CVE-2021-47454

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

powerpc/smp: do not decrement idle task preempt count in CPU offline

With PREEMPT_COUNT=y, when a CPU is offlined and then onlined again, we get:

BUG: scheduling while atomic: swapper/1/0/0x00000000 no locks held by swapper/1/0. CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.15.0-rc2+ #100 Call Trace: dump_stack_lvl+0xac/0x108 __schedule_bug+0xac/0xe0 __schedule+0xcf8/0x10d0 schedule_idle+0x3c/0x70 do_idle+0x2d8/0x4a0 cpu_startup_entry+0x38/0x40 start_secondary+0x2ec/0x3a0 start_secondary_prolog+0x10/0x14

This is because powerpcs arch_cpu_idle_dead() decrements the idle tasks preempt count, for reasons explained in commit a7c2bb8279d2 (powerpc: Re-enable preemption before cpu_die()), specifically start_secondary() expects a preempt_count() of 0.

However, since commit 2c669ef6979c (powerpc/preempt: Dont touch the idle tasks preempt_count during hotplug) and commit f1a0a376ca0c (sched/core: Initialize the idle task with preemption disabled), that justification no longer holds.

The idle task isnt supposed to re-enable preemption, so remove the vestigial preempt_enable() from the CPU offline path.

Tested with pseries and powernv in qemu, and pseries on PowerVM.

Affected Software

References

• https://git.kernel.org/stable/c/3ea0b497a7a2fff6a4b7090310c9f52c91975934
• https://git.kernel.org/stable/c/53770a411559cf7bc0906d1df319cc533d2f4f58
• https://git.kernel.org/stable/c/787252a10d9422f3058df9a4821f389e5326c440