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x86/apm: Don't access __preempt_count with zeroed fs
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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 call outside the APM_DO_SAVE_SEGS/APM_DO_RESTORE_SEGS
invocations so that fs is actually restored before calling
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 ]---

Fixes: dd84441 ("x86/speculation: Use IBRS if available before calling into firmware")
Signed-off-by: Ville Syrjälä <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: [email protected]
Cc:  David Woodhouse <[email protected]>
Cc:  "H. Peter Anvin" <[email protected]>
Cc:  [email protected]
Cc: David Woodhouse <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
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@vsyrjala @KAGA-KOKO
vsyrjala authored and KAGA-KOKO committed Jul 16, 2018
1 parent 2c991e4 commit 6f6060a
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Showing 2 changed files with 5 additions and 6 deletions.
6 changes: 0 additions & 6 deletions arch/x86/include/asm/apm.h
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Original file line number Diff line number Diff line change
Expand Up @@ -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" \
Expand All @@ -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"
Expand All @@ -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,
Expand All @@ -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"
Expand All @@ -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;
}

Expand Down
5 changes: 5 additions & 0 deletions arch/x86/kernel/apm_32.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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);
Expand Down Expand Up @@ -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();
Expand Down Expand Up @@ -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();
Expand Down

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