dlm: prevent NPD when writing a positive value to event_done

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: dlm: prevent NPD when writing a positive value to event_done do_uevent returns the value written to event_done. In case it is a positive value, new_lockspace would undo all the work, and lockspace would not be set. __dlm_new_lockspace, however, would treat that positive value as a success due to commit 8511a2728ab8 ("dlm: fix use count with multiple joins"). Down the line, device_create_lockspace would pass that NULL lockspace to dlm_find_lockspace_local, leading to a NULL pointer dereference. Treating such positive values as successes prevents the problem. Given this has been broken for so long, this is unlikely to break userspace expectations.

References

• https://git.kernel.org/stable/c/a1c41aebb184d9228a440dcb6761224e65b0e49a
• https://git.kernel.org/stable/c/ee28d99d789b077565cbe0377374d1e826c64d93
• https://git.kernel.org/stable/c/c7837e2c96559663c33f43da403d9cf3cf77cfa7
• https://git.kernel.org/stable/c/7109d69bec6edce546dc870e66bd2b668a3d5549
• https://git.kernel.org/stable/c/10b7a59814765d18d43555c9cef4eb3048b7e8a3
• https://git.kernel.org/stable/c/b73c4ad4d387fe5bc988145bd9f1bc0de76afd5c
• https://git.kernel.org/stable/c/8e2bad543eca5c25cd02cbc63d72557934d45f13