Fix a NULL pointer dereference #111
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Background
In the Linux kernel before 5.0.6, drop_sysctl_table() in fs/proc/proc_sysctl.c can hit a NULL pointer dereference in a cleanup path related to put_links (aka CID-23da9588037e), leading to a local denial-of-service; this is tracked as CVE-2019-20054.
Changes
Harden the drop_sysctl_table()/put_links teardown path so it does not dereference NULL (ensure the relevant pointers/links are validated/initialized before use and handle the missing-link case safely).
Security Impact
Prevents a local-triggerable kernel crash (DoS) caused by a NULL pointer dereference in proc sysctl table teardown, addressing CVE-2019-20054.
References
Upstream commit: 89189557b47b35683a27c80ee78aef18248eefb4; CVE: CVE-2019-20054