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Subject: Re: [PATCHv2 6/9] staging: vme_user: return -EFAULT on __copy_*_user errors
From: Dmitry Kalinkin <dmitry.kalinkin@gmail.com>
In-Reply-To: <20150625112758.GB13366@sudip-PC>
Date: Thu, 25 Jun 2015 15:05:27 +0300
Cc: linux-kernel@vger.kernel.org, devel@driverdev.osuosl.org,
        Martyn Welch <martyn.welch@ge.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Manohar Vanga <manohar.vanga@gmail.com>
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To: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
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> On 25 Jun 2015, at 14:27, Sudip Mukherjee <sudipm.mukherjee@gmail.com> wrote:
> 
> On Tue, Jun 23, 2015 at 07:03:36PM +0300, Dmitry Kalinkin wrote:
>> Signed-off-by: Dmitry Kalinkin <dmitry.kalinkin@gmail.com>
>> ---
>> drivers/staging/vme/devices/vme_user.c | 47 ++++++++--------------------------
>> 1 file changed, 11 insertions(+), 36 deletions(-)
>> 
> <snip>
>> @@ -178,38 +167,24 @@ static ssize_t buffer_to_user(unsigned int minor, char __user *buf,
>> 			      size_t count, loff_t *ppos)
>> {
>> 	void *image_ptr;
>> -	ssize_t retval;
>> 
>> 	image_ptr = image[minor].kern_buf + *ppos;
>> +	if (__copy_to_user(buf, image_ptr, (unsigned long)count))
>> +		return -EFAULT;
>> 
>> -	retval = __copy_to_user(buf, image_ptr, (unsigned long)count);
>> -	if (retval != 0) {
>> -		retval = (count - retval);
>> -		pr_warn("Partial copy to userspace\n");
>> -	} else
>> -		retval = count;
>> -
>> -	/* Return number of bytes successfully read */
>> -	return retval;
>> +	return count;
> will it not affect the userspace code?
> previously number of bytes successfully read was returned, now incase of
> partial read -EFAULT is being returned.
Exactly.

Practically there is an access_ok() call in vfs_read() and vfs_write() that
will catch this first.  I don’t know exactly what is the condition for
__copy_to_user to fail, but it is probably some rare arch-specific thing (and
we only care for x86/powerpc here). But when it happens it better be returning
proper error codes. This is why I think this is not a “we broke userspace”
situation.

Cheers,
Dmitry--
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