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 powerpc’s arch_cpu_idle_dead() decrements the idle task’s
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: Don’t touch the idle
task’s preempt_count during hotplug”) and commit f1a0a376ca0c (“sched/core:
Initialize the idle task with preemption disabled”), that justification no
longer holds.
The idle task isn’t 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.
[
{
"product": "Linux",
"vendor": "Linux",
"defaultStatus": "unaffected",
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"programFiles": [
"arch/powerpc/kernel/smp.c"
],
"versions": [
{
"version": "bdf4d33e8342",
"lessThan": "53770a411559",
"status": "affected",
"versionType": "git"
},
{
"version": "2c669ef6979c",
"lessThan": "3ea0b497a7a2",
"status": "affected",
"versionType": "git"
},
{
"version": "2c669ef6979c",
"lessThan": "787252a10d94",
"status": "affected",
"versionType": "git"
}
]
},
{
"product": "Linux",
"vendor": "Linux",
"defaultStatus": "affected",
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"programFiles": [
"arch/powerpc/kernel/smp.c"
],
"versions": [
{
"version": "5.14",
"status": "affected"
},
{
"version": "0",
"lessThan": "5.14",
"status": "unaffected",
"versionType": "custom"
},
{
"version": "5.10.76",
"lessThanOrEqual": "5.10.*",
"status": "unaffected",
"versionType": "custom"
},
{
"version": "5.14.15",
"lessThanOrEqual": "5.14.*",
"status": "unaffected",
"versionType": "custom"
},
{
"version": "5.15",
"lessThanOrEqual": "*",
"status": "unaffected",
"versionType": "original_commit_for_fix"
}
]
}
]