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

[Lorenzo Stoakes]

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).

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.

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.

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
>