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Subject[RFC 09/10] x86/enter: Create macros to restrict/unrestrict Indirect Branch Speculation
Date
From: Tim Chen <tim.c.chen@linux.intel.com>

Create macros to control Indirect Branch Speculation.

Name them so they reflect what they are actually doing.
The macros are used to restrict and unrestrict the indirect branch speculation.
They do not *disable* (or *enable*) indirect branch speculation. A trip back to
user-space after *restricting* speculation would still affect the BTB.

Quoting from a commit by Tim Chen:

"""
If IBRS is set, near returns and near indirect jumps/calls will not allow
their predicted target address to be controlled by code that executed in a
less privileged prediction mode *BEFORE* the IBRS mode was last written with
a value of 1 or on another logical processor so long as all Return Stack
Buffer (RSB) entries from the previous less privileged prediction mode are
overwritten.

Thus a near indirect jump/call/return may be affected by code in a less
privileged prediction mode that executed *AFTER* IBRS mode was last written
with a value of 1.
"""

[ tglx: Changed macro names and rewrote changelog ]
[ karahmed: changed macro names *again* and rewrote changelog ]

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Arjan Van De Ven <arjan.van.de.ven@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Ashok Raj <ashok.raj@intel.com>
Link: https://lkml.kernel.org/r/3aab341725ee6a9aafd3141387453b45d788d61a.1515542293.git.tim.c.chen@linux.intel.com
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
arch/x86/entry/calling.h | 73 ++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 73 insertions(+)

diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index 3f48f69..5aafb51 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -6,6 +6,8 @@
#include <asm/percpu.h>
#include <asm/asm-offsets.h>
#include <asm/processor-flags.h>
+#include <asm/msr-index.h>
+#include <asm/cpufeatures.h>

/*

@@ -349,3 +351,74 @@ For 32-bit we have the following conventions - kernel is built with
.Lafter_call_\@:
#endif
.endm
+
+/*
+ * IBRS related macros
+ */
+.macro PUSH_MSR_REGS
+ pushq %rax
+ pushq %rcx
+ pushq %rdx
+.endm
+
+.macro POP_MSR_REGS
+ popq %rdx
+ popq %rcx
+ popq %rax
+.endm
+
+.macro WRMSR_ASM msr_nr:req edx_val:req eax_val:req
+ movl \msr_nr, %ecx
+ movl \edx_val, %edx
+ movl \eax_val, %eax
+ wrmsr
+.endm
+
+.macro RESTRICT_IB_SPEC
+ ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_IBRS
+ PUSH_MSR_REGS
+ WRMSR_ASM $MSR_IA32_SPEC_CTRL, $0, $SPEC_CTRL_IBRS
+ POP_MSR_REGS
+.Lskip_\@:
+.endm
+
+.macro UNRESTRICT_IB_SPEC
+ ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_IBRS
+ PUSH_MSR_REGS
+ WRMSR_ASM $MSR_IA32_SPEC_CTRL, $0, $0
+ POP_MSR_REGS
+.Lskip_\@:
+.endm
+
+.macro RESTRICT_IB_SPEC_CLOBBER
+ ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_IBRS
+ WRMSR_ASM $MSR_IA32_SPEC_CTRL, $0, $SPEC_CTRL_IBRS
+.Lskip_\@:
+.endm
+
+.macro UNRESTRICT_IB_SPEC_CLOBBER
+ ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_IBRS
+ WRMSR_ASM $MSR_IA32_SPEC_CTRL, $0, $0
+.Lskip_\@:
+.endm
+
+.macro RESTRICT_IB_SPEC_SAVE_AND_CLOBBER save_reg:req
+ ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_IBRS
+ movl $MSR_IA32_SPEC_CTRL, %ecx
+ rdmsr
+ movl %eax, \save_reg
+ movl $0, %edx
+ movl $SPEC_CTRL_IBRS, %eax
+ wrmsr
+.Lskip_\@:
+.endm
+
+.macro RESTORE_IB_SPEC_CLOBBER save_reg:req
+ ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_IBRS
+ /* Set IBRS to the value saved in the save_reg */
+ movl $MSR_IA32_SPEC_CTRL, %ecx
+ movl $0, %edx
+ movl \save_reg, %eax
+ wrmsr
+.Lskip_\@:
+.endm
--
2.7.4
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