Re: [PATCH v2] xen/privcmd: fix error exit of privcmd_ioctl_dm_op()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.08.2022 11:26, Juergen Gross wrote:
> 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 patches pinned by pin_user_pages_fast()

Nit: s/patches/pages/

> matching the expected number of pages.
> 
> Cc: <stable@xxxxxxxxxxxxxxx>
> Fixes: ab520be8cd5d ("xen/privcmd: Add IOCTL_PRIVCMD_DM_OP")
> Reported-by: Rustam Subkhankulov <subkhankulov@xxxxxxxxx>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

I have a question / suggestion, though:

> --- a/drivers/xen/privcmd.c
> +++ b/drivers/xen/privcmd.c
> @@ -602,6 +602,10 @@ static int lock_pages(
>               *pinned += page_count;
>               nr_pages -= page_count;
>               pages += page_count;
> +
> +             /* Exact reason isn't known, EFAULT is one possibility. */
> +             if (page_count < requested)
> +                     return -EFAULT;
>       }

I don't really know the inner workings of pin_user_pages_fast()
nor what future plans there are with it. To be as independent of
its behavior as possible, how about bailing here only when
page_count actually is zero (i.e. no forward progress)?

Jan