mm/page_alloc: fix race condition in unaccepted memory handling

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: mm/page_alloc: fix race condition in unaccepted memory handling The page allocator tracks the number of zones that have unaccepted memory using static_branch_enc/dec() and uses that static branch in hot paths to determine if it needs to deal with unaccepted memory. Borislav and Thomas pointed out that the tracking is racy: operations on static_branch are not serialized against adding/removing unaccepted pages to/from the zone. Sanity checks inside static_branch machinery detects it: WARNING: CPU: 0 PID: 10 at kernel/jump_label.c:276 __static_key_slow_dec_cpuslocked+0x8e/0xa0 The comment around the WARN() explains the problem: /* * Warn about the '-1' case though; since that means a * decrement is concurrent with a first (0->1) increment. IOW * people are trying to disable something that wasn't yet fully * enabled. This suggests an ordering problem on the user side. */ The effect of this static_branch optimization is only visible on microbenchmark. Instead of adding more complexity around it, remove it altogether.

References

• https://git.kernel.org/stable/c/98fdd2f612e949c652693f6df00442c81037776d
• https://git.kernel.org/stable/c/74953f93f47a45296cc2a3fd04e2a3202ff3fa53
• https://git.kernel.org/stable/c/71dda1cb10702dc2859f00eb789b0502de2176a9
• https://git.kernel.org/stable/c/fefc075182275057ce607effaa3daa9e6e3bdc73