugetlbfs mmap
>>> to validate length and return -EINVAL. However, I think we really want to
>>> do the 'round up' earlier in mmap. This is because the man page says:
>>>
>>> Huge page (Huge TLB) mappings
>>> For mappings that employ huge pages, the requirements for the arguments
>>> of mmap() and munmap() differ somewhat from the requirements for map‐
>>> pings that use the native system page size.
>>>
>>> For mmap(), offset must be a multiple of the underlying huge page size.
>>> The system automatically aligns length to be a multiple of the underly‐
>>> ing huge page size.
>>>
>>> Since the location for the mapping is chosen BEFORE getting to the hugetlbfs
>>> mmap routine, we can not wait until then to round up the length. Is there a
>>> defined way to go from a struct file * to the underlying filesystem so we
>>> can continue to do the 'round up' in early mmap code?
>>
>> That's easy enough:
>>
>> static inline struct file *real_file(struct file *file)
>> {
>> return file->f_op != ovl_file_operations ? file : file->private_data;
>> }
>>
>> But adding more filesystem specific code to generic code does not
>> sound like the cleanest way to solve this...
>
> We can incorporate the above 'real_file' functionality in the filesystem
> specific routine is_file_hugepages(), and I think that would address this
> specific issue. I'll code that up.
>
>>> One other thing I noticed with overlayfs is that it does not contain a
>>> specific get_unmapped_area file_operations routine. I would expect it to at
>>> least check for and use the get_unmapped_area of the underlying filesystem?
>>> Can someone comment if this is by design?
>>
>> Not sure. What exactly is f_op->get_unmapped_area supposed to do?
>>
>
> IIUC, filesystems can define their own routines to get addresses for mmap
> operations. Quite a few filesystems define get_unmapped_area.
>
> The generic mmap code does the following,
>
> get_area = current->mm->get_unmapped_area;
> if (file) {
> if (file->f_op->get_unmapped_area)
> get_area = file->f_op->get_unmapped_area;
> } else if (flags & MAP_SHARED) {
> /*
> * mmap_region() will call shmem_zero_setup() to create a file,
> * so use shmem's get_unmapped_area in case it can be huge.
> * do_mmap_pgoff() will clear pgoff, so match alignment.
> */
> pgoff = 0;
> get_area = shmem_get_unmapped_area;
> }
>
> addr = get_area(file, addr, len, pgoff, flags);
>
> If the filesystem provides a get_unmapped_area, it will use it. I beleive
> overlayfs prevents this from happening for the underlying filesystem.
>
> Perhaps we do need to add something like a call 'real_file' to this generic
> code? I can't think of any other way to get to the underlying filesystem
> get_unmapped_area here.
I started going down the path of creating a get_unmapped_area f_op for
overlayfs. That is pretty straight forward and works well. But that
did not take care of the is_file_hugepages() routine. Recall that
is_file_hugepages simply does if (file->f_op == &hugetlbfs_file_operations).
I suppose I could add a specific overlayfs check like real_file here. But,
that does not seem like a clean solution.
I also discovered other routines doing comparisons like
if (file->f_op == <expected_fops>), they are:
is_dma_buf_file()
is_file_shm_hugepages()
get_pipe_info()
is_file_epoll()
So, it seems that these routines are also impacted if operating on files
in an overlayfs?
Any suggestions on how to move forward? It seems like there may be the
need for a real_file() routine? I see a d_real dentry_op was added to
deal with this issue for dentries. Might we need something similiar for
files (f_real)?
Looking for suggestions as I do not normally work with this code.
--
Mike Kravetz
