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Subject[PATCH] x86/apm: Don't access __preempt_count with zeroed fs
Date
From: Ville Syrjälä <ville.syrjala@linux.intel.com>

APM_DO_POP_SEGS does not restore fs/gs which were zeroed by
APM_DO_ZERO_SEGS. Trying to access __preempt_count with
zeroed fs doesn't really work. Move the ibrs stuff outside
the APM_DO_SAVE_SEGS/APM_DO_RESTORE_SEGS invocations so that
fs is actually restored before we call preempt_enable().

Fixes the following sort of oopses:
[ 0.313581] general protection fault: 0000 [#1] PREEMPT SMP
[ 0.313803] Modules linked in:
[ 0.314040] CPU: 0 PID: 268 Comm: kapmd Not tainted 4.16.0-rc1-triton-bisect-00090-gdd84441a7971 #19
[ 0.316161] EIP: __apm_bios_call_simple+0xc8/0x170
[ 0.316161] EFLAGS: 00210016 CPU: 0
[ 0.316161] EAX: 00000102 EBX: 00000000 ECX: 00000102 EDX: 00000000
[ 0.316161] ESI: 0000530e EDI: dea95f64 EBP: dea95f18 ESP: dea95ef0
[ 0.316161] DS: 007b ES: 007b FS: 0000 GS: 0000 SS: 0068
[ 0.316161] CR0: 80050033 CR2: 00000000 CR3: 015d3000 CR4: 000006d0
[ 0.316161] Call Trace:
[ 0.316161] ? cpumask_weight.constprop.15+0x20/0x20
[ 0.316161] on_cpu0+0x44/0x70
[ 0.316161] apm+0x54e/0x720
[ 0.316161] ? __switch_to_asm+0x26/0x40
[ 0.316161] ? __schedule+0x17d/0x590
[ 0.316161] kthread+0xc0/0xf0
[ 0.316161] ? proc_apm_show+0x150/0x150
[ 0.316161] ? kthread_create_worker_on_cpu+0x20/0x20
[ 0.316161] ret_from_fork+0x2e/0x38
[ 0.316161] Code: da 8e c2 8e e2 8e ea 57 55 2e ff 1d e0 bb 5d b1 0f 92 c3 5d 5f 07 1f 89 47 0c 90 8d b4 26 00 00 00 00 90 8d b4 26 00 00 00 00 90 <64> ff 0d 84 16 5c b1 74 7f 8b 45 dc 8e e0 8b 45 d8 8e e8 8b 45
[ 0.316161] EIP: __apm_bios_call_simple+0xc8/0x170 SS:ESP: 0068:dea95ef0
[ 0.316161] ---[ end trace 656253db2deaa12c ]---

Cc: stable@vger.kernel.org
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Fixes: dd84441a7971 ("x86/speculation: Use IBRS if available before calling into firmware")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
---
arch/x86/include/asm/apm.h | 6 ------
arch/x86/kernel/apm_32.c | 5 +++++
2 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/apm.h b/arch/x86/include/asm/apm.h
index c356098b6fb9..4d4015ddcf26 100644
--- a/arch/x86/include/asm/apm.h
+++ b/arch/x86/include/asm/apm.h
@@ -7,8 +7,6 @@
#ifndef _ASM_X86_MACH_DEFAULT_APM_H
#define _ASM_X86_MACH_DEFAULT_APM_H

-#include <asm/nospec-branch.h>
-
#ifdef APM_ZERO_SEGS
# define APM_DO_ZERO_SEGS \
"pushl %%ds\n\t" \
@@ -34,7 +32,6 @@ static inline void apm_bios_call_asm(u32 func, u32 ebx_in, u32 ecx_in,
* N.B. We do NOT need a cld after the BIOS call
* because we always save and restore the flags.
*/
- firmware_restrict_branch_speculation_start();
__asm__ __volatile__(APM_DO_ZERO_SEGS
"pushl %%edi\n\t"
"pushl %%ebp\n\t"
@@ -47,7 +44,6 @@ static inline void apm_bios_call_asm(u32 func, u32 ebx_in, u32 ecx_in,
"=S" (*esi)
: "a" (func), "b" (ebx_in), "c" (ecx_in)
: "memory", "cc");
- firmware_restrict_branch_speculation_end();
}

static inline bool apm_bios_call_simple_asm(u32 func, u32 ebx_in,
@@ -60,7 +56,6 @@ static inline bool apm_bios_call_simple_asm(u32 func, u32 ebx_in,
* N.B. We do NOT need a cld after the BIOS call
* because we always save and restore the flags.
*/
- firmware_restrict_branch_speculation_start();
__asm__ __volatile__(APM_DO_ZERO_SEGS
"pushl %%edi\n\t"
"pushl %%ebp\n\t"
@@ -73,7 +68,6 @@ static inline bool apm_bios_call_simple_asm(u32 func, u32 ebx_in,
"=S" (si)
: "a" (func), "b" (ebx_in), "c" (ecx_in)
: "memory", "cc");
- firmware_restrict_branch_speculation_end();
return error;
}

diff --git a/arch/x86/kernel/apm_32.c b/arch/x86/kernel/apm_32.c
index 5d0de79fdab0..ec00d1ff5098 100644
--- a/arch/x86/kernel/apm_32.c
+++ b/arch/x86/kernel/apm_32.c
@@ -240,6 +240,7 @@
#include <asm/olpc.h>
#include <asm/paravirt.h>
#include <asm/reboot.h>
+#include <asm/nospec-branch.h>

#if defined(CONFIG_APM_DISPLAY_BLANK) && defined(CONFIG_VT)
extern int (*console_blank_hook)(int);
@@ -614,11 +615,13 @@ static long __apm_bios_call(void *_call)
gdt[0x40 / 8] = bad_bios_desc;

apm_irq_save(flags);
+ firmware_restrict_branch_speculation_start();
APM_DO_SAVE_SEGS;
apm_bios_call_asm(call->func, call->ebx, call->ecx,
&call->eax, &call->ebx, &call->ecx, &call->edx,
&call->esi);
APM_DO_RESTORE_SEGS;
+ firmware_restrict_branch_speculation_end();
apm_irq_restore(flags);
gdt[0x40 / 8] = save_desc_40;
put_cpu();
@@ -690,10 +693,12 @@ static long __apm_bios_call_simple(void *_call)
gdt[0x40 / 8] = bad_bios_desc;

apm_irq_save(flags);
+ firmware_restrict_branch_speculation_start();
APM_DO_SAVE_SEGS;
error = apm_bios_call_simple_asm(call->func, call->ebx, call->ecx,
&call->eax);
APM_DO_RESTORE_SEGS;
+ firmware_restrict_branch_speculation_end();
apm_irq_restore(flags);
gdt[0x40 / 8] = save_desc_40;
put_cpu();
--
2.16.4
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