Re: [PATCH v1 1/4] mm: Fix uprobe pte be overwritten when expanding vma

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Thu, May 29, 2025 at 03:56:47PM +0000, Pu Lehui wrote:
> From: Pu Lehui <pulehui@huawei.com>
>
> We encountered a BUG alert triggered by Syzkaller as follows:
>    BUG: Bad rss-counter state mm:00000000b4a60fca type:MM_ANONPAGES val:1
>
> And we can reproduce it with the following steps:
> 1. register uprobe on file at zero offset
> 2. mmap the file at zero offset:
>    addr1 = mmap(NULL, 2 * 4096, PROT_NONE, MAP_PRIVATE, fd, 0);
> 3. mremap part of vma1 to new vma2:
>    addr2 = mremap(addr1, 4096, 2 * 4096, MREMAP_MAYMOVE);
> 4. mremap back to orig addr1:
>    mremap(addr2, 4096, 4096, MREMAP_MAYMOVE | MREMAP_FIXED, addr1);
>
> In the step 3, the vma1 range [addr1, addr1 + 4096] will be remap to new
> vma2 with range [addr2, addr2 + 8192], and remap uprobe anon page from
> the vma1 to vma2, then unmap the vma1 range [addr1, addr1 + 4096]. In
> tht step 4, the vma2 range [addr2, addr2 + 4096] will be remap back to
> the addr range [addr1, addr1 + 4096]. Since the addr range
> [addr1 + 4096, addr1 + 8192] still maps the file, it will take
> vma_merge_new_range to expand the range, and then do uprobe_mmap in
> vma_complete. Since the merged vma pgoff is also zero offset, it will
> install uprobe anon page to the merged vma. However, the upcomming
> move_page_tables step, which use set_pte_at to remap the vma2 uprobe pte
> to the merged vma, will overwrite the newly uprobe pte in the merged
> vma, and lead that pte to be orphan.
>
> Since the uprobe pte will be remapped to the merged vma, we can
> remove the unnecessary uprobe_mmap upon merged vma.
>
> This problem was first find in linux-6.6.y and also exists in the
> community syzkaller:
> https://lore.kernel.org/all/000000000000ada39605a5e71711@google.com/T/
>
> CC: stable@vger.kernel.org
> Fixes: 2b1444983508 ("uprobes, mm, x86: Add the ability to install and remove uprobes breakpoints")
> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Signed-off-by: Pu Lehui <pulehui@huawei.com>

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  mm/vma.c | 20 +++++++++++++++++---
>  mm/vma.h |  7 +++++++
>  2 files changed, 24 insertions(+), 3 deletions(-)
>
> diff --git a/mm/vma.c b/mm/vma.c
> index 1c6595f282e5..b2d7c03d8aa4 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -169,6 +169,9 @@ static void init_multi_vma_prep(struct vma_prepare *vp,
>  	vp->file = vma->vm_file;
>  	if (vp->file)
>  		vp->mapping = vma->vm_file->f_mapping;
> +
> +	if (vmg && vmg->skip_vma_uprobe)
> +		vp->skip_vma_uprobe = true;
>  }
>
>  /*
> @@ -358,10 +361,13 @@ static void vma_complete(struct vma_prepare *vp, struct vma_iterator *vmi,
>
>  	if (vp->file) {
>  		i_mmap_unlock_write(vp->mapping);
> -		uprobe_mmap(vp->vma);
>
> -		if (vp->adj_next)
> -			uprobe_mmap(vp->adj_next);
> +		if (!vp->skip_vma_uprobe) {
> +			uprobe_mmap(vp->vma);
> +
> +			if (vp->adj_next)
> +				uprobe_mmap(vp->adj_next);
> +		}
>  	}
>
>  	if (vp->remove) {
> @@ -1823,6 +1829,14 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
>  		faulted_in_anon_vma = false;
>  	}
>
> +	/*
> +	 * If the VMA we are copying might contain a uprobe PTE, ensure
> +	 * that we do not establish one upon merge. Otherwise, when mremap()
> +	 * moves page tables, it will orphan the newly created PTE.
> +	 */
> +	if (vma->vm_file)
> +		vmg.skip_vma_uprobe = true;
> +
>  	new_vma = find_vma_prev(mm, addr, &vmg.prev);
>  	if (new_vma && new_vma->vm_start < addr + len)
>  		return NULL;	/* should never get here */
> diff --git a/mm/vma.h b/mm/vma.h
> index 9a8af9be29a8..0db066e7a45d 100644
> --- a/mm/vma.h
> +++ b/mm/vma.h
> @@ -19,6 +19,8 @@ struct vma_prepare {
>  	struct vm_area_struct *insert;
>  	struct vm_area_struct *remove;
>  	struct vm_area_struct *remove2;
> +
> +	bool skip_vma_uprobe :1;
>  };
>
>  struct unlink_vma_file_batch {
> @@ -120,6 +122,11 @@ struct vma_merge_struct {
>  	 */
>  	bool give_up_on_oom :1;
>
> +	/*
> +	 * If set, skip uprobe_mmap upon merged vma.
> +	 */
> +	bool skip_vma_uprobe :1;
> +
>  	/* Internal flags set during merge process: */
>
>  	/*
> --
> 2.34.1
>