Tyrel Datwyler <tyreld@xxxxxxxxxxxxx> 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
