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Subject: Re: [PATCH] rpadlpar: fix potential drc_name corruption in store
 functions
To:     =?UTF-8?Q?Michal_Such=c3=a1nek?= <msuchanek@suse.de>
Cc:     bhelgaas@google.com, linux-pci@vger.kernel.org, mmc@linux.ibm.com,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
References: <20210310223021.423155-1-tyreld@linux.ibm.com>
 <20210313091751.GM6564@kitsune.suse.cz>
From:   Tyrel Datwyler <tyreld@linux.ibm.com>
Message-ID: <a67af978-1c47-c66b-47f0-3d754da738f9@linux.ibm.com>
Date:   Sun, 14 Mar 2021 15:32:07 -0700
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On 3/13/21 1:17 AM, Michal Suchánek wrote:
> On Wed, Mar 10, 2021 at 04:30:21PM -0600, Tyrel Datwyler wrote:
>> Both add_slot_store() and remove_slot_store() try to fix up the drc_name
>> copied from the store buffer by placing a NULL terminator at nbyte + 1
>> or in place of a '\n' if present. However, the static buffer that we
>> copy the drc_name data into is not zeored and can contain anything past
>> the n-th byte. This is problematic if a '\n' byte appears in that buffer
>> after nbytes and the string copied into the store buffer was not NULL
>> terminated to start with as the strchr() search for a '\n' byte will mark
>> this incorrectly as the end of the drc_name string resulting in a drc_name
>> string that contains garbage data after the n-th byte. The following
>> debugging shows an example of the drmgr utility writing "PHB 4543" to
>> the add_slot sysfs attribute, but add_slot_store logging a corrupted
>> string value.
>>
>> [135823.702864] drmgr: drmgr: -c phb -a -s PHB 4543 -d 1
>> [135823.702879] add_slot_store: drc_name = PHB 4543°|<82>!, rc = -19
>>
>> Fix this by NULL terminating the string when we copy it into our static
>> buffer by coping nbytes + 1 of data from the store buffer. The code has
> Why is it OK to copy nbytes + 1 and why is it expected that the buffer
> contains a nul after the content?

It is my understanding that the store function buffer is allocated as a
zeroed-page which the kernel copies up to at most (PAGE_SIZE - 1) of user data
into. Anything after nbytes would therefore be zeroed.

> 
> Isn't it much saner to just nul terminate the string after copying?

At the cost of an extra line of code, sure.

-Tyrel

> 
> diff --git a/drivers/pci/hotplug/rpadlpar_sysfs.c b/drivers/pci/hotplug/rpadlpar_sysfs.c
> index cdbfa5df3a51..cfbad67447da 100644
> --- a/drivers/pci/hotplug/rpadlpar_sysfs.c
> +++ b/drivers/pci/hotplug/rpadlpar_sysfs.c
> @@ -35,11 +35,11 @@ static ssize_t add_slot_store(struct kobject *kobj, struct kobj_attribute *attr,
>  		return 0;
>  
>  	memcpy(drc_name, buf, nbytes);
> +	&drc_name[nbytes] = '\0';
>  
>  	end = strchr(drc_name, '\n');
> -	if (!end)
> -		end = &drc_name[nbytes];
> -	*end = '\0';
> +	if (end)
> +		*end = '\0';
>  
>  	rc = dlpar_add_slot(drc_name);
>  	if (rc)
> @@ -66,11 +66,11 @@ static ssize_t remove_slot_store(struct kobject *kobj,
>  		return 0;
>  
>  	memcpy(drc_name, buf, nbytes);
> +	&drc_name[nbytes] = '\0';
>  
>  	end = strchr(drc_name, '\n');
> -	if (!end)
> -		end = &drc_name[nbytes];
> -	*end = '\0';
> +	if (end)
> +		*end = '\0';
>  
>  	rc = dlpar_remove_slot(drc_name);
>  	if (rc)
> 
> Thanks
> 
> Michal
> 
>> already made sure that nbytes is not >= MAX_DRC_NAME_LEN and the store
>> buffer is guaranteed to be zeroed beyond the nth-byte of data copied
>> from the user. Further, since the string is now NULL terminated the code
>> only needs to change '\n' to '\0' when present.
>>
>> Signed-off-by: Tyrel Datwyler <tyreld@linux.ibm.com>
>> ---
>>  drivers/pci/hotplug/rpadlpar_sysfs.c | 14 ++++++--------
>>  1 file changed, 6 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/pci/hotplug/rpadlpar_sysfs.c b/drivers/pci/hotplug/rpadlpar_sysfs.c
>> index cdbfa5df3a51..375087921284 100644
>> --- a/drivers/pci/hotplug/rpadlpar_sysfs.c
>> +++ b/drivers/pci/hotplug/rpadlpar_sysfs.c
>> @@ -34,12 +34,11 @@ static ssize_t add_slot_store(struct kobject *kobj, struct kobj_attribute *attr,
>>  	if (nbytes >= MAX_DRC_NAME_LEN)
>>  		return 0;
>>  
>> -	memcpy(drc_name, buf, nbytes);
>> +	memcpy(drc_name, buf, nbytes + 1);
>>  
>>  	end = strchr(drc_name, '\n');
>> -	if (!end)
>> -		end = &drc_name[nbytes];
>> -	*end = '\0';
>> +	if (end)
>> +		*end = '\0';
>>  
>>  	rc = dlpar_add_slot(drc_name);
>>  	if (rc)
>> @@ -65,12 +64,11 @@ static ssize_t remove_slot_store(struct kobject *kobj,
>>  	if (nbytes >= MAX_DRC_NAME_LEN)
>>  		return 0;
>>  
>> -	memcpy(drc_name, buf, nbytes);
>> +	memcpy(drc_name, buf, nbytes + 1);
>>  
>>  	end = strchr(drc_name, '\n');
>> -	if (!end)
>> -		end = &drc_name[nbytes];
>> -	*end = '\0';
>> +	if (end)
>> +		*end = '\0';
>>  
>>  	rc = dlpar_remove_slot(drc_name);
>>  	if (rc)
>> -- 
>> 2.27.0
>>