[PATCH v3 07/32] KVM: arm64: Introduce a BSS section for use at Hyp

Quentin Perret
2021-03-03T14:09:09Z

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.

Signed-off-by: Quentin Perret <qperret@google.com>
---
arch/arm64/include/asm/sections.h | 1 +
arch/arm64/kernel/vmlinux.lds.S | 52 ++++++++++++++++++++-----------
arch/arm64/kvm/arm.c | 14 ++++++++-
arch/arm64/kvm/hyp/nvhe/hyp.lds.S | 1 +
4 files changed, 49 insertions(+), 19 deletions(-)

diff --git a/arch/arm64/include/asm/sections.h b/arch/arm64/include/asm/sections.h
index 2f36b16a5b5d..e4ad9db53af1 100644
--- a/arch/arm64/include/asm/sections.h
+++ b/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[];
diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index 7eea7888bb02..e96173ce211b 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/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.
*/
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 2d1e7ef69c04..3f8bcf8db036 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -1770,7 +1770,19 @@ 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");
diff --git a/arch/arm64/kvm/hyp/nvhe/hyp.lds.S b/arch/arm64/kvm/hyp/nvhe/hyp.lds.S
index cd119d82d8e3..f4562f417d3f 100644
--- a/arch/arm64/kvm/hyp/nvhe/hyp.lds.S
+++ b/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)
}