linux-kernel - Re: [PATCH v2] xen/privcmd: fix error exit of privcmd_ioctl_dm

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Message-ID: <2c762b15-fd7e-f14e-fcc9-a083af683e4f@suse.com>
Date:   Thu, 25 Aug 2022 11:50:25 +0200
From:   Jan Beulich <jbeulich@...e.com>
To:     Juergen Gross <jgross@...e.com>
Cc:     Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        stable@...r.kernel.org,
        Rustam Subkhankulov <subkhankulov@...ras.ru>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] xen/privcmd: fix error exit of privcmd_ioctl_dm_op()

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@...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>

Reviewed-by: Jan Beulich <jbeulich@...e.com>

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