This function was originally implemented in commit dc2bc768a009 ("stack
overflow safe kdump: safe_smp_processor_id()") to mitigate the CPU
number corruption on a stack overflow.  At the time, x86-32 stored the
CPU number in thread_struct, which was located at the bottom of the task
stack and thus vulnerable to an overflow.  The CPU number is now located
in percpu memory, so this workaround is no longer needed.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
---
 arch/x86/include/asm/cpu.h |  1 -
 arch/x86/include/asm/smp.h |  6 ------
 arch/x86/kernel/apic/ipi.c | 30 ------------------------------
 arch/x86/kernel/crash.c    |  2 +-
 arch/x86/kernel/reboot.c   |  2 +-
 5 files changed, 2 insertions(+), 39 deletions(-)

diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
index 0c8ec62789a1..ad235dda1ded 100644
--- a/arch/x86/include/asm/cpu.h
+++ b/arch/x86/include/asm/cpu.h
@@ -12,7 +12,6 @@
 #ifndef CONFIG_SMP
 #define cpu_physical_id(cpu)			boot_cpu_physical_apicid
 #define cpu_acpi_id(cpu)			0
-#define safe_smp_processor_id()			0
 #endif /* CONFIG_SMP */
 
 #ifdef CONFIG_HOTPLUG_CPU
diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
index c8508d78ef3e..abf84e3bcb09 100644
--- a/arch/x86/include/asm/smp.h
+++ b/arch/x86/include/asm/smp.h
@@ -137,12 +137,6 @@ __visible void smp_call_function_single_interrupt(struct pt_regs *r);
 #define raw_smp_processor_id()  this_cpu_read(pcpu_hot.cpu_number)
 #define __smp_processor_id() __this_cpu_read(pcpu_hot.cpu_number)
 
-#ifdef CONFIG_X86_32
-extern int safe_smp_processor_id(void);
-#else
-# define safe_smp_processor_id()	smp_processor_id()
-#endif
-
 static inline struct cpumask *cpu_llc_shared_mask(int cpu)
 {
 	return per_cpu(cpu_llc_shared_map, cpu);
diff --git a/arch/x86/kernel/apic/ipi.c b/arch/x86/kernel/apic/ipi.c
index 942168da7195..98a57cb4aa86 100644
--- a/arch/x86/kernel/apic/ipi.c
+++ b/arch/x86/kernel/apic/ipi.c
@@ -288,34 +288,4 @@ void default_send_IPI_mask_logical(const struct cpumask *cpumask, int vector)
 	__default_send_IPI_dest_field(mask, vector, APIC_DEST_LOGICAL);
 	local_irq_restore(flags);
 }
-
-#ifdef CONFIG_SMP
-static int convert_apicid_to_cpu(u32 apic_id)
-{
-	int i;
-
-	for_each_possible_cpu(i) {
-		if (per_cpu(x86_cpu_to_apicid, i) == apic_id)
-			return i;
-	}
-	return -1;
-}
-
-int safe_smp_processor_id(void)
-{
-	u32 apicid;
-	int cpuid;
-
-	if (!boot_cpu_has(X86_FEATURE_APIC))
-		return 0;
-
-	apicid = read_apic_id();
-	if (apicid == BAD_APICID)
-		return 0;
-
-	cpuid = convert_apicid_to_cpu(apicid);
-
-	return cpuid >= 0 ? cpuid : 0;
-}
-#endif
 #endif
diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
index 340af8155658..0be61c45400c 100644
--- a/arch/x86/kernel/crash.c
+++ b/arch/x86/kernel/crash.c
@@ -140,7 +140,7 @@ void native_machine_crash_shutdown(struct pt_regs *regs)
 	x86_platform.guest.enc_kexec_begin();
 	x86_platform.guest.enc_kexec_finish();
 
-	crash_save_cpu(regs, safe_smp_processor_id());
+	crash_save_cpu(regs, smp_processor_id());
 }
 
 #if defined(CONFIG_KEXEC_FILE) || defined(CONFIG_CRASH_HOTPLUG)
diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
index 9aaac1f9f45b..964f6b0a3d68 100644
--- a/arch/x86/kernel/reboot.c
+++ b/arch/x86/kernel/reboot.c
@@ -921,7 +921,7 @@ void nmi_shootdown_cpus(nmi_shootdown_cb callback)
 		return;
 
 	/* Make a note of crashing cpu. Will be used in NMI callback. */
-	crashing_cpu = safe_smp_processor_id();
+	crashing_cpu = smp_processor_id();
 
 	shootdown_callback = callback;
 

base-commit: 693c8502970a533363e9ece482c80bb6db0c12a5
-- 
2.48.1

The following commit has been merged into the x86/core branch of tip:

Commit-ID:     604ea3e90b17f27928a64d86259c57710c254438
Gitweb:        https://git.kernel.org/tip/604ea3e90b17f27928a64d86259c57710c254438
Author:        Brian Gerst <brgerst@gmail.com>
AuthorDate:    Mon, 03 Mar 2025 12:01:15 -05:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Mon, 03 Mar 2025 20:30:09 +01:00

x86/smp/32: Remove safe_smp_processor_id()

The safe_smp_processor_id() function was originally implemented in:

  dc2bc768a009 ("stack overflow safe kdump: safe_smp_processor_id()")

to mitigate the CPU number corruption on a stack overflow.  At the time,
x86-32 stored the CPU number in thread_struct, which was located at the
bottom of the task stack and thus vulnerable to an overflow.

The CPU number is now located in percpu memory, so this workaround
is no longer needed.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Uros Bizjak <ubizjak@gmail.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/r/20250303170115.2176553-1-brgerst@gmail.com
---
 arch/x86/include/asm/cpu.h |  1 -
 arch/x86/include/asm/smp.h |  6 ------
 arch/x86/kernel/apic/ipi.c | 30 ------------------------------
 arch/x86/kernel/crash.c    |  2 +-
 arch/x86/kernel/reboot.c   |  2 +-
 5 files changed, 2 insertions(+), 39 deletions(-)

diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
index 98eced5..f44bbce 100644
--- a/arch/x86/include/asm/cpu.h
+++ b/arch/x86/include/asm/cpu.h
@@ -12,7 +12,6 @@
 #ifndef CONFIG_SMP
 #define cpu_physical_id(cpu)			boot_cpu_physical_apicid
 #define cpu_acpi_id(cpu)			0
-#define safe_smp_processor_id()			0
 #endif /* CONFIG_SMP */
 
 #ifdef CONFIG_HOTPLUG_CPU
diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
index ca073f4..2419e50 100644
--- a/arch/x86/include/asm/smp.h
+++ b/arch/x86/include/asm/smp.h
@@ -136,12 +136,6 @@ __visible void smp_call_function_single_interrupt(struct pt_regs *r);
 #define raw_smp_processor_id()  this_cpu_read(pcpu_hot.cpu_number)
 #define __smp_processor_id() __this_cpu_read(pcpu_hot.cpu_number)
 
-#ifdef CONFIG_X86_32
-extern int safe_smp_processor_id(void);
-#else
-# define safe_smp_processor_id()	smp_processor_id()
-#endif
-
 static inline struct cpumask *cpu_llc_shared_mask(int cpu)
 {
 	return per_cpu(cpu_llc_shared_map, cpu);
diff --git a/arch/x86/kernel/apic/ipi.c b/arch/x86/kernel/apic/ipi.c
index 5da693d..23025a3 100644
--- a/arch/x86/kernel/apic/ipi.c
+++ b/arch/x86/kernel/apic/ipi.c
@@ -287,34 +287,4 @@ void default_send_IPI_mask_logical(const struct cpumask *cpumask, int vector)
 	__default_send_IPI_dest_field(mask, vector, APIC_DEST_LOGICAL);
 	local_irq_restore(flags);
 }
-
-#ifdef CONFIG_SMP
-static int convert_apicid_to_cpu(u32 apic_id)
-{
-	int i;
-
-	for_each_possible_cpu(i) {
-		if (per_cpu(x86_cpu_to_apicid, i) == apic_id)
-			return i;
-	}
-	return -1;
-}
-
-int safe_smp_processor_id(void)
-{
-	u32 apicid;
-	int cpuid;
-
-	if (!boot_cpu_has(X86_FEATURE_APIC))
-		return 0;
-
-	apicid = read_apic_id();
-	if (apicid == BAD_APICID)
-		return 0;
-
-	cpuid = convert_apicid_to_cpu(apicid);
-
-	return cpuid >= 0 ? cpuid : 0;
-}
-#endif
 #endif
diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
index 340af81..0be61c4 100644
--- a/arch/x86/kernel/crash.c
+++ b/arch/x86/kernel/crash.c
@@ -140,7 +140,7 @@ void native_machine_crash_shutdown(struct pt_regs *regs)
 	x86_platform.guest.enc_kexec_begin();
 	x86_platform.guest.enc_kexec_finish();
 
-	crash_save_cpu(regs, safe_smp_processor_id());
+	crash_save_cpu(regs, smp_processor_id());
 }
 
 #if defined(CONFIG_KEXEC_FILE) || defined(CONFIG_CRASH_HOTPLUG)
diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
index 9aaac1f..964f6b0 100644
--- a/arch/x86/kernel/reboot.c
+++ b/arch/x86/kernel/reboot.c
@@ -921,7 +921,7 @@ void nmi_shootdown_cpus(nmi_shootdown_cb callback)
 		return;
 
 	/* Make a note of crashing cpu. Will be used in NMI callback. */
-	crashing_cpu = safe_smp_processor_id();
+	crashing_cpu = smp_processor_id();
 
 	shootdown_callback = callback;