linux-hardening - [PATCH v5 12/32] ARM: assembler: add optimized ldr/str macros to load variables from memory

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Date:   Mon, 24 Jan 2022 18:47:24 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     linux@...linux.org.uk, linux-arm-kernel@...ts.infradead.org
Cc:     linux-hardening@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
        Nicolas Pitre <nico@...xnic.net>,
        Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>,
        Keith Packard <keithpac@...zon.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tony Lindgren <tony@...mide.com>,
        Marc Zyngier <maz@...nel.org>,
        Vladimir Murzin <vladimir.murzin@....com>,
        Jesse Taube <mr.bossman075@...il.com>
Subject: [PATCH v5 12/32] ARM: assembler: add optimized ldr/str macros to load variables from memory

We will be adding variable loads to various hot paths, so it makes sense
to add a helper macro that can load variables from asm code without the
use of literal pool entries. On v7 or later, we can simply use MOVW/MOVT
pairs, but on earlier cores, this requires a bit of hackery to emit a
instruction sequence that implements this using a sequence of ADD/LDR
instructions.

Acked-by: Linus Walleij <linus.walleij@...aro.org>
Acked-by: Nicolas Pitre <nico@...xnic.net>
Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
Tested-by: Marc Zyngier <maz@...nel.org>
Tested-by: Vladimir Murzin <vladimir.murzin@....com> # ARMv7M
---
 arch/arm/Kconfig                 | 11 +++++
 arch/arm/include/asm/assembler.h | 48 ++++++++++++++++++--
 2 files changed, 55 insertions(+), 4 deletions(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 99ac5d75dcec..9586636289d2 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -139,6 +139,17 @@ config ARM
 	  Europe.  There is an ARM Linux project with a web page at
 	  <http://www.arm.linux.org.uk/>.
 
+config ARM_HAS_GROUP_RELOCS
+	def_bool y
+	depends on !LD_IS_LLD || LLD_VERSION >= 140000
+	depends on !COMPILE_TEST
+	help
+	  Whether or not to use R_ARM_ALU_PC_Gn or R_ARM_LDR_PC_Gn group
+	  relocations, which have been around for a long time, but were not
+	  supported in LLD until version 14. The combined range is -/+ 256 MiB,
+	  which is usually sufficient, but not for allyesconfig, so we disable
+	  this feature when doing compile testing.
+
 config ARM_HAS_SG_CHAIN
 	bool
 
diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
index 7d23d4bb2168..7a4e292b68e4 100644
--- a/arch/arm/include/asm/assembler.h
+++ b/arch/arm/include/asm/assembler.h
@@ -564,12 +564,12 @@ THUMB(	orr	\reg , \reg , #PSR_T_BIT	)
 	/*
 	 * mov_l - move a constant value or [relocated] address into a register
 	 */
-	.macro		mov_l, dst:req, imm:req
+	.macro		mov_l, dst:req, imm:req, cond
 	.if		__LINUX_ARM_ARCH__ < 7
-	ldr		\dst, =\imm
+	ldr\cond	\dst, =\imm
 	.else
-	movw		\dst, #:lower16:\imm
-	movt		\dst, #:upper16:\imm
+	movw\cond	\dst, #:lower16:\imm
+	movt\cond	\dst, #:upper16:\imm
 	.endif
 	.endm
 
@@ -607,6 +607,46 @@ THUMB(	orr	\reg , \reg , #PSR_T_BIT	)
 	__adldst_l	str, \src, \sym, \tmp, \cond
 	.endm
 
+	.macro		__ldst_va, op, reg, tmp, sym, offset=0, cond
+#if __LINUX_ARM_ARCH__ >= 7 || \
+    !defined(CONFIG_ARM_HAS_GROUP_RELOCS) || \
+    (defined(MODULE) && defined(CONFIG_ARM_MODULE_PLTS))
+	mov_l		\tmp, \sym, \cond
+#else
+	/*
+	 * Avoid a literal load, by emitting a sequence of ADD/LDR instructions
+	 * with the appropriate relocations. The combined sequence has a range
+	 * of -/+ 256 MiB, which should be sufficient for the core kernel and
+	 * for modules loaded into the module region.
+	 */
+	.globl		\sym
+	.reloc		.L0_\@, R_ARM_ALU_PC_G0_NC, \sym
+	.reloc		.L1_\@, R_ARM_ALU_PC_G1_NC, \sym
+	.reloc		.L2_\@, R_ARM_LDR_PC_G2, \sym
+.L0_\@: sub\cond	\tmp, pc, #8 - \offset
+.L1_\@: sub\cond	\tmp, \tmp, #4 - \offset
+#endif
+.L2_\@: \op\cond	\reg, [\tmp, #\offset]
+	.endm
+
+	/*
+	 * ldr_va - load a 32-bit word from the virtual address of \sym
+	 */
+	.macro		ldr_va, rd:req, sym:req, cond, tmp, offset
+	.ifb		\tmp
+	__ldst_va	ldr, \rd, \rd, \sym, \offset, \cond
+	.else
+	__ldst_va	ldr, \rd, \tmp, \sym, \offset, \cond
+	.endif
+	.endm
+
+	/*
+	 * str_va - store a 32-bit word to the virtual address of \sym
+	 */
+	.macro		str_va, rn:req, sym:req, tmp:req
+	__ldst_va	str, \rn, \tmp, \sym
+	.endm
+
 	/*
 	 * rev_l - byte-swap a 32-bit value
 	 *
-- 
2.30.2