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

["Michal Suchánek" <msuchanek@suse.de>]

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
>