[v2] arm64: entry: simplify trampoline data page

Commit Message

Ard Biesheuvel June 22, 2022, 4:10 p.m. UTC

Get rid of some clunky open coded arithmetic on section addresses, by
emitting the trampoline data variables into a separate, dedicated r/o
data section, and putting it at the next page boundary. This way, we can
access the literals via single LDR instruction.

While at it, get rid of other, implicit literals, and use ADRP/ADD or
MOVZ/MOVK sequences, as appropriate. Note that the latter are only
supported for CONFIG_RELOCATABLE=n (which is usually the case if
CONFIG_RANDOMIZE_BASE=n), so update the CPP conditionals to reflect
this.

Acked-by: Mark Rutland <[e-mail]>
Signed-off-by: Ard Biesheuvel <[e-mail]>
---
v2: incorporate suggested comment by Mark and add his ack

 arch/arm64/include/asm/fixmap.h |  4 +-
 arch/arm64/kernel/entry.S       | 53 ++++++++------------
 arch/arm64/kernel/vmlinux.lds.S |  3 +-
 arch/arm64/mm/mmu.c             | 10 ++--
 4 files changed, 30 insertions(+), 40 deletions(-)

Comments

Will Deacon June 24, 2022, 5:13 p.m. UTC | #1

On Wed, 22 Jun 2022 18:10:10 +0200, Ard Biesheuvel wrote:
> Get rid of some clunky open coded arithmetic on section addresses, by
> emitting the trampoline data variables into a separate, dedicated r/o
> data section, and putting it at the next page boundary. This way, we can
> access the literals via single LDR instruction.
> 
> While at it, get rid of other, implicit literals, and use ADRP/ADD or
> MOVZ/MOVK sequences, as appropriate. Note that the latter are only
> supported for CONFIG_RELOCATABLE=n (which is usually the case if
> CONFIG_RANDOMIZE_BASE=n), so update the CPP conditionals to reflect
> this.
> 
> [...]

Applied to arm64 (for-next/kpti), thanks!

[1/1] arm64: entry: simplify trampoline data page
      https://git.kernel.org/arm64/c/1c9a8e876808

Cheers,

Patch

diff --git a/arch/arm64/include/asm/fixmap.h b/arch/arm64/include/asm/fixmap.h
index daff882883f9..71ed5fdf718b 100644
--- a/arch/arm64/include/asm/fixmap.h
+++ b/arch/arm64/include/asm/fixmap.h
@@ -62,10 +62,12 @@  enum fixed_addresses {
 #endif /* CONFIG_ACPI_APEI_GHES */
 
 #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
+#ifdef CONFIG_RELOCATABLE
+	FIX_ENTRY_TRAMP_TEXT4,	/* one extra slot for the data page */
+#endif
 	FIX_ENTRY_TRAMP_TEXT3,
 	FIX_ENTRY_TRAMP_TEXT2,
 	FIX_ENTRY_TRAMP_TEXT1,
-	FIX_ENTRY_TRAMP_DATA,
 #define TRAMP_VALIAS		(__fix_to_virt(FIX_ENTRY_TRAMP_TEXT1))
 #endif /* CONFIG_UNMAP_KERNEL_AT_EL0 */
 	__end_of_permanent_fixed_addresses,
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 5b82b9292400..254fe31c03a0 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -636,18 +636,28 @@  alternative_else_nop_endif
 	 */
 	.endm
 
-	.macro tramp_data_page	dst
-	adr_l	\dst, .entry.tramp.text
-	sub	\dst, \dst, PAGE_SIZE
-	.endm
-
-	.macro tramp_data_read_var	dst, var
-#ifdef CONFIG_RANDOMIZE_BASE
-	tramp_data_page		\dst
-	add	\dst, \dst, #:lo12:__entry_tramp_data_\var
-	ldr	\dst, [\dst]
+	.macro		tramp_data_read_var	dst, var
+#ifdef CONFIG_RELOCATABLE
+	ldr		\dst, .L__tramp_data_\var
+	.ifndef		.L__tramp_data_\var
+	.pushsection	".entry.tramp.rodata", "a", %progbits
+	.align		3
+.L__tramp_data_\var:
+	.quad		\var
+	.popsection
+	.endif
 #else
-	ldr	\dst, =\var
+	/*
+	 * As !RELOCATABLE implies !RANDOMIZE_BASE the address is always a
+	 * compile time constant (and hence not secret and not worth hiding).
+	 *
+	 * As statically allocated kernel code and data always live in the top
+	 * 47 bits of the address space we can sign-extend bit 47 and avoid an
+	 * instruction to load the upper 16 bits (which must be 0xFFFF).
+	 */
+	movz		\dst, :abs_g2_s:\var
+	movk		\dst, :abs_g1_nc:\var
+	movk		\dst, :abs_g0_nc:\var
 #endif
 	.endm
 
@@ -695,7 +705,7 @@  alternative_else_nop_endif
 	msr	vbar_el1, x30
 	isb
 	.else
-	ldr	x30, =vectors
+	adr_l	x30, vectors
 	.endif // \kpti == 1
 
 	.if	\bhb == BHB_MITIGATION_FW
@@ -764,24 +774,7 @@  SYM_CODE_END(tramp_exit_native)
 SYM_CODE_START(tramp_exit_compat)
 	tramp_exit	32
 SYM_CODE_END(tramp_exit_compat)
-
-	.ltorg
 	.popsection				// .entry.tramp.text
-#ifdef CONFIG_RANDOMIZE_BASE
-	.pushsection ".rodata", "a"
-	.align PAGE_SHIFT
-SYM_DATA_START(__entry_tramp_data_start)
-__entry_tramp_data_vectors:
-	.quad	vectors
-#ifdef CONFIG_ARM_SDE_INTERFACE
-__entry_tramp_data___sdei_asm_handler:
-	.quad	__sdei_asm_handler
-#endif /* CONFIG_ARM_SDE_INTERFACE */
-__entry_tramp_data_this_cpu_vector:
-	.quad	this_cpu_vector
-SYM_DATA_END(__entry_tramp_data_start)
-	.popsection				// .rodata
-#endif /* CONFIG_RANDOMIZE_BASE */
 #endif /* CONFIG_UNMAP_KERNEL_AT_EL0 */
 
 /*
@@ -932,7 +925,6 @@  NOKPROBE(call_on_irq_stack)
  * This clobbers x4, __sdei_handler() will restore this from firmware's
  * copy.
  */
-.ltorg
 .pushsection ".entry.tramp.text", "ax"
 SYM_CODE_START(__sdei_asm_entry_trampoline)
 	mrs	x4, ttbr1_el1
@@ -967,7 +959,6 @@  SYM_CODE_START(__sdei_asm_exit_trampoline)
 1:	sdei_handler_exit exit_mode=x2
 SYM_CODE_END(__sdei_asm_exit_trampoline)
 NOKPROBE(__sdei_asm_exit_trampoline)
-	.ltorg
 .popsection		// .entry.tramp.text
 #endif /* CONFIG_UNMAP_KERNEL_AT_EL0 */
 
diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index 2d4a8f995175..8a078c0ee140 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -115,7 +115,8 @@  jiffies = jiffies_64;
 	__entry_tramp_text_start = .;			\
 	*(.entry.tramp.text)				\
 	. = ALIGN(PAGE_SIZE);				\
-	__entry_tramp_text_end = .;
+	__entry_tramp_text_end = .;			\
+	*(.entry.tramp.rodata)
 #else
 #define TRAMP_TEXT
 #endif
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 626ec32873c6..be4d6c3f5692 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -665,13 +665,9 @@  static int __init map_entry_trampoline(void)
 		__set_fixmap(FIX_ENTRY_TRAMP_TEXT1 - i,
 			     pa_start + i * PAGE_SIZE, prot);
 
-	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) {
-		extern char __entry_tramp_data_start[];
-
-		__set_fixmap(FIX_ENTRY_TRAMP_DATA,
-			     __pa_symbol(__entry_tramp_data_start),
-			     PAGE_KERNEL_RO);
-	}
+	if (IS_ENABLED(CONFIG_RELOCATABLE))
+		__set_fixmap(FIX_ENTRY_TRAMP_TEXT1 - i,
+			     pa_start + i * PAGE_SIZE, PAGE_KERNEL_RO);
 
 	return 0;
 }