Re: [PATCH] mm: fix the race on huge alloc failed

[Matthew Wilcox]

On Sat, Aug 29, 2026 at 07:00:34AM -0300, Guilherme Giacomo Simoes wrote:
> The race occurs because the reader (__vmf_anon_prepare()) checks
> `vma->anon->vma` without holding the mmap_lock and withou the
> READ_ONCE() macro. Since the writer (__anon_vma_prepare()) is holding the
> mmap_lock and updating the pointer, it creates a data race as the two
> access are not properly synchronized.
> 
> Use READ_ONCE() on the reader side and WRITE_ONCE() on the writer side
> to tell to compiler treat these memory access carefully and not to
> optimize them leading to inconsistent read.
> 
> Reported-by: syzbot+395b7abe9696862fc188@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=395b7abe9696862fc188
> Fixes: 164b06f238b9 ("mm: call wp_page_copy() under the VMA lock")

what makes you think this is the right commit for fixes?

> Signed-off-by: Guilherme Giacomo Simoes <trintaeoitogc@xxxxxxxxx>
> ---
>  mm/memory.c | 2 +-
>  mm/rmap.c   | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 6b8280cfc1db..33c1fbd30cdd 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3820,7 +3820,7 @@ vm_fault_t __vmf_anon_prepare(struct vm_fault *vmf)
>  	struct vm_area_struct *vma = vmf->vma;
>  	vm_fault_t ret = 0;
>  
> -	if (likely(vma->anon_vma))
> +	if (likely(READ_ONCE(vma->anon_vma)))
>  		return 0;
>  	if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
>  		if (!mmap_read_trylock(vma->vm_mm))
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 1c77d5dc06e9..9d64d776b8c5 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -209,7 +209,7 @@ int __anon_vma_prepare(struct vm_area_struct *vma)
>  	/* page_table_lock to protect against threads */
>  	spin_lock(&mm->page_table_lock);
>  	if (likely(!vma->anon_vma)) {
> -		vma->anon_vma = anon_vma;
> +		WRITE_ONCE(vma->anon_vma, anon_vma);
>  		anon_vma_chain_assign(vma, avc, anon_vma);
>  		anon_vma_interval_tree_insert(avc, &anon_vma->rb_root);
>  		anon_vma->num_active_vmas++;
> -- 
> 2.52.0
> 
>