NULL Pointer Dereference Affecting kernel-debuginfo-common-x86_64 package, versions <0:4.14.232-176.381.amzn2

Note: Versions mentioned in the description apply only to the upstream kernel-debuginfo-common-x86_64 package and not the kernel-debuginfo-common-x86_64 package as distributed by Amazon-Linux. See How to fix? for Amazon-Linux:2 relevant fixed versions and status.

In the Linux kernel, the following vulnerability has been resolved:

net: hso: fix null-ptr-deref during tty device unregistration

Multiple ttys try to claim the same the minor number causing a double unregistration of the same device. The first unregistration succeeds but the next one results in a null-ptr-deref.

The get_free_serial_index() function returns an available minor number but doesn't assign it immediately. The assignment is done by the caller later. But before this assignment, calls to get_free_serial_index() would return the same minor number.

Fix this by modifying get_free_serial_index to assign the minor number immediately after one is found to be and rename it to obtain_minor() to better reflect what it does. Similary, rename set_serial_by_index() to release_minor() and modify it to free up the minor number of the given hso_serial. Every obtain_minor() should have corresponding release_minor() call.

• http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-46904
• https://git.kernel.org/stable/c/145c89c441d27696961752bf51b323f347601bee
• https://git.kernel.org/stable/c/388d05f70f1ee0cac4a2068fd295072f1a44152a
• https://git.kernel.org/stable/c/4a2933c88399c0ebc738db39bbce3ae89786d723
• https://git.kernel.org/stable/c/8a12f8836145ffe37e9c8733dce18c22fb668b66
• https://git.kernel.org/stable/c/92028d7a31e55d53e41cff679156b9432cffcb36
• https://git.kernel.org/stable/c/a462067d7c8e6953a733bf5ade8db947b1bb5449
• https://git.kernel.org/stable/c/caf5ac93b3b5d5fac032fc11fbea680e115421b4
• https://git.kernel.org/stable/c/dc195928d7e4ec7b5cfc6cd10dc4c8d87a7c72ac