Re: [PATCH] xarray: fix index jumping backwards in xas_find()

[Mike Rapoport]

On Thu, Sep 03, 2026 at 11:22:51AM -0700, Andrew Morton wrote:
> On Thu,  3 Sep 2026 13:48:55 +0000 (UTC) "syzbot" <syzbot@xxxxxxxxxx> wrote:
> 
> > From: Krystian Kaniewski <krystianmkaniewski@xxxxxxxxx>
> > 
> > A bug in the XArray iterator xas_find() causes the iterator's index
> > (xas->xa_index) to jump backwards when iterating over a multi-index entry
> > (like a THP) that resides in a non-leaf node and is concurrently split.
> > 
> > When iterating over a multi-index entry in a non-leaf node, xas_load() sets
> > xas->xa_offset to the base offset of the entry, but leaves xas->xa_index at
> > the requested index. When the caller subsequently wants to advance to the
> > next entry, xas_find() is called. xas_find() attempts to synchronize
> > xas->xa_offset with xas->xa_index before advancing. However, the fixup
> > logic was incorrectly restricted to leaf nodes (!xas->xa_node->shift).
> > Because the THP resides in a non-leaf node, the fixup is skipped.
> > 
> > As a result, xas_find() simply increments xas->xa_offset and recalculates
> > xas->xa_index based on this new offset. This causes xas->xa_index to jump
> > backwards. If the THP was concurrently split, the entry at the new offset
> > is a node pointer, so xas_find() descends into it and returns the folio at
> > the backwards index. The caller (filemap_map_pages()) then calculates the
> > PTE pointer based on this backwards index, resulting in an invalid memory
> > access such as an out-of-bounds read or use-after-free on a page-table page
> > freed via tlb_remove_table_rcu().
> > 
> > To fix this, check if xas->xa_offset matches get_offset(xas->xa_index,
> > xas->xa_node). If it does not and the node is a non-leaf node, set
> > xas->xa_offset to get_offset(xas->xa_index, xas->xa_node) before advancing.
> > Also add test cases in test_xarray to verify xas_find() behavior when
> > iterating over and splitting multi-index entries.
> 
> Thanks.  But the changelog omits vital information: a description of
> the userspace-visible runtime effects of the bug.
> 
> > Fixes: b803b42823d0 ("xarray: Add XArray iterators")
> > Assisted-by: Gemini:gemini-3.7-flash syzbot
> > Reported-by: syzbot+b72767277f29b6407083@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://syzkaller.appspot.com/bug?extid=b72767277f29b6407083
> 
> I see from there that it was a use-after-free, detected by KASAN.  I'll
> update the changelog and I'll add cc:stable.
> 
> I'll queue the patch for testing while awaiting review input.

The testing shows there are issues with this patch:

https://github.com/linux-mm/linux-mm/actions/runs/33815757662/job/100847425973

And since it's in mm-unstable now it breaks all CI jobs :(

> > Link: https://syzkaller.appspot.com/ai_job?id=a01c56bd-74d0-411c-afb4-ee6f0cb6cb61
> 
> 

-- 
Sincerely yours,
Mike.