On 3/14/21 7:52 PM, Michael Ellerman wrote: > 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. 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 >
