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From:   Alexandre Ghiti <alexghiti@rivosinc.com>
To:     Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Ard Biesheuvel <ardb@kernel.org>,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-efi@vger.kernel.org, linux-arm-kernel@lists.infradead.org
Cc:     Alexandre Ghiti <alexghiti@rivosinc.com>
Subject: [PATCH v2 1/4] riscv: Introduce virtual kernel mapping KASLR
Date:   Wed, 29 Mar 2023 07:29:23 +0200
Message-Id: <20230329052926.69632-2-alexghiti@rivosinc.com>
In-Reply-To: <20230329052926.69632-1-alexghiti@rivosinc.com>
References: <20230329052926.69632-1-alexghiti@rivosinc.com>
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KASLR implementation relies on a relocatable kernel so that we can move
the kernel mapping.

The seed needed to virtually move the kernel is taken from the device tree,
so we rely on the bootloader to provide a correct seed. Zkr could be used
unconditionnally instead if implemented, but that's for another patch.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
---
 arch/riscv/Kconfig                   | 18 ++++++++++++++
 arch/riscv/include/asm/page.h        |  3 +++
 arch/riscv/kernel/pi/Makefile        |  2 +-
 arch/riscv/kernel/pi/cmdline_early.c | 13 ++++++++++
 arch/riscv/kernel/pi/fdt_early.c     | 30 +++++++++++++++++++++++
 arch/riscv/mm/init.c                 | 36 +++++++++++++++++++++++++++-
 6 files changed, 100 insertions(+), 2 deletions(-)
 create mode 100644 arch/riscv/kernel/pi/fdt_early.c

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 6ff9f574195d..af23750543c7 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -576,6 +576,24 @@ config RELOCATABLE
 
           If unsure, say N.
 
+config RANDOMIZE_BASE
+        bool "Randomize the address of the kernel image"
+        select RELOCATABLE
+        help
+          Randomizes the virtual address at which the kernel image is
+          loaded, as a security feature that deters exploit attempts
+          relying on knowledge of the location of kernel internals.
+
+          It is the bootloader's job to provide entropy, by passing a
+          random u64 value in /chosen/kaslr-seed at kernel entry.
+
+          When booting via the UEFI stub, it will invoke the firmware's
+          EFI_RNG_PROTOCOL implementation (if available) to supply entropy
+          to the kernel proper. In addition, it will randomise the physical
+          location of the kernel Image as well.
+
+          If unsure, say N.
+
 endmenu # "Kernel features"
 
 menu "Boot options"
diff --git a/arch/riscv/include/asm/page.h b/arch/riscv/include/asm/page.h
index ea1a0e237211..e0c8a1c8b22a 100644
--- a/arch/riscv/include/asm/page.h
+++ b/arch/riscv/include/asm/page.h
@@ -107,6 +107,7 @@ typedef struct page *pgtable_t;
 struct kernel_mapping {
 	unsigned long page_offset;
 	unsigned long virt_addr;
+	unsigned long virt_offset;
 	uintptr_t phys_addr;
 	uintptr_t size;
 	/* Offset between linear mapping virtual address and kernel load address */
@@ -186,6 +187,8 @@ extern phys_addr_t __phys_addr_symbol(unsigned long x);
 
 #define sym_to_pfn(x)           __phys_to_pfn(__pa_symbol(x))
 
+unsigned long kaslr_offset(void);
+
 #endif /* __ASSEMBLY__ */
 
 #define virt_addr_valid(vaddr)	({						\
diff --git a/arch/riscv/kernel/pi/Makefile b/arch/riscv/kernel/pi/Makefile
index 42c58f4ab53b..edb3218bfb36 100644
--- a/arch/riscv/kernel/pi/Makefile
+++ b/arch/riscv/kernel/pi/Makefile
@@ -35,5 +35,5 @@ $(obj)/string.o: $(srctree)/lib/string.c FORCE
 $(obj)/ctype.o: $(srctree)/lib/ctype.c FORCE
 	$(call if_changed_rule,cc_o_c)
 
-obj-y		:= cmdline_early.pi.o string.pi.o ctype.pi.o lib-fdt.pi.o lib-fdt_ro.pi.o
+obj-y		:= cmdline_early.pi.o fdt_early.pi.o string.pi.o ctype.pi.o lib-fdt.pi.o lib-fdt_ro.pi.o
 extra-y		:= $(patsubst %.pi.o,%.o,$(obj-y))
diff --git a/arch/riscv/kernel/pi/cmdline_early.c b/arch/riscv/kernel/pi/cmdline_early.c
index 05652d13c746..68e786c84c94 100644
--- a/arch/riscv/kernel/pi/cmdline_early.c
+++ b/arch/riscv/kernel/pi/cmdline_early.c
@@ -14,6 +14,7 @@ static char early_cmdline[COMMAND_LINE_SIZE];
  * LLVM complain because the function is actually unused in this file).
  */
 u64 set_satp_mode_from_cmdline(uintptr_t dtb_pa);
+bool set_nokaslr_from_cmdline(uintptr_t dtb_pa);
 
 static char *get_early_cmdline(uintptr_t dtb_pa)
 {
@@ -60,3 +61,15 @@ u64 set_satp_mode_from_cmdline(uintptr_t dtb_pa)
 
 	return match_noXlvl(cmdline);
 }
+
+static bool match_nokaslr(char *cmdline)
+{
+	return strstr(cmdline, "nokaslr");
+}
+
+bool set_nokaslr_from_cmdline(uintptr_t dtb_pa)
+{
+	char *cmdline = get_early_cmdline(dtb_pa);
+
+	return match_nokaslr(cmdline);
+}
diff --git a/arch/riscv/kernel/pi/fdt_early.c b/arch/riscv/kernel/pi/fdt_early.c
new file mode 100644
index 000000000000..899610e042ab
--- /dev/null
+++ b/arch/riscv/kernel/pi/fdt_early.c
@@ -0,0 +1,30 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <linux/types.h>
+#include <linux/init.h>
+#include <linux/libfdt.h>
+
+/*
+ * Declare the functions that are exported (but prefixed) here so that LLVM
+ * does not complain it lacks the 'static' keyword (which, if added, makes
+ * LLVM complain because the function is actually unused in this file).
+ */
+u64 get_kaslr_seed(uintptr_t dtb_pa);
+
+u64 get_kaslr_seed(uintptr_t dtb_pa)
+{
+	int node, len;
+	fdt64_t *prop;
+	u64 ret;
+
+	node = fdt_path_offset((void *)dtb_pa, "/chosen");
+	if (node < 0)
+		return 0;
+
+	prop = fdt_getprop_w((void *)dtb_pa, node, "kaslr-seed", &len);
+	if (!prop || len != sizeof(u64))
+		return 0;
+
+	ret = fdt64_to_cpu(*prop);
+	*prop = 0;
+	return ret;
+}
diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index 3ad771571c2d..3e535a88a065 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -1033,11 +1033,45 @@ static void __init pt_ops_set_late(void)
 #endif
 }
 
+#ifdef CONFIG_RANDOMIZE_BASE
+extern bool __init __pi_set_nokaslr_from_cmdline(uintptr_t dtb_pa);
+extern u64 __init __pi_get_kaslr_seed(uintptr_t dtb_pa);
+
+static int __init print_nokaslr(char *p)
+{
+	pr_info("Disabled KASLR");
+	return 0;
+}
+early_param("nokaslr", print_nokaslr);
+
+unsigned long kaslr_offset(void)
+{
+	return kernel_map.virt_offset;
+}
+#endif
+
 asmlinkage void __init setup_vm(uintptr_t dtb_pa)
 {
 	pmd_t __maybe_unused fix_bmap_spmd, fix_bmap_epmd;
 
-	kernel_map.virt_addr = KERNEL_LINK_ADDR;
+#ifdef CONFIG_RANDOMIZE_BASE
+	if (!__pi_set_nokaslr_from_cmdline(dtb_pa)) {
+		u64 kaslr_seed = __pi_get_kaslr_seed(dtb_pa);
+		u32 kernel_size = (uintptr_t)(&_end) - (uintptr_t)(&_start);
+		u32 nr_pos;
+
+		/*
+		 * Compute the number of positions available: we are limited
+		 * by the early page table that only has one PUD and we must
+		 * be aligned on PMD_SIZE.
+		 */
+		nr_pos = (PUD_SIZE - kernel_size) / PMD_SIZE;
+
+		kernel_map.virt_offset = (kaslr_seed % nr_pos) * PMD_SIZE;
+	}
+#endif
+
+	kernel_map.virt_addr = KERNEL_LINK_ADDR + kernel_map.virt_offset;
 	kernel_map.page_offset = _AC(CONFIG_PAGE_OFFSET, UL);
 
 #ifdef CONFIG_XIP_KERNEL
-- 
2.37.2