A kernel panic is observed when handling machine check exceptions from
real mode.
BUG: Unable to handle kernel data access on read at 0xc00000006be21300
Oops: Kernel access of bad area, sig: 11 [#1]
MSR: 8000000000001003 <SF,ME,RI,LE> CR: 88222248 XER: 00000005
CFAR: c00000000003ffc4 DAR: c00000006be21300 DSISR: 40000000 IRQMASK: 0
NIP [c000000000029e40] arch_irq_work_raise+0x10/0x70
LR [c00000000003ffc8] machine_check_queue_event+0xa8/0x150
Call Trace:
[c0000000179d3c70] [c00000000003ff64] machine_check_queue_event+0x44/0x150
[c0000000179d3d30] [c0000000000084e0] machine_check_early_common+0x1f0/0x2c0
The crash occurs because arch_irq_work_raise() calls preempt_disable()
from machine check exception (MCE) handlers running in real mode. In
this context, accessing the preempt_count can fault, leading to the panic.
The preempt_disable()/preempt_enable() pair in arch_irq_work_raise()
was originally added by commit 0fe1ac48bef0 ("powerpc/perf_event: Fix
oops due to perf_event_do_pending call") to avoid races while raising
irq work from exception context.
Later, commit 471ba0e686cb ("irq_work: Do not raise an IPI when
queueing work on the local CPU") added preemption protection in
irq_work_queue() path, while commit 20b876918c06 ("irq_work: Use per
cpu atomics instead of regular atomics") added equivalent
protection in irq_work_queue_on() before reaching arch_irq_work_raise():
irq_work_queue() / irq_work_queue_on()
-> preempt_disable()
-> __irq_work_queue_local()
-> irq_work_raise()
-> arch_irq_work_raise()
As a result, callers other than mce_irq_work_raise() already execute
with preemption disabled, making the additional
preempt_disable()/preempt_enable() pair in arch_irq_work_raise()
redundant.
The arch_irq_work_raise() function executes in NMI context when called
from MCE handler. Hence we will not be preempted or scheduled out since
we are in NMI context with MSR[EE]=0. Therefore, it is safe to remove
the preempt_disable()/preempt_enable() calls from here.
Remove it to avoid accessing preempt_count from real mode context.
Fixes: cc15ff327569 ("powerpc/mce: Avoid using irq_work_queue() in realmode")
Suggested-by: Mahesh Salgaonkar <[email protected]>
Acked-by: Shrikanth Hegde <[email protected]>
Reviewed-by: Ritesh Harjani (IBM) <[email protected]>
Signed-off-by: Sayali Patil <[email protected]>
---
v1->v2
- Split the patch series into two parts; v2 part 2 of remaining two patches
will be sent later due to changes required in
"lkdtm/powerpc: add PPC_RADIX_TLBIEL test for radix MCE validation".
- Updated commit message as per review feedback.
- Added comment for arch_irq_work_raise() to indicate it must be called
with preemption disabled, as per review feedback.
- Added Acked-By from Shrikanth and Reviewed-by tag from Ritesh.
v1: https://lore.kernel.org/all/[email protected]/
---
arch/powerpc/kernel/time.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index 4bbeb8644d3d..b4472288e0d4 100644
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -458,6 +458,10 @@ DEFINE_PER_CPU(u8, irq_work_pending);
#endif /* 32 vs 64 bit */
+/*
+ * Must be called with preemption disabled since it updates
+ * per-CPU irq_work state and programs the local CPU decrementer.
+ */
void arch_irq_work_raise(void)
{
/*
@@ -471,10 +475,8 @@ void arch_irq_work_raise(void)
* which could get tangled up if we're messing with the same state
* here.
*/
- preempt_disable();
set_irq_work_pending_flag();
set_dec(1);
- preempt_enable();
}
static void set_dec_or_work(u64 val)
--
2.52.0
