KVM: arm64: Introduce a BSS section for use at Hyp

Currently, the hyp code cannot make full use of a bss, as the kernel
section is mapped read-only.

While this mapping could simply be changed to read-write, it would
intermingle even more the hyp and kernel state than they currently are.
Instead, introduce a __hyp_bss section, that uses reserved pages, and
create the appropriate RW hyp mappings during KVM init.
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210319100146.1149909-8-qperret@google.com

arch/arm64/include/asm/sections.h

@@ -13,6 +13,7 @@ extern char __hyp_idmap_text_start[], __hyp_idmap_text_end[];
 extern char __hyp_text_start[], __hyp_text_end[];
 extern char __hyp_rodata_start[], __hyp_rodata_end[];
 extern char __hyp_reloc_begin[], __hyp_reloc_end[];
+extern char __hyp_bss_start[], __hyp_bss_end[];
 extern char __idmap_text_start[], __idmap_text_end[];
 extern char __initdata_begin[], __initdata_end[];
 extern char __inittext_begin[], __inittext_end[];

arch/arm64/kernel/vmlinux.lds.S

@@ -5,24 +5,7 @@
  * Written by Martin Mares <mj@atrey.karlin.mff.cuni.cz>
  */
 
-#define RO_EXCEPTION_TABLE_ALIGN	8
-#define RUNTIME_DISCARD_EXIT
-
-#include <asm-generic/vmlinux.lds.h>
-#include <asm/cache.h>
 #include <asm/hyp_image.h>
-#include <asm/kernel-pgtable.h>
-#include <asm/memory.h>
-#include <asm/page.h>
-
-#include "image.h"
-
-OUTPUT_ARCH(aarch64)
-ENTRY(_text)
-
-jiffies = jiffies_64;
-
-
 #ifdef CONFIG_KVM
 #define HYPERVISOR_EXTABLE					\
 	. = ALIGN(SZ_8);					\
@@ -51,13 +34,43 @@ jiffies = jiffies_64;
 		__hyp_reloc_end = .;				\
 	}
 
+#define BSS_FIRST_SECTIONS					\
+	__hyp_bss_start = .;					\
+	*(HYP_SECTION_NAME(.bss))				\
+	. = ALIGN(PAGE_SIZE);					\
+	__hyp_bss_end = .;
+
+/*
+ * We require that __hyp_bss_start and __bss_start are aligned, and enforce it
+ * with an assertion. But the BSS_SECTION macro places an empty .sbss section
+ * between them, which can in some cases cause the linker to misalign them. To
+ * work around the issue, force a page alignment for __bss_start.
+ */
+#define SBSS_ALIGN			PAGE_SIZE
 #else /* CONFIG_KVM */
 #define HYPERVISOR_EXTABLE
 #define HYPERVISOR_DATA_SECTIONS
 #define HYPERVISOR_PERCPU_SECTION
 #define HYPERVISOR_RELOC_SECTION
+#define SBSS_ALIGN			0
 #endif
 
+#define RO_EXCEPTION_TABLE_ALIGN	8
+#define RUNTIME_DISCARD_EXIT
+
+#include <asm-generic/vmlinux.lds.h>
+#include <asm/cache.h>
+#include <asm/kernel-pgtable.h>
+#include <asm/memory.h>
+#include <asm/page.h>
+
+#include "image.h"
+
+OUTPUT_ARCH(aarch64)
+ENTRY(_text)
+
+jiffies = jiffies_64;
+
 #define HYPERVISOR_TEXT					\
 	/*						\
 	 * Align to 4 KB so that			\
@@ -276,7 +289,7 @@ SECTIONS
 	__pecoff_data_rawsize = ABSOLUTE(. - __initdata_begin);
 	_edata = .;
 
-	BSS_SECTION(0, 0, 0)
+	BSS_SECTION(SBSS_ALIGN, 0, 0)
 
 	. = ALIGN(PAGE_SIZE);
 	init_pg_dir = .;
@@ -324,6 +337,9 @@ ASSERT(__hibernate_exit_text_end - (__hibernate_exit_text_start & ~(SZ_4K - 1))
 ASSERT((__entry_tramp_text_end - __entry_tramp_text_start) == PAGE_SIZE,
 	"Entry trampoline text too big")
 #endif
+#ifdef CONFIG_KVM
+ASSERT(__hyp_bss_start == __bss_start, "HYP and Host BSS are misaligned")
+#endif
 /*
  * If padding is applied before .head.text, virt<->phys conversions will fail.
  */

arch/arm64/kvm/arm.c

@@ -1775,7 +1775,19 @@ static int init_hyp_mode(void)
 		goto out_err;
 	}
 
-	err = create_hyp_mappings(kvm_ksym_ref(__bss_start),
+	/*
+	 * .hyp.bss is guaranteed to be placed at the beginning of the .bss
+	 * section thanks to an assertion in the linker script. Map it RW and
+	 * the rest of .bss RO.
+	 */
+	err = create_hyp_mappings(kvm_ksym_ref(__hyp_bss_start),
+				  kvm_ksym_ref(__hyp_bss_end), PAGE_HYP);
+	if (err) {
+		kvm_err("Cannot map hyp bss section: %d\n", err);
+		goto out_err;
+	}
+
+	err = create_hyp_mappings(kvm_ksym_ref(__hyp_bss_end),
 				  kvm_ksym_ref(__bss_stop), PAGE_HYP_RO);
 	if (err) {
 		kvm_err("Cannot map bss section\n");

arch/arm64/kvm/hyp/nvhe/hyp.lds.S

@@ -25,4 +25,5 @@ SECTIONS {
 	BEGIN_HYP_SECTION(.data..percpu)
 		PERCPU_INPUT(L1_CACHE_BYTES)
 	END_HYP_SECTION
+	HYP_SECTION(.bss)
 }