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From: Sasha Levin <sashal@kernel.org>
To: linux-kernel@vger.kernel.org, tglx@linutronix.de, bp@alien8.de,
	luto@kernel.org
Cc: hpa@zytor.com, dave.hansen@intel.com, tony.luck@intel.com,
	ak@linux.intel.com, ravi.v.shankar@intel.com,
	chang.seok.bae@intel.com,
	Vegard Nossum <vegard.nossum@oracle.com>,
	Sasha Levin <sashal@kernel.org>
Subject: [PATCH v10 04/18] x86/entry/64: Clean up paranoid exit
Date: Thu, 23 Apr 2020 19:21:53 -0400	[thread overview]
Message-ID: <20200423232207.5797-5-sashal@kernel.org> (raw)
In-Reply-To: <20200423232207.5797-1-sashal@kernel.org>

From: Andy Lutomirski <luto@kernel.org>

All that paranoid exit needs to do is to disable IRQs, handle IRQ tracing,
then restore CR3, and restore GS base. Simply do those actions in that
order. Cleaning up the spaghetti code.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/x86/entry/entry_64.S | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 0e9504fabe526..3adb3c8e2409b 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1266,19 +1266,25 @@ SYM_CODE_END(paranoid_entry)
 SYM_CODE_START_LOCAL(paranoid_exit)
 	UNWIND_HINT_REGS
 	DISABLE_INTERRUPTS(CLBR_ANY)
+
+	/*
+	 * The order of operations is important. IRQ tracing requires
+	 * kernel GS base and CR3. RESTORE_CR3 requires kernel GS base.
+	 *
+	 * NB to anyone to try to optimize this code: this code does
+	 * not execute at all for exceptions from user mode. Those
+	 * exceptions go through error_exit instead.
+	 */
 	TRACE_IRQS_OFF_DEBUG
-	testl	%ebx, %ebx			/* swapgs needed? */
-	jnz	.Lparanoid_exit_no_swapgs
-	TRACE_IRQS_IRETQ
-	/* Always restore stashed CR3 value (see paranoid_entry) */
-	RESTORE_CR3	scratch_reg=%rbx save_reg=%r14
+	RESTORE_CR3	scratch_reg=%rax save_reg=%r14
+
+	/* If EBX is 0, SWAPGS is required */
+	testl	%ebx, %ebx
+	jnz	restore_regs_and_return_to_kernel
+
+	/* We are returning to a context with user GS base */
 	SWAPGS_UNSAFE_STACK
 	jmp	restore_regs_and_return_to_kernel
-.Lparanoid_exit_no_swapgs:
-	TRACE_IRQS_IRETQ_DEBUG
-	/* Always restore stashed CR3 value (see paranoid_entry) */
-	RESTORE_CR3	scratch_reg=%rbx save_reg=%r14
-	jmp restore_regs_and_return_to_kernel
 SYM_CODE_END(paranoid_exit)
 
 /*
-- 
2.20.1


  parent reply	other threads:[~2020-04-23 23:22 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-23 23:21 [PATCH v10 00/18] Enable FSGSBASE instructions Sasha Levin
2020-04-23 23:21 ` [PATCH v10 01/18] x86/ptrace: Prevent ptrace from clearing the FS/GS selector Sasha Levin
2020-04-25 22:46   ` Andy Lutomirski
2020-04-23 23:21 ` [PATCH v10 02/18] selftests/x86/fsgsbase: Test GS selector on ptracer-induced GS base write Sasha Levin
2020-04-23 23:21 ` [PATCH v10 03/18] x86/cpu: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE Sasha Levin
2020-04-23 23:21 ` Sasha Levin [this message]
2020-04-23 23:21 ` [PATCH v10 05/18] x86/entry/64: Switch CR3 before SWAPGS in paranoid entry Sasha Levin
2020-04-23 23:21 ` [PATCH v10 06/18] x86/entry/64: Introduce the FIND_PERCPU_BASE macro Sasha Levin
2020-04-23 23:21 ` [PATCH v10 07/18] x86/entry/64: Handle FSGSBASE enabled paranoid entry/exit Sasha Levin
2020-04-23 23:21 ` [PATCH v10 08/18] x86/entry/64: Document GSBASE handling in the paranoid path Sasha Levin
2020-04-23 23:21 ` [PATCH v10 09/18] x86/fsgsbase/64: Add intrinsics for FSGSBASE instructions Sasha Levin
2020-04-23 23:21 ` [PATCH v10 10/18] x86/fsgsbase/64: Enable FSGSBASE instructions in helper functions Sasha Levin
2020-04-23 23:22 ` [PATCH v10 11/18] x86/fsgsbase/64: Use FSGSBASE in switch_to() if available Sasha Levin
2020-04-23 23:22 ` [PATCH v10 12/18] x86/fsgsbase/64: move save_fsgs to header file Sasha Levin
2020-04-23 23:22 ` [PATCH v10 13/18] x86/fsgsbase/64: Use FSGSBASE instructions on thread copy and ptrace Sasha Levin
2020-04-23 23:22 ` [PATCH v10 14/18] x86/speculation/swapgs: Check FSGSBASE in enabling SWAPGS mitigation Sasha Levin
2020-04-23 23:22 ` [PATCH v10 15/18] selftests/x86/fsgsbase: Test ptracer-induced GS base write with FSGSBASE Sasha Levin
2020-04-23 23:22 ` [PATCH v10 16/18] x86/fsgsbase/64: Enable FSGSBASE on 64bit by default and add a chicken bit Sasha Levin
2020-04-23 23:22 ` [PATCH v10 17/18] x86/elf: Enumerate kernel FSGSBASE capability in AT_HWCAP2 Sasha Levin
2020-04-23 23:22 ` [PATCH v10 18/18] Documentation/x86/64: Add documentation for GS/FS addressing mode Sasha Levin
2020-05-10  8:09 ` [PATCH v10 00/18] Enable FSGSBASE instructions Vegard Nossum
2020-05-10  8:29   ` Vegard Nossum
2020-05-10 10:15     ` Thomas Gleixner
2020-05-10 14:17       ` Sasha Levin
2020-05-11  0:48     ` Andi Kleen
2020-05-11  0:50       ` Andi Kleen
2020-05-11  5:03         ` Sasha Levin

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