Messages in this thread | | | Date | Wed, 17 Nov 2021 11:52:34 +0000 | From | Mark Rutland <> | Subject | Re: [PATCH] Reset task stack state in bringup_cpu() |
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On Tue, Nov 16, 2021 at 11:31:40AM -0500, Qian Cai wrote: > On Mon, Nov 15, 2021 at 11:33:10AM +0000, Mark Rutland wrote: > > To hot unplug a CPU, the idle task on that CPU calls a few layers of C > > code before finally leaving the kernel. When KASAN is in use, poisoned > > shadow is left around for each of the active stack frames, and when > > shadow call stacks are in use. When shadow call stacks are in use the > > task's SCS SP is left pointing at an arbitrary point within the task's > > shadow call stack. > > > > When an offlines CPU is hotlpugged back into the kernel, this stale > > state can adversely affect the newly onlined CPU. Stale KASAN shadow can > > alias new stackframes and result in bogus KASAN warnings. A stale SCS SP > > is effectively a memory leak, and prevents a portion of the shadow call > > stack being used. Across a number of hotplug cycles the task's entire > > shadow call stack can become unusable. > > > > We previously fixed the KASAN issue in commit: > > > > e1b77c92981a5222 ("sched/kasan: remove stale KASAN poison after hotplug") > > > > In commit: > > > > f1a0a376ca0c4ef1 ("sched/core: Initialize the idle task with preemption disabled") > > > > ... we broke both KASAN and SCS, with SCS being fixed up in commit: > > > > 63acd42c0d4942f7 ("sched/scs: Reset the shadow stack when idle_task_exit") > > > > ... but as this runs in the context of the idle task being offlines it's > > potentially fragile. > > > > Fix both of these consistently and more robustly by resetting the SCS SP > > and KASAN shadow immediately before we online a CPU. This ensures the > > idle task always has a consistent state, and removes the need to do so > > when initializing an idle task or when unplugging an idle task. > > > > I've tested this with both GCC and clang, with reelvant options enabled, > > offlining and online CPUs with: > > > > | while true; do > > | for C in /sys/devices/system/cpu/cpu*/online; do > > | echo 0 > $C; > > | echo 1 > $C; > > | done > > | done > > > > Link: https://lore.kernel.org/lkml/20211012083521.973587-1-woodylin@google.com/ > > Link: https://lore.kernel.org/linux-arm-kernel/YY9ECKyPtDbD9q8q@qian-HP-Z2-SFF-G5-Workstation/ > > Fixes: 1a0a376ca0c4ef1 ("sched/core: Initialize the idle task with preemption disabled") > > Reported-by: Qian Cai <quic_qiancai@quicinc.com> > > Signed-off-by: Mark Rutland <mark.rutland@arm.com> > > Thanks for fixing this quickly, Mark. Triggering an user-after-free in > user namespace but don't think it is related. I'll investigate that > first since it is blocking the rest of regression testing.
Cool; are you happy to provide a Tested-by tag for this patch? :)
Thanks, Mark.
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