Re: [PATCH] mm/mmap: Fix race in mmap_region() with ftrucate()

[Liam R. Howlett]

* Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> [241015 14:23]:
> On Tue, Oct 15, 2024 at 12:11:35PM -0400, Liam R. Howlett wrote:
> > From: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
> >
> > Avoiding the zeroing of the vma tree in mmap_region() introduced a race
> > with truncate in the page table walk.  To avoid any races, create a hole
> > in the rmap during the operation by clearing the pagetable entries
> > earlier under the mmap write lock and (critically) before the new vma is
> > installed into the vma tree.  The result is that the old vma is still in
> > the vma tree, but the page tables are cleared while holding the
> > i_mmap_rwsem.
> 
> Nit: old VMAs (can be more than one).

right, okay.

> 
> Hmm this is not correct I don't think. We are not holding the i_mmap_rwsem
> (nor could we as free_pgtables() ends up manipulating this lock.

I meant we hold i_mmap_rwsem when clearing the ptes by calling
free_pgtables(), which does what we need in regards to locking.

> 
> Rather, free_pgtables() unlinks the old VMAs from the rmap before removing
> their page tables, the key point here is that we have not yet established a
> new VMA that makes page tables within the range valid and accessible.
> 
> Before we did so _prior_ to this, making it possible for a racing
> ftruncate() to grab a PMD right before we free it and then UAF.

You have totally lost me in your wording as well, but I get what you are
saying.  I'll try to make the change log more clear.

> 
> Now we make that impossible.
> 
> So I'd reword this to that effect.
> 
> >
> > This change extends the fix required for hugetblfs and the call_mmap()
> > function by moving the cleanup higher in the function and running it
> > unconditionally.
> >
> > Cc: Jann Horn <jannh@xxxxxxxxxx>
> > Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > Cc: David Hildenbrand <david@xxxxxxxxxx>
> > Fixes: f8d112a4e657 ("mm/mmap: avoid zeroing vma tree in mmap_region()")
> > Reported-by: Jann Horn <jannh@xxxxxxxxxx>
> > Closes: https://lore.kernel.org/all/CAG48ez0ZpGzxi=-5O_uGQ0xKXOmbjeQ0LjZsRJ1Qtf2X5eOr1w@xxxxxxxxxxxxxx/
> > Link: https://lore.kernel.org/all/CAG48ez0ZpGzxi=-5O_uGQ0xKXOmbjeQ0LjZsRJ1Qtf2X5eOr1w@xxxxxxxxxxxxxx/
> > Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> 
> Other than that, LGTM, so:
> 
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> 
> > ---
> >  mm/mmap.c | 12 +++++++-----
> >  1 file changed, 7 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index dd4b35a25aeb..a20998fb633c 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -1413,6 +1413,13 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> >  		vmg.flags = vm_flags;
> >  	}
> >
> > +	/*
> > +	 * clear PTEs while the vma is still in the tree so that rmap
> > +	 * cannot race with the freeing later in the truncate scenario.
> > +	 * This is also needed for call_mmap(), which is why vm_ops
> > +	 * close function is called.
> > +	 */
> > +	vms_clean_up_area(&vms, &mas_detach);
> >  	vma = vma_merge_new_range(&vmg);
> >  	if (vma)
> >  		goto expanded;
> > @@ -1432,11 +1439,6 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> >
> >  	if (file) {
> >  		vma->vm_file = get_file(file);
> > -		/*
> > -		 * call_mmap() may map PTE, so ensure there are no existing PTEs
> > -		 * and call the vm_ops close function if one exists.
> > -		 */
> > -		vms_clean_up_area(&vms, &mas_detach);
> >  		error = call_mmap(file, vma);
> >  		if (error)
> >  			goto unmap_and_free_vma;
> > --
> > 2.43.0
> >