linux-kernel - [PATCH v3 2/6] x86/uaccess: Avoid barrier_nospec() in 64-bit __get

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Message-ID: <82cbb9983fef5ecf6f1cb33661e977172d40a7e6.1730166635.git.jpoimboe@kernel.org>
Date: Mon, 28 Oct 2024 18:56:15 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	Waiman Long <longman@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	linuxppc-dev@...ts.ozlabs.org,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Mark Rutland <mark.rutland@....com>,
	"Kirill A . Shutemov" <kirill@...temov.name>
Subject: [PATCH v3 2/6] x86/uaccess: Avoid barrier_nospec() in 64-bit __get_user()

The barrier_nospec() in 64-bit __get_user() is slow.  Instead use
pointer masking to force the user pointer to all 1's if a previous
access_ok() mispredicted true for an invalid address.

Note that for safety on some AMD CPUs, this relies on recent commit
86e6b1547b3d ("x86: fix user address masking non-canonical speculation
issue").

Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
---
 arch/x86/lib/getuser.S | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/arch/x86/lib/getuser.S b/arch/x86/lib/getuser.S
index 4357ec2a0bfc..998d5be6b794 100644
--- a/arch/x86/lib/getuser.S
+++ b/arch/x86/lib/getuser.S
@@ -112,8 +112,12 @@ EXPORT_SYMBOL(__get_user_8)
 
 /* .. and the same for __get_user, just without the range checks */
 SYM_FUNC_START(__get_user_nocheck_1)
-	ASM_STAC
+#ifdef CONFIG_X86_64
+	check_range size=1
+#else
 	ASM_BARRIER_NOSPEC
+#endif
+	ASM_STAC
 	UACCESS movzbl (%_ASM_AX),%edx
 	xor %eax,%eax
 	ASM_CLAC
@@ -122,8 +126,12 @@ SYM_FUNC_END(__get_user_nocheck_1)
 EXPORT_SYMBOL(__get_user_nocheck_1)
 
 SYM_FUNC_START(__get_user_nocheck_2)
-	ASM_STAC
+#ifdef CONFIG_X86_64
+	check_range size=2
+#else
 	ASM_BARRIER_NOSPEC
+#endif
+	ASM_STAC
 	UACCESS movzwl (%_ASM_AX),%edx
 	xor %eax,%eax
 	ASM_CLAC
@@ -132,8 +140,12 @@ SYM_FUNC_END(__get_user_nocheck_2)
 EXPORT_SYMBOL(__get_user_nocheck_2)
 
 SYM_FUNC_START(__get_user_nocheck_4)
-	ASM_STAC
+#ifdef CONFIG_X86_64
+	check_range size=4
+#else
 	ASM_BARRIER_NOSPEC
+#endif
+	ASM_STAC
 	UACCESS movl (%_ASM_AX),%edx
 	xor %eax,%eax
 	ASM_CLAC
@@ -142,8 +154,12 @@ SYM_FUNC_END(__get_user_nocheck_4)
 EXPORT_SYMBOL(__get_user_nocheck_4)
 
 SYM_FUNC_START(__get_user_nocheck_8)
-	ASM_STAC
+#ifdef CONFIG_X86_64
+	check_range size=8
+#else
 	ASM_BARRIER_NOSPEC
+#endif
+	ASM_STAC
 #ifdef CONFIG_X86_64
 	UACCESS movq (%_ASM_AX),%rdx
 #else
-- 
2.47.0