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Subject[PATCH] x86/apic/x2apic: Fix a NULL pointer deref when handling a dying cpu
Date
Check that the per-cpu cluster mask pointer has been set prior to
clearing a dying cpu's bit. The per-cpu pointer is not set until the
target cpu reaches smp_callin() during CPUHP_BRINGUP_CPU, whereas the
teardown function, x2apic_dead_cpu(), is associated with the earlier
CPUHP_X2APIC_PREPARE. If an error occurs before the cpu is awakened,
e.g. if do_boot_cpu() itself fails, x2apic_dead_cpu() will dereference
the NULL pointer and cause a panic.

smpboot: do_boot_cpu failed(-22) to wakeup CPU#1
BUG: kernel NULL pointer dereference, address: 0000000000000008
#PF: supervisor write access in kernel mode
#PF: error_code(0x0002) - not-present page
PGD 0 P4D 0
Oops: 0002 [#1] SMP PTI
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.2.0+ #120
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
RIP: 0010:x2apic_dead_cpu+0x1a/0x30
Code: <f0> 48 0f b3 78 08 31 c0 c3 0f 1f 00 66 2e 0f 1f 84 00 00 00 00 00
RSP: 0000:ffffc9000001be58 EFLAGS: 00010286
RAX: 0000000000000000 RBX: 0000000000000001 RCX: 0000000000000000
RDX: ffff88817bb00000 RSI: 0000000000000027 RDI: 0000000000000001
RBP: 0000000000000027 R08: 0000000000000000 R09: 0000000000000001
R10: 0000000000000266 R11: 0000000000000002 R12: 0000000000000000
R13: ffffffff81038c00 R14: 00000000fffffffb R15: 0000000000000000
FS: 0000000000000000(0000) GS:ffff88817ba00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000008 CR3: 000000000500a001 CR4: 0000000000160ef0
Call Trace:
cpuhp_invoke_callback+0x9a/0x580
_cpu_up+0x10d/0x140
do_cpu_up+0x69/0xb0
smp_init+0x63/0xa9
kernel_init_freeable+0xd7/0x229
? rest_init+0xa0/0xa0
kernel_init+0xa/0x100
ret_from_fork+0x35/0x40

Fixes: 023a611748fd5 ("x86/apic/x2apic: Simplify cluster management")
Cc: stable@vger.kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---

The do_boot_cpu() -EINVAL failure is due to unrelated code that isn't in
mainline, i.e. there is no underlying bug that needs to be investigated.

arch/x86/kernel/apic/x2apic_cluster.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/apic/x2apic_cluster.c b/arch/x86/kernel/apic/x2apic_cluster.c
index 45e92cba92f5..b0889c48a2ac 100644
--- a/arch/x86/kernel/apic/x2apic_cluster.c
+++ b/arch/x86/kernel/apic/x2apic_cluster.c
@@ -156,7 +156,8 @@ static int x2apic_dead_cpu(unsigned int dead_cpu)
{
struct cluster_mask *cmsk = per_cpu(cluster_masks, dead_cpu);

- cpumask_clear_cpu(dead_cpu, &cmsk->mask);
+ if (cmsk)
+ cpumask_clear_cpu(dead_cpu, &cmsk->mask);
free_cpumask_var(per_cpu(ipi_mask, dead_cpu));
return 0;
}
--
2.22.0
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