Patch "powerpc: Avoid nmi_enter/nmi_exit in real mode interrupt." has been added to the 6.1-stable tree

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This is a note to let you know that I've just added the patch titled

    powerpc: Avoid nmi_enter/nmi_exit in real mode interrupt.

to the 6.1-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     powerpc-avoid-nmi_enter-nmi_exit-in-real-mode-interr.patch
and it can be found in the queue-6.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit cfa724e7c8c0c510754739ca01136a06b8fee283
Author: Mahesh Salgaonkar <mahesh@xxxxxxxxxxxxx>
Date:   Wed Apr 10 10:00:06 2024 +0530

    powerpc: Avoid nmi_enter/nmi_exit in real mode interrupt.
    
    [ Upstream commit 0db880fc865ffb522141ced4bfa66c12ab1fbb70 ]
    
    nmi_enter()/nmi_exit() touches per cpu variables which can lead to kernel
    crash when invoked during real mode interrupt handling (e.g. early HMI/MCE
    interrupt handler) if percpu allocation comes from vmalloc area.
    
    Early HMI/MCE handlers are called through DEFINE_INTERRUPT_HANDLER_NMI()
    wrapper which invokes nmi_enter/nmi_exit calls. We don't see any issue when
    percpu allocation is from the embedded first chunk. However with
    CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK enabled there are chances where percpu
    allocation can come from the vmalloc area.
    
    With kernel command line "percpu_alloc=page" we can force percpu allocation
    to come from vmalloc area and can see kernel crash in machine_check_early:
    
    [    1.215714] NIP [c000000000e49eb4] rcu_nmi_enter+0x24/0x110
    [    1.215717] LR [c0000000000461a0] machine_check_early+0xf0/0x2c0
    [    1.215719] --- interrupt: 200
    [    1.215720] [c000000fffd73180] [0000000000000000] 0x0 (unreliable)
    [    1.215722] [c000000fffd731b0] [0000000000000000] 0x0
    [    1.215724] [c000000fffd73210] [c000000000008364] machine_check_early_common+0x134/0x1f8
    
    Fix this by avoiding use of nmi_enter()/nmi_exit() in real mode if percpu
    first chunk is not embedded.
    
    Reviewed-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
    Tested-by: Shirisha Ganta <shirisha@xxxxxxxxxxxxx>
    Signed-off-by: Mahesh Salgaonkar <mahesh@xxxxxxxxxxxxx>
    Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
    Link: https://msgid.link/20240410043006.81577-1-mahesh@xxxxxxxxxxxxx
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/arch/powerpc/include/asm/interrupt.h b/arch/powerpc/include/asm/interrupt.h
index 6d8492b6e2b83..4999de47b4a38 100644
--- a/arch/powerpc/include/asm/interrupt.h
+++ b/arch/powerpc/include/asm/interrupt.h
@@ -355,6 +355,14 @@ static inline void interrupt_nmi_enter_prepare(struct pt_regs *regs, struct inte
 	if (IS_ENABLED(CONFIG_KASAN))
 		return;
 
+	/*
+	 * Likewise, do not use it in real mode if percpu first chunk is not
+	 * embedded. With CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK enabled there
+	 * are chances where percpu allocation can come from vmalloc area.
+	 */
+	if (percpu_first_chunk_is_paged)
+		return;
+
 	/* Otherwise, it should be safe to call it */
 	nmi_enter();
 }
@@ -370,6 +378,8 @@ static inline void interrupt_nmi_exit_prepare(struct pt_regs *regs, struct inter
 		// no nmi_exit for a pseries hash guest taking a real mode exception
 	} else if (IS_ENABLED(CONFIG_KASAN)) {
 		// no nmi_exit for KASAN in real mode
+	} else if (percpu_first_chunk_is_paged) {
+		// no nmi_exit if percpu first chunk is not embedded
 	} else {
 		nmi_exit();
 	}
diff --git a/arch/powerpc/include/asm/percpu.h b/arch/powerpc/include/asm/percpu.h
index 8e5b7d0b851c6..634970ce13c6b 100644
--- a/arch/powerpc/include/asm/percpu.h
+++ b/arch/powerpc/include/asm/percpu.h
@@ -15,6 +15,16 @@
 #endif /* CONFIG_SMP */
 #endif /* __powerpc64__ */
 
+#if defined(CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK) && defined(CONFIG_SMP)
+#include <linux/jump_label.h>
+DECLARE_STATIC_KEY_FALSE(__percpu_first_chunk_is_paged);
+
+#define percpu_first_chunk_is_paged	\
+		(static_key_enabled(&__percpu_first_chunk_is_paged.key))
+#else
+#define percpu_first_chunk_is_paged	false
+#endif /* CONFIG_PPC64 && CONFIG_SMP */
+
 #include <asm-generic/percpu.h>
 
 #include <asm/paca.h>
diff --git a/arch/powerpc/kernel/setup_64.c b/arch/powerpc/kernel/setup_64.c
index b2e0d3ce4261c..7662265f24337 100644
--- a/arch/powerpc/kernel/setup_64.c
+++ b/arch/powerpc/kernel/setup_64.c
@@ -834,6 +834,7 @@ static __init int pcpu_cpu_to_node(int cpu)
 
 unsigned long __per_cpu_offset[NR_CPUS] __read_mostly;
 EXPORT_SYMBOL(__per_cpu_offset);
+DEFINE_STATIC_KEY_FALSE(__percpu_first_chunk_is_paged);
 
 void __init setup_per_cpu_areas(void)
 {
@@ -876,6 +877,7 @@ void __init setup_per_cpu_areas(void)
 	if (rc < 0)
 		panic("cannot initialize percpu area (err=%d)", rc);
 
+	static_key_enable(&__percpu_first_chunk_is_paged.key);
 	delta = (unsigned long)pcpu_base_addr - (unsigned long)__per_cpu_start;
 	for_each_possible_cpu(cpu) {
                 __per_cpu_offset[cpu] = delta + pcpu_unit_offsets[cpu];




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