linux-kernel - Re: [PATCH] xen/gntdev.c: Replace vm_map_pages() with vm_map_pages

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Date:   Tue, 30 Jul 2019 15:56:51 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Souptick Joarder <jrdr.linux@...il.com>, jgross@...e.com,
        sstabellini@...nel.org, marmarek@...isiblethingslab.com
Cc:     willy@...radead.org, akpm@...ux-foundation.org,
        linux@...linux.org.uk, linux-mm@...ck.org,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, gregkh@...uxfoundation.org
Subject: Re: [PATCH] xen/gntdev.c: Replace vm_map_pages() with
 vm_map_pages_zero()

On 7/30/19 2:34 PM, Souptick Joarder wrote:
> 'commit df9bde015a72 ("xen/gntdev.c: convert to use vm_map_pages()")'
> breaks gntdev driver. If vma->vm_pgoff > 0, vm_map_pages()
> will:
>  - use map->pages starting at vma->vm_pgoff instead of 0
>  - verify map->count against vma_pages()+vma->vm_pgoff instead of just
>    vma_pages().
>
> In practice, this breaks using a single gntdev FD for mapping multiple
> grants.
>
> relevant strace output:
> [pid   857] ioctl(7, IOCTL_GNTDEV_MAP_GRANT_REF, 0x7ffd3407b6d0) = 0
> [pid   857] mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, 7, 0) =
> 0x777f1211b000
> [pid   857] ioctl(7, IOCTL_GNTDEV_SET_UNMAP_NOTIFY, 0x7ffd3407b710) = 0
> [pid   857] ioctl(7, IOCTL_GNTDEV_MAP_GRANT_REF, 0x7ffd3407b6d0) = 0
> [pid   857] mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, 7,
> 0x1000) = -1 ENXIO (No such device or address)
>
> details here:
> https://github.com/QubesOS/qubes-issues/issues/5199
>
> The reason is -> ( copying Marek's word from discussion)
>
> vma->vm_pgoff is used as index passed to gntdev_find_map_index. It's
> basically using this parameter for "which grant reference to map".
> map struct returned by gntdev_find_map_index() describes just the pages
> to be mapped. Specifically map->pages[0] should be mapped at
> vma->vm_start, not vma->vm_start+vma->vm_pgoff*PAGE_SIZE.
>
> When trying to map grant with index (aka vma->vm_pgoff) > 1,
> __vm_map_pages() will refuse to map it because it will expect map->count
> to be at least vma_pages(vma)+vma->vm_pgoff, while it is exactly
> vma_pages(vma).
>
> Converting vm_map_pages() to use vm_map_pages_zero() will fix the
> problem.
>
> Marek has tested and confirmed the same.

Cc: stable@...r.kernel.org # v5.2+
Fixes: df9bde015a72 ("xen/gntdev.c: convert to use vm_map_pages()")

> Reported-by: Marek Marczykowski-Górecki <marmarek@...isiblethingslab.com>
> Signed-off-by: Souptick Joarder <jrdr.linux@...il.com>
> Tested-by: Marek Marczykowski-Górecki <marmarek@...isiblethingslab.com>


Reviewed-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>


> ---
>  drivers/xen/gntdev.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> index 4c339c7..a446a72 100644
> --- a/drivers/xen/gntdev.c
> +++ b/drivers/xen/gntdev.c
> @@ -1143,7 +1143,7 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
>  		goto out_put_map;
>  
>  	if (!use_ptemod) {
> -		err = vm_map_pages(vma, map->pages, map->count);
> +		err = vm_map_pages_zero(vma, map->pages, map->count);
>  		if (err)
>  			goto out_put_map;
>  	} else {