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

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Date:   Thu, 25 Aug 2022 10:07:46 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <jbeulich@...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] xen/privcmd: fix error exit of privcmd_ioctl_dm_op()

On 25.08.22 09:38, Jan Beulich wrote:
> On 24.08.2022 16:26, 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.
>>
>> Fix that by calling unlock_pages only if lock_pages() was at least
>> partially successful.
>>
>> 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>
> albeit I wonder whether you did consider the variant actually
> reducing code size (and avoiding the need for yet another label),
> ...
> 
>> --- a/drivers/xen/privcmd.c
>> +++ b/drivers/xen/privcmd.c
>> @@ -679,7 +679,7 @@ 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;
> 
> ... dropping this line and ...
> 
>> -		goto out;
>> +		goto unlock;
>>   	}
>>   
>>   	for (i = 0; i < kdata.num; i++) {
>> @@ -691,8 +691,9 @@ static long privcmd_ioctl_dm_op(struct file *file, void __user *udata)
>>   	rc = HYPERVISOR_dm_op(kdata.dom, kdata.num, xbufs);
>>   	xen_preemptible_hcall_end();
>>   
>> -out:
>> + unlock:
>>   	unlock_pages(pages, nr_pages);
> 
> ... passing "pinned" here.

Looking into this I found another problem: NOT using pinned is wrong, as
lock_pages() doesn't guarantee that all pages were really locked. I think
lock_pages() should return an error, in case pin_user_pages_fast() didn't
lock as many pages es expected.


Juergen

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