linux-kernel - Re: [Xen-devel] [PATCH] xen/privcmd: fix static checker warning

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Message-ID: <8666edd3-2d4a-82f5-8354-47ae0906681f@citrix.com>
Date:   Thu, 7 Jun 2018 11:28:26 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Paul Durrant <paul.durrant@...rix.com>,
        <xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>
CC:     Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: [Xen-devel] [PATCH] xen/privcmd: fix static checker warning

On 07/06/18 11:21, Paul Durrant wrote:
> Commit 3ad0876554ca ("xen/privcmd: add IOCTL_PRIVCMD_MMAP_RESOURCE")
> introduced a static checker warning:
>
>   drivers/xen/privcmd.c:827 privcmd_ioctl_mmap_resource()
>   warn: passing casted pointer 'pfns' to 'xen_remap_domain_mfn_array()'
>         64 vs 32.
>
> caused by this cast:
>
>    827  num = xen_remap_domain_mfn_array(vma,
>    828                                   kdata.addr & PAGE_MASK,
>    829                                   pfns, kdata.num, (int *)pfns,
>                                                           ^^^^^^^^^^^
>
> The reason for the cast is that xen_remap_domain_mfn_array() requires an
> array of ints to store error codes. It is actually safe to re-use the
> pfns array for this purpose but it does look odd (as well as leading to
> the warning). It would also be easy for a future implementation change
> to make this re-use unsafe so this patch modifies privcmd to use a
> separately allocated array for error codes.
>
> Reported-by: Dan Carpenter <dan.carpenter@...cle.com>
> Signed-off-by: Paul Durrant <paul.durrant@...rix.com>

It may be safe to reuse pfns[] as the storage space for the errs array,
but code is incorrect when sizeof(pfn) != sizeof(int).  In such a case,
you skip over every other err, and second half of pfns[] is junk from
the point of view of the errs loop.

~Andrew

> ---
> Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> Cc: Juergen Gross <jgross@...e.com>
> Cc: Dan Carpenter <dan.carpenter@...cle.com>
> ---
>  drivers/xen/privcmd.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
> index 8ae0349..8507c13 100644
> --- a/drivers/xen/privcmd.c
> +++ b/drivers/xen/privcmd.c
> @@ -822,11 +822,18 @@ static long privcmd_ioctl_mmap_resource(struct file *file, void __user *udata)
>  		unsigned int domid =
>  			(xdata.flags & XENMEM_rsrc_acq_caller_owned) ?
>  			DOMID_SELF : kdata.dom;
> +		int *errs;
>  		int num;
>  
> +		errs = kcalloc(kdata.num, sizeof(*errs), GFP_KERNEL);
> +		if (!errs) {
> +			rc = -ENOMEM;
> +			goto out;
> +		}
> +
>  		num = xen_remap_domain_mfn_array(vma,
>  						 kdata.addr & PAGE_MASK,
> -						 pfns, kdata.num, (int *)pfns,
> +						 pfns, kdata.num, errs,
>  						 vma->vm_page_prot,
>  						 domid,
>  						 vma->vm_private_data);
> @@ -836,12 +843,14 @@ static long privcmd_ioctl_mmap_resource(struct file *file, void __user *udata)
>  			unsigned int i;
>  
>  			for (i = 0; i < num; i++) {
> -				rc = pfns[i];
> +				rc = errs[i];
>  				if (rc < 0)
>  					break;
>  			}
>  		} else
>  			rc = 0;
> +
> +		kfree(errs);
>  	}
>  
>  out: