padata: Fix pd UAF once and for all

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: padata: Fix pd UAF once and for all There is a race condition/UAF in padata_reorder that goes back to the initial commit. A reference count is taken at the start of the process in padata_do_parallel, and released at the end in padata_serial_worker. This reference count is (and only is) required for padata_replace to function correctly. If padata_replace is never called then there is no issue. In the function padata_reorder which serves as the core of padata, as soon as padata is added to queue->serial.list, and the associated spin lock released, that padata may be processed and the reference count on pd would go away. Fix this by getting the next padata before the squeue->serial lock is released. In order to make this possible, simplify padata_reorder by only calling it once the next padata arrives.

References

• https://git.kernel.org/stable/c/a2048e475e22b13dc3e53d485b7e6e11464ed9a6
• https://git.kernel.org/stable/c/73f132e60857038416540c3599b1de6033d7575a
• https://git.kernel.org/stable/c/609e59193fc6d9dd323f1c6ae1fdd721f1c79680
• https://git.kernel.org/stable/c/a11a12a9880ab37342b73c93cfe1a3ada02ff0db
• https://git.kernel.org/stable/c/f231d5d001ec75f5886c02d496a4c79edc383d45
• https://git.kernel.org/stable/c/dbe3e911a59bda6de96e7cae387ff882c2c177fa
• https://git.kernel.org/stable/c/cdf79bd2e1ecb3cc75631c73d8f4149be6019a52
• https://git.kernel.org/stable/c/71203f68c7749609d7fc8ae6ad054bdedeb24f91