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Date:   Sat, 7 Jan 2023 12:10:27 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        Kostya Serebryany <kcc@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Taras Madan <tarasmadan@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Bharata B Rao <bharata@....com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv13 05/16] x86/uaccess: Provide untagged_addr() and remove
 tags before address check

On Fri, Dec 30, 2022 at 04:42:05PM -0800, Linus Torvalds wrote:
> The one thing that that "shift by 63 and bitwise or" trick does
> require is that the _ASM_EXTABLE_UA() thing for getuser/putuser would
> have to have an extra annotation to shut up the
> 
>         WARN_ONCE(trapnr == X86_TRAP_GP, "General protection fault in
> user access. Non-canonical address?");
> 
> in ex_handler_uaccess() for the GP trap that users can now cause by
> giving a non-canonical address with the high bit clear. So we'd
> probably just want a new EX_TYPE_* for these cases, but that still
> looks fairly straightforward.

Plain _ASM_EXTABLE() seems does the trick.

> Hmm?

Here's what I've come up with:

diff --git a/arch/x86/lib/getuser.S b/arch/x86/lib/getuser.S
index b70d98d79a9d..3e69e3727769 100644
--- a/arch/x86/lib/getuser.S
+++ b/arch/x86/lib/getuser.S
@@ -37,22 +37,22 @@
 
 #define ASM_BARRIER_NOSPEC ALTERNATIVE "", "lfence", X86_FEATURE_LFENCE_RDTSC
 
-#ifdef CONFIG_X86_5LEVEL
-#define LOAD_TASK_SIZE_MINUS_N(n) \
-	ALTERNATIVE __stringify(mov $((1 << 47) - 4096 - (n)),%rdx), \
-		    __stringify(mov $((1 << 56) - 4096 - (n)),%rdx), X86_FEATURE_LA57
-#else
-#define LOAD_TASK_SIZE_MINUS_N(n) \
-	mov $(TASK_SIZE_MAX - (n)),%_ASM_DX
-#endif
+.macro check_range size:req
+.if IS_ENABLED(CONFIG_X86_64)
+	mov %rax, %rdx
+	shr $63, %rdx
+	or %rdx, %rax
+.else
+	cmp $TASK_SIZE_MAX-\size+1, %eax
+	jae .Lbad_get_user
+	sbb %edx, %edx		/* array_index_mask_nospec() */
+	and %edx, %eax
+.endif
+.endm
 
 	.text
 SYM_FUNC_START(__get_user_1)
-	LOAD_TASK_SIZE_MINUS_N(0)
-	cmp %_ASM_DX,%_ASM_AX
-	jae bad_get_user
-	sbb %_ASM_DX, %_ASM_DX		/* array_index_mask_nospec() */
-	and %_ASM_DX, %_ASM_AX
+	check_range size=1
 	ASM_STAC
 1:	movzbl (%_ASM_AX),%edx
 	xor %eax,%eax
@@ -62,11 +62,7 @@ SYM_FUNC_END(__get_user_1)
 EXPORT_SYMBOL(__get_user_1)
 
 SYM_FUNC_START(__get_user_2)
-	LOAD_TASK_SIZE_MINUS_N(1)
-	cmp %_ASM_DX,%_ASM_AX
-	jae bad_get_user
-	sbb %_ASM_DX, %_ASM_DX		/* array_index_mask_nospec() */
-	and %_ASM_DX, %_ASM_AX
+	check_range size=2
 	ASM_STAC
 2:	movzwl (%_ASM_AX),%edx
 	xor %eax,%eax
@@ -76,11 +72,7 @@ SYM_FUNC_END(__get_user_2)
 EXPORT_SYMBOL(__get_user_2)
 
 SYM_FUNC_START(__get_user_4)
-	LOAD_TASK_SIZE_MINUS_N(3)
-	cmp %_ASM_DX,%_ASM_AX
-	jae bad_get_user
-	sbb %_ASM_DX, %_ASM_DX		/* array_index_mask_nospec() */
-	and %_ASM_DX, %_ASM_AX
+	check_range size=4
 	ASM_STAC
 3:	movl (%_ASM_AX),%edx
 	xor %eax,%eax
@@ -90,30 +82,17 @@ SYM_FUNC_END(__get_user_4)
 EXPORT_SYMBOL(__get_user_4)
 
 SYM_FUNC_START(__get_user_8)
-#ifdef CONFIG_X86_64
-	LOAD_TASK_SIZE_MINUS_N(7)
-	cmp %_ASM_DX,%_ASM_AX
-	jae bad_get_user
-	sbb %_ASM_DX, %_ASM_DX		/* array_index_mask_nospec() */
-	and %_ASM_DX, %_ASM_AX
+	check_range size=8
 	ASM_STAC
+#ifdef CONFIG_X86_64
 4:	movq (%_ASM_AX),%rdx
-	xor %eax,%eax
-	ASM_CLAC
-	RET
 #else
-	LOAD_TASK_SIZE_MINUS_N(7)
-	cmp %_ASM_DX,%_ASM_AX
-	jae bad_get_user_8
-	sbb %_ASM_DX, %_ASM_DX		/* array_index_mask_nospec() */
-	and %_ASM_DX, %_ASM_AX
-	ASM_STAC
 4:	movl (%_ASM_AX),%edx
 5:	movl 4(%_ASM_AX),%ecx
+#endif
 	xor %eax,%eax
 	ASM_CLAC
 	RET
-#endif
 SYM_FUNC_END(__get_user_8)
 EXPORT_SYMBOL(__get_user_8)
 
@@ -166,7 +145,7 @@ EXPORT_SYMBOL(__get_user_nocheck_8)
 
 SYM_CODE_START_LOCAL(.Lbad_get_user_clac)
 	ASM_CLAC
-bad_get_user:
+.Lbad_get_user:
 	xor %edx,%edx
 	mov $(-EFAULT),%_ASM_AX
 	RET
@@ -184,23 +163,23 @@ SYM_CODE_END(.Lbad_get_user_8_clac)
 #endif
 
 /* get_user */
-	_ASM_EXTABLE_UA(1b, .Lbad_get_user_clac)
-	_ASM_EXTABLE_UA(2b, .Lbad_get_user_clac)
-	_ASM_EXTABLE_UA(3b, .Lbad_get_user_clac)
+	_ASM_EXTABLE(1b, .Lbad_get_user_clac)
+	_ASM_EXTABLE(2b, .Lbad_get_user_clac)
+	_ASM_EXTABLE(3b, .Lbad_get_user_clac)
 #ifdef CONFIG_X86_64
-	_ASM_EXTABLE_UA(4b, .Lbad_get_user_clac)
+	_ASM_EXTABLE(4b, .Lbad_get_user_clac)
 #else
-	_ASM_EXTABLE_UA(4b, .Lbad_get_user_8_clac)
-	_ASM_EXTABLE_UA(5b, .Lbad_get_user_8_clac)
+	_ASM_EXTABLE(4b, .Lbad_get_user_8_clac)
+	_ASM_EXTABLE(5b, .Lbad_get_user_8_clac)
 #endif
 
 /* __get_user */
-	_ASM_EXTABLE_UA(6b, .Lbad_get_user_clac)
-	_ASM_EXTABLE_UA(7b, .Lbad_get_user_clac)
-	_ASM_EXTABLE_UA(8b, .Lbad_get_user_clac)
+	_ASM_EXTABLE(6b, .Lbad_get_user_clac)
+	_ASM_EXTABLE(7b, .Lbad_get_user_clac)
+	_ASM_EXTABLE(8b, .Lbad_get_user_clac)
 #ifdef CONFIG_X86_64
-	_ASM_EXTABLE_UA(9b, .Lbad_get_user_clac)
+	_ASM_EXTABLE(9b, .Lbad_get_user_clac)
 #else
-	_ASM_EXTABLE_UA(9b, .Lbad_get_user_8_clac)
-	_ASM_EXTABLE_UA(10b, .Lbad_get_user_8_clac)
+	_ASM_EXTABLE(9b, .Lbad_get_user_8_clac)
+	_ASM_EXTABLE(10b, .Lbad_get_user_8_clac)
 #endif
diff --git a/arch/x86/lib/putuser.S b/arch/x86/lib/putuser.S
index 32125224fcca..0ec57997a764 100644
--- a/arch/x86/lib/putuser.S
+++ b/arch/x86/lib/putuser.S
@@ -33,20 +33,20 @@
  * as they get called from within inline assembly.
  */
 
-#ifdef CONFIG_X86_5LEVEL
-#define LOAD_TASK_SIZE_MINUS_N(n) \
-	ALTERNATIVE __stringify(mov $((1 << 47) - 4096 - (n)),%rbx), \
-		    __stringify(mov $((1 << 56) - 4096 - (n)),%rbx), X86_FEATURE_LA57
-#else
-#define LOAD_TASK_SIZE_MINUS_N(n) \
-	mov $(TASK_SIZE_MAX - (n)),%_ASM_BX
-#endif
+.macro check_range size:req
+.if IS_ENABLED(CONFIG_X86_64)
+	movq %rcx, %rbx
+	shrq $63, %rbx
+	orq %rbx, %rcx
+.else
+	cmp $TASK_SIZE_MAX-\size+1, %ecx
+	jae .Lbad_put_user
+.endif
+.endm
 
 .text
 SYM_FUNC_START(__put_user_1)
-	LOAD_TASK_SIZE_MINUS_N(0)
-	cmp %_ASM_BX,%_ASM_CX
-	jae .Lbad_put_user
+	check_range size=1
 	ASM_STAC
 1:	movb %al,(%_ASM_CX)
 	xor %ecx,%ecx
@@ -66,9 +66,7 @@ SYM_FUNC_END(__put_user_nocheck_1)
 EXPORT_SYMBOL(__put_user_nocheck_1)
 
 SYM_FUNC_START(__put_user_2)
-	LOAD_TASK_SIZE_MINUS_N(1)
-	cmp %_ASM_BX,%_ASM_CX
-	jae .Lbad_put_user
+	check_range size=2
 	ASM_STAC
 3:	movw %ax,(%_ASM_CX)
 	xor %ecx,%ecx
@@ -88,9 +86,7 @@ SYM_FUNC_END(__put_user_nocheck_2)
 EXPORT_SYMBOL(__put_user_nocheck_2)
 
 SYM_FUNC_START(__put_user_4)
-	LOAD_TASK_SIZE_MINUS_N(3)
-	cmp %_ASM_BX,%_ASM_CX
-	jae .Lbad_put_user
+	check_range size=4
 	ASM_STAC
 5:	movl %eax,(%_ASM_CX)
 	xor %ecx,%ecx
@@ -110,9 +106,7 @@ SYM_FUNC_END(__put_user_nocheck_4)
 EXPORT_SYMBOL(__put_user_nocheck_4)
 
 SYM_FUNC_START(__put_user_8)
-	LOAD_TASK_SIZE_MINUS_N(7)
-	cmp %_ASM_BX,%_ASM_CX
-	jae .Lbad_put_user
+	check_range size=8
 	ASM_STAC
 7:	mov %_ASM_AX,(%_ASM_CX)
 #ifdef CONFIG_X86_32
@@ -144,15 +138,15 @@ SYM_CODE_START_LOCAL(.Lbad_put_user_clac)
 	RET
 SYM_CODE_END(.Lbad_put_user_clac)
 
-	_ASM_EXTABLE_UA(1b, .Lbad_put_user_clac)
-	_ASM_EXTABLE_UA(2b, .Lbad_put_user_clac)
-	_ASM_EXTABLE_UA(3b, .Lbad_put_user_clac)
-	_ASM_EXTABLE_UA(4b, .Lbad_put_user_clac)
-	_ASM_EXTABLE_UA(5b, .Lbad_put_user_clac)
-	_ASM_EXTABLE_UA(6b, .Lbad_put_user_clac)
-	_ASM_EXTABLE_UA(7b, .Lbad_put_user_clac)
-	_ASM_EXTABLE_UA(9b, .Lbad_put_user_clac)
+	_ASM_EXTABLE(1b, .Lbad_put_user_clac)
+	_ASM_EXTABLE(2b, .Lbad_put_user_clac)
+	_ASM_EXTABLE(3b, .Lbad_put_user_clac)
+	_ASM_EXTABLE(4b, .Lbad_put_user_clac)
+	_ASM_EXTABLE(5b, .Lbad_put_user_clac)
+	_ASM_EXTABLE(6b, .Lbad_put_user_clac)
+	_ASM_EXTABLE(7b, .Lbad_put_user_clac)
+	_ASM_EXTABLE(9b, .Lbad_put_user_clac)
 #ifdef CONFIG_X86_32
-	_ASM_EXTABLE_UA(8b, .Lbad_put_user_clac)
-	_ASM_EXTABLE_UA(10b, .Lbad_put_user_clac)
+	_ASM_EXTABLE(8b, .Lbad_put_user_clac)
+	_ASM_EXTABLE(10b, .Lbad_put_user_clac)
 #endif
-- 
  Kiryl Shutsemau / Kirill A. Shutemov

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