linux-kernel - Re: [PATCH v4] xen/privcmd: fix error exit of privcmd_ioctl_dm

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Message-ID: <5e749d5e-08d9-81c9-0f46-a06be2d752ce@epam.com>
Date:   Fri, 26 Aug 2022 09:08:11 +0000
From:   Oleksandr Tyshchenko <Oleksandr_Tyshchenko@...m.com>
To:     Juergen Gross <jgross@...e.com>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Stefano Stabellini <sstabellini@...nel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Rustam Subkhankulov <subkhankulov@...ras.ru>
Subject: Re: [PATCH v4] xen/privcmd: fix error exit of privcmd_ioctl_dm_op()


On 25.08.22 17:19, Juergen Gross wrote:

Hello Juergen

> The error exit of privcmd_ioctl_dm_op() is calling unlock_pages()
> potentially with pages being NULL, leading to a NULL dereference.
>
> Additionally lock_pages() doesn't check for pin_user_pages_fast()
> having been completely successful, resulting in potentially not
> locking all pages into memory. This could result in sporadic failures
> when using the related memory in user mode.
>
> Fix all of that by calling unlock_pages() always with the real number
> of pinned pages, which will be zero in case pages being NULL, and by
> checking the number of pages pinned by pin_user_pages_fast() matching
> the expected number of pages.
>
> Cc: <stable@...r.kernel.org>
> Fixes: ab520be8cd5d ("xen/privcmd: Add IOCTL_PRIVCMD_DM_OP")
> Reported-by: Rustam Subkhankulov <subkhankulov@...ras.ru>
> Signed-off-by: Juergen Gross <jgross@...e.com>


I haven't spotted any issues:

Reviewed-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>


> ---
> V2:
> - use "pinned" as parameter for unlock_pages() (Jan Beulich)
> - drop label "unlock" again (Jan Beulich)
> - add check for complete success of pin_user_pages_fast()
> V3:
> - continue after partial success of pin_user_pages_fast() (Jan Beulich)
> V4:
> - fix case of multiple partial successes for one buffer (Jan Beulich)
> ---
>   drivers/xen/privcmd.c | 21 +++++++++++----------
>   1 file changed, 11 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
> index 3369734108af..e88e8f6f0a33 100644
> --- a/drivers/xen/privcmd.c
> +++ b/drivers/xen/privcmd.c
> @@ -581,27 +581,30 @@ static int lock_pages(
>   	struct privcmd_dm_op_buf kbufs[], unsigned int num,
>   	struct page *pages[], unsigned int nr_pages, unsigned int *pinned)
>   {
> -	unsigned int i;
> +	unsigned int i, off = 0;
>   
> -	for (i = 0; i < num; i++) {
> +	for (i = 0; i < num; ) {
>   		unsigned int requested;
>   		int page_count;
>   
>   		requested = DIV_ROUND_UP(
>   			offset_in_page(kbufs[i].uptr) + kbufs[i].size,
> -			PAGE_SIZE);
> +			PAGE_SIZE) - off;
>   		if (requested > nr_pages)
>   			return -ENOSPC;
>   
>   		page_count = pin_user_pages_fast(
> -			(unsigned long) kbufs[i].uptr,
> +			(unsigned long)kbufs[i].uptr + off * PAGE_SIZE,
>   			requested, FOLL_WRITE, pages);
> -		if (page_count < 0)
> -			return page_count;
> +		if (page_count <= 0)
> +			return page_count ? : -EFAULT;


[not related to the current patch]

I just wonder, whether drivers/xen/gntdev.c:gntdev_get_page() really 
wants to gain the same check?

index 59ffea800079..45e16031204d 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -740,8 +740,8 @@ static int gntdev_get_page(struct gntdev_copy_batch 
*batch, void __user *virt,
         int ret;

         ret = pin_user_pages_fast(addr, 1, batch->writeable ? 
FOLL_WRITE : 0, &page);
-       if (ret < 0)
-               return ret;
+       if (ret <= 0)
+               return ret ? : -EFAULT;

         batch->pages[batch->nr_pages++] = page;



>   
>   		*pinned += page_count;
>   		nr_pages -= page_count;
>   		pages += page_count;
> +
> +		off = (requested == page_count) ? 0 : off + page_count;
> +		i += !off;
>   	}
>   
>   	return 0;
> @@ -677,10 +680,8 @@ static long privcmd_ioctl_dm_op(struct file *file, void __user *udata)
>   	}
>   
>   	rc = lock_pages(kbufs, kdata.num, pages, nr_pages, &pinned);
> -	if (rc < 0) {
> -		nr_pages = pinned;
> +	if (rc < 0)
>   		goto out;
> -	}
>   
>   	for (i = 0; i < kdata.num; i++) {
>   		set_xen_guest_handle(xbufs[i].h, kbufs[i].uptr);
> @@ -692,7 +693,7 @@ static long privcmd_ioctl_dm_op(struct file *file, void __user *udata)
>   	xen_preemptible_hcall_end();
>   
>   out:
> -	unlock_pages(pages, nr_pages);
> +	unlock_pages(pages, pinned);
>   	kfree(xbufs);
>   	kfree(pages);
>   	kfree(kbufs);

-- 
Regards,

Oleksandr Tyshchenko