Re: set_page_dirty vs truncate

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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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