Re: set_page_dirty vs truncate

[John Hubbard <jhubbard@nvidia.com>]

On 12/18/20 9:18 PM, Matthew Wilcox wrote:
> On Fri, Dec 18, 2020 at 10:03:16PM +0000, Matthew Wilcox wrote:
>> On Fri, Dec 18, 2020 at 04:05:31PM +0000, Matthew Wilcox wrote:
>>> A number of implementations of ->set_page_dirty check whether the page
>>> has been truncated (ie page->mapping has become NULL since entering
>>> set_page_dirty()).  Several other implementations assume that they can do
>>> page->mapping->host to get to the inode.  So either some implementations
>>> are doing unnecessary checks or others are vulnerable to a NULL pointer
>>> dereference if truncate() races with set_page_dirty().
>>>
>>> I'm touching ->set_page_dirty() anyway as part of the page folio
>>> conversion.  I'm thinking about passing in the mapping so there's no
>>> need to look at page->mapping.
>>>
>>> The comments on set_page_dirty() and set_page_dirty_lock() suggests
>>> there's no consistency in whether truncation is blocked or not; we're
>>> only guaranteed that the inode itself won't go away.  But maybe the
>>> comments are stale.
>>
>> The comments are, I believe, not stale.  Here's some syzbot
>> reports which indicate that ext4 is seeing races between set_page_dirty()
>> and truncate():
>>
>>   https://groups.google.com/g/syzkaller-lts-bugs/c/s9fHu162zhQ/m/Phnf6ucaAwAJ
>>
>> The reproducer includes calls to ftruncate(), so that would suggest
>> that's what's going on.
> 
> Hmmm ... looks like __set_page_dirty_nobuffers() has a similar problem:
> 
> {
>          lock_page_memcg(page);
>          if (!TestSetPageDirty(page)) {
>                  struct address_space *mapping = page_mapping(page);
>                  unsigned long flags;
> 
>                  if (!mapping) {
>                          unlock_page_memcg(page);
>                          return 1;
>                  }
> 
>                  xa_lock_irqsave(&mapping->i_pages, flags);
>                  BUG_ON(page_mapping(page) != mapping);
> 
> sure, we check that the page wasn't truncated between set_page_dirty()
> and the call to TestSetPageDirty(), but we can truncate dirty pages
> with no problem.  So between the call to TestSetPageDirty() and
> the call to xa_lock_irqsave(), the page can be truncated, and the
> BUG_ON should fire.
> 
> I haven't been able to find any examples of this, but maybe it's just a very
> narrow race.  Does anyone recognise this signature?  Adding the filesystems
> which use __set_page_dirty_nobuffers() directly without extra locking.


That sounds like the same *kind* of failure that Jan Kara and I were
seeing on live systems[1], that led eventually to the gup-to-pup
conversion exercise.

That crash happened due to calling set_page_dirty() on pages that had no
buffers on them [2]. And that sounds like *exactly* the same thing as
calling __set_page_dirty_nobuffers() without extra locking. So I'd
expect that it's Just Wrong To Do, for the same reasons as Jan spells
out very clearly in [1].

Hope that helps.


[1] https://www.spinics.net/lists/linux-mm/msg142700.html

[2] which triggered this assertion:

#define page_buffers(page)					\
	({							\
		BUG_ON(!PagePrivate(page));			\
		((struct buffer_head *)page_private(page));	\
	})


> 
> $ git grep set_page_dirty.*=.*__set_page_dirty_nobuffers
> fs/9p/vfs_addr.c:       .set_page_dirty = __set_page_dirty_nobuffers,
> fs/cifs/file.c: .set_page_dirty = __set_page_dirty_nobuffers,
> fs/cifs/file.c: .set_page_dirty = __set_page_dirty_nobuffers,
> fs/fuse/file.c: .set_page_dirty = __set_page_dirty_nobuffers,
> fs/hostfs/hostfs_kern.c:        .set_page_dirty = __set_page_dirty_nobuffers,
> fs/jfs/jfs_metapage.c:  .set_page_dirty = __set_page_dirty_nobuffers,
> fs/nfs/file.c:  .set_page_dirty = __set_page_dirty_nobuffers,
> fs/ntfs/aops.c: .set_page_dirty = __set_page_dirty_nobuffers,   /* Set the page dirty
> fs/orangefs/inode.c:    .set_page_dirty = __set_page_dirty_nobuffers,
> fs/vboxsf/file.c:       .set_page_dirty = __set_page_dirty_nobuffers,
> 

...wow, long list of these.

thanks,
-- 
John Hubbard
NVIDIA

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