Re: [PATCH v2] mm/mmap: Fix race in mmap_region() with ftrucate()

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Vlastimil Babka <vbabka@suse.cz> [241016 05:45]:
> Subject has a typo, should say ftruncate()
> 
> Also let's explicitly note it's a 6.12 hotfix.
> 
> On 10/16/24 03:34, Liam R. Howlett wrote:
> > From: "Liam R. Howlett" <Liam.Howlett@Oracle.com>
> > 
> > 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(s) are left
> > in the vma tree, but free_pgtables() removes them from the rmap and
> > clears the ptes while holding the necessary locks.
> 
> And no parallel page faults can reinstate any PTEs as the vma's are marked
> as detached, right.

Right, it is detached and waiting to be freed so the per-vma page
faulting will fall through and wait for the mmap read lock to continue
page faulting, but the MAP_FIXED call has to drop the mmap write lock.

> 
> > 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@google.com>
> > Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> > Cc: Vlastimil Babka <vbabka@suse.cz>
> > Cc: Matthew Wilcox <willy@infradead.org>
> > Cc: David Hildenbrand <david@redhat.com>
> > Fixes: f8d112a4e657 ("mm/mmap: avoid zeroing vma tree in mmap_region()")
> > Reported-by: Jann Horn <jannh@google.com>
> > Closes: https://lore.kernel.org/all/CAG48ez0ZpGzxi=-5O_uGQ0xKXOmbjeQ0LjZsRJ1Qtf2X5eOr1w@mail.gmail.com/
> > Link: https://lore.kernel.org/all/CAG48ez0ZpGzxi=-5O_uGQ0xKXOmbjeQ0LjZsRJ1Qtf2X5eOr1w@mail.gmail.com/
> > Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
> > Reviewed-by: Jann Horn <jannh@google.com>
> > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> 
> Acked-by: Vlastimil Babka <vbabka@suse.cz>

Thanks!

> 
> > ---
> >  mm/mmap.c | 12 +++++++-----
> >  1 file changed, 7 insertions(+), 5 deletions(-)
> > 
> > v1: https://lore.kernel.org/all/20241015161135.2133951-1-Liam.Howlett@oracle.com/
> > 
> > Changes since v1:
> >   Updated commit message - Thanks Lorenzo
> > 
> > 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;
>