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

[Vlastimil Babka <vbabka@suse.cz>]

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.

> 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>

> ---
>  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;