[PATCH] rpadlpar: fix potential drc_name corruption in store functions

[PATCH] rpadlpar: fix potential drc_name corruption in store functions

[Tyrel Datwyler]

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
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

Re: [PATCH] rpadlpar: fix potential drc_name corruption in store functions

[Michal Suchánek]

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?

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

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
> 

Re: [PATCH] rpadlpar: fix potential drc_name corruption in store functions

[Tyrel Datwyler]

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
>>

Re: [PATCH] rpadlpar: fix potential drc_name corruption in store functions

[Michael Ellerman]

Tyrel Datwyler <tyreld@linux.ibm.com> writes:
> 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.

I think that's true, but it would be nice if we didn't have to rely on
that obscure detail in order for this code to be correct & understandable.

>> Isn't it much saner to just nul terminate the string after copying?
>
> At the cost of an extra line of code, sure.

Is there a reason we can't use strscpy()? That should deal with all the
corner cases around the string copy, and then all you have to do is look
for a newline and turn it into nul.

cheers

Re: [PATCH] rpadlpar: fix potential drc_name corruption in store functions

[Tyrel Datwyler]

On 3/14/21 7:52 PM, Michael Ellerman wrote:
> Tyrel Datwyler <tyreld@linux.ibm.com> writes:
>> 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.
> 
> I think that's true, but it would be nice if we didn't have to rely on
> that obscure detail in order for this code to be correct & understandable.

I think its a security guarantee, but I guess barring a comment that explicitly
outlines the correctness it probably isn't obvious.

> 
>>> Isn't it much saner to just nul terminate the string after copying?
>>
>> At the cost of an extra line of code, sure.
> 
> Is there a reason we can't use strscpy()? That should deal with all the
> corner cases around the string copy, and then all you have to do is look
> for a newline and turn it into nul.

Fine with me. I'll spin v2 with strscpy().

-Tyrel

> 
> cheers
>