[RESEND PATCH v6 08/12] x86/fsgsbase/64: Use the per-CPU base as GSBASE at the paranoid_entry

[Chang S. Bae]

The FSGSBASE instructions allow fast accesses on GSBASE.  Now, at the
paranoid_entry, the per-CPU base value can be always copied to GSBASE.
And the original GSBASE value will be restored at the exit.

So far, GSBASE modification has not been directly allowed from userspace.
So, swapping GSBASE has been conditionally executed according to the
kernel-enforced convention that a negative GSBASE indicates a kernel value.
But when FSGSBASE is enabled, userspace can put an arbitrary value in
GSBASE. The change will secure a correct GSBASE value with FSGSBASE.

Also, factor out the RDMSR-based GSBASE read into a new macro,
READ_MSR_GSBASE.

Suggested-by: H. Peter Anvin <hpa@xxxxxxxxx>
Suggested-by: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
---
 arch/x86/entry/entry_64.S       | 71 +++++++++++++++++++++++++++------
 arch/x86/include/asm/fsgsbase.h |  9 +++++
 2 files changed, 67 insertions(+), 13 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 1f0efdb7b629..9df528565e40 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -38,6 +38,7 @@
 #include <asm/export.h>
 #include <asm/frame.h>
 #include <asm/nospec-branch.h>
+#include <asm/fsgsbase.h>
 #include <linux/err.h>
 
 #include "calling.h"
@@ -934,10 +935,14 @@ ENTRY(\sym)
 	addq	$EXCEPTION_STKSZ, CPU_TSS_IST(\shift_ist)
 	.endif
 
-	/* these procedures expect "no swapgs" flag in ebx */
 	.if \paranoid
+	/*
+	 * With FSGSBASE, original GSBASE is stored in %rbx
+	 * Without FSGSBASE, expect "no swapgs" flag in %ebx
+	 */
 	jmp	paranoid_exit
 	.else
+	/* Expect "no swapgs" flag in %ebx */
 	jmp	error_exit
 	.endif
 
@@ -1151,22 +1156,24 @@ idtentry machine_check		do_mce			has_error_code=0	paranoid=1
 #endif
 
 /*
- * Save all registers in pt_regs, and switch gs if needed.
- * Use slow, but surefire "are we in kernel?" check.
- * Return: ebx=0: need swapgs on exit, ebx=1: otherwise
+ * Save all registers in pt_regs.
+ *
+ * When FSGSBASE enabled, current GSBASE is always copied to %rbx.
+ *
+ * Without FSGSBASE, SWAPGS is needed when entering from userspace.
+ * A positive GSBASE means it is a user value and a negative GSBASE
+ * means it is a kernel value.
+ *
+ * Return:
+ * 	With FSGSBASE, %rbx has current GSBASE.
+ * 	Without that,
+ *		%ebx=0: need SWAPGS on exit, %ebx=1: otherwise
  */
 ENTRY(paranoid_entry)
 	UNWIND_HINT_FUNC
 	cld
 	PUSH_AND_CLEAR_REGS save_ret=1
 	ENCODE_FRAME_POINTER 8
-	movl	$1, %ebx
-	movl	$MSR_GS_BASE, %ecx
-	rdmsr
-	testl	%edx, %edx
-	js	1f				/* negative -> in kernel */
-	SWAPGS
-	xorl	%ebx, %ebx
 
 1:
 	/*
@@ -1178,9 +1185,38 @@ ENTRY(paranoid_entry)
 	 * This is also why CS (stashed in the "iret frame" by the
 	 * hardware at entry) can not be used: this may be a return
 	 * to kernel code, but with a user CR3 value.
+	 *
+	 * As long as this PTI macro doesn't depend on kernel GSBASE,
+	 * we can do it early. This is because FIND_PERCPU_BASE
+	 * references data in kernel space.
 	 */
 	SAVE_AND_SWITCH_TO_KERNEL_CR3 scratch_reg=%rax save_reg=%r14
 
+	/*
+	 * Read GSBASE by RDGSBASE. Kernel GSBASE is found
+	 * from the per-CPU offset table with a CPU NR.
+	 */
+	ALTERNATIVE "jmp .Lparanoid_entry_no_fsgsbase",	"",\
+		X86_FEATURE_FSGSBASE
+	rdgsbase	%rbx
+	FIND_PERCPU_BASE	%rax
+	wrgsbase	%rax
+	ret
+
+.Lparanoid_entry_no_fsgsbase:
+	movl	$1, %ebx
+	/*
+	 * FSGSBASE is not in use, so depend on the kernel-enforced
+	 * convention that a negative GSBASE indicates a kernel value.
+	 */
+	READ_MSR_GSBASE save_reg=%edx
+	testl	%edx, %edx	/* Negative -> in kernel */
+	jns	.Lparanoid_entry_swapgs
+	ret
+
+.Lparanoid_entry_swapgs:
+	SWAPGS
+	xorl	%ebx, %ebx
 	ret
 END(paranoid_entry)
 
@@ -1194,12 +1230,21 @@ END(paranoid_entry)
  * be complicated.  Fortunately, we there's no good reason
  * to try to handle preemption here.
  *
- * On entry, ebx is "no swapgs" flag (1: don't need swapgs, 0: need it)
+ * On entry,
+ *	With FSGSBASE,
+ *		%rbx is original GSBASE that needs to be restored on the exit
+ *	Without that,
+ * 		%ebx is "no swapgs" flag (1: don't need swapgs, 0: need it)
  */
 ENTRY(paranoid_exit)
 	UNWIND_HINT_REGS
 	DISABLE_INTERRUPTS(CLBR_ANY)
 	TRACE_IRQS_OFF_DEBUG
+	ALTERNATIVE "jmp .Lparanoid_exit_no_fsgsbase",	"nop",\
+		X86_FEATURE_FSGSBASE
+	wrgsbase	%rbx
+	jmp	.Lparanoid_exit_no_swapgs;
+.Lparanoid_exit_no_fsgsbase:
 	testl	%ebx, %ebx			/* swapgs needed? */
 	jnz	.Lparanoid_exit_no_swapgs
 	TRACE_IRQS_IRETQ
@@ -1212,7 +1257,7 @@ ENTRY(paranoid_exit)
 	/* Always restore stashed CR3 value (see paranoid_entry) */
 	RESTORE_CR3	scratch_reg=%rbx save_reg=%r14
 .Lparanoid_exit_restore:
-	jmp restore_regs_and_return_to_kernel
+	jmp	restore_regs_and_return_to_kernel
 END(paranoid_exit)
 
 /*
diff --git a/arch/x86/include/asm/fsgsbase.h b/arch/x86/include/asm/fsgsbase.h
index 5e3dfbe8c1bf..ba7a444ab5c8 100644
--- a/arch/x86/include/asm/fsgsbase.h
+++ b/arch/x86/include/asm/fsgsbase.h
@@ -117,6 +117,15 @@ extern void x86_gsbase_write_cpu_inactive(unsigned long gsbase);
 
 #endif /* CONFIG_SMP */
 
+.macro READ_MSR_GSBASE save_reg:req
+	movl	$MSR_GS_BASE, %ecx
+	/* Read MSR specified by %ecx into %edx:%eax */
+	rdmsr
+	.ifnc \save_reg, %edx
+	movl	%edx, \save_reg
+	.endif
+.endm
+
 #endif /* CONFIG_X86_64 */
 
 #endif /* __ASSEMBLY__ */
-- 
2.19.1