[RFC PATCH 07/12] riscv/mm: prepare shadow stack for init task for kernel cfi

Deepak Gupta
2024-04-09T06:12:12Z

Under CONFIG_SHADOW_CALL_STACK, shadow call stack goes into data section.
Although with CONFIG_DYNAMIC_SCS on riscv, hardware assisted shadow stack
are used. Hardware assisted shadow stack on riscv uses PTE.R=0, PTE.W=1 &
PTE.X=0 encodings. Without CONFIG_DYNAMIC_SCS, shadow stack for init is
placed in data section and thus regular read/write encodings are applied
to it. Although with with CONFIG_DYNAMIC_SCS, they need to go into
different section. This change places it into `.shadowstack` section.
As part of this change early boot code (`setup_vm`), applies appropriate
PTE encodings to shadow call stack for init placed in `.shadowstack`
section.

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
---
arch/riscv/include/asm/pgtable.h | 4 ++++
arch/riscv/include/asm/sections.h | 22 +++++++++++++++++++++
arch/riscv/include/asm/thread_info.h | 10 ++++++++--
arch/riscv/kernel/vmlinux.lds.S | 12 ++++++++++++
arch/riscv/mm/init.c | 29 +++++++++++++++++++++-------
5 files changed, 68 insertions(+), 9 deletions(-)

diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
index 9f8ea0e33eb1..3409b250390d 100644
--- a/arch/riscv/include/asm/pgtable.h
+++ b/arch/riscv/include/asm/pgtable.h
@@ -197,6 +197,10 @@ extern struct pt_alloc_ops pt_ops __initdata;
#define PAGE_KERNEL_READ_EXEC __pgprot((_PAGE_KERNEL & ~_PAGE_WRITE) \
| _PAGE_EXEC)

+#ifdef CONFIG_DYNAMIC_SCS
+#define PAGE_KERNEL_SHADOWSTACK __pgprot(_PAGE_KERNEL & ~(_PAGE_READ | _PAGE_EXEC))
+#endif
+
#define PAGE_TABLE __pgprot(_PAGE_TABLE)

#define _PAGE_IOREMAP ((_PAGE_KERNEL & ~_PAGE_MTMASK) | _PAGE_IO)
diff --git a/arch/riscv/include/asm/sections.h b/arch/riscv/include/asm/sections.h
index a393d5035c54..4c4154d0021e 100644
--- a/arch/riscv/include/asm/sections.h
+++ b/arch/riscv/include/asm/sections.h
@@ -14,6 +14,10 @@ extern char __init_data_begin[], __init_data_end[];
extern char __init_text_begin[], __init_text_end[];
extern char __alt_start[], __alt_end[];
extern char __exittext_begin[], __exittext_end[];
+#ifdef CONFIG_DYNAMIC_SCS
+extern char __init_shstk_start[], __init_shstk_end[];
+#endif
+extern char __end_srodata[];

static inline bool is_va_kernel_text(uintptr_t va)
{
@@ -31,4 +35,22 @@ static inline bool is_va_kernel_lm_alias_text(uintptr_t va)
return va >= start && va < end;
}

+#ifdef CONFIG_DYNAMIC_SCS
+static inline bool is_va_init_shadow_stack_early(uintptr_t va)
+{
+ uintptr_t start = (uintptr_t)(kernel_mapping_pa_to_va(__init_shstk_start));
+ uintptr_t end = (uintptr_t)(kernel_mapping_pa_to_va(__init_shstk_end));
+
+ return va >= start && va < end;
+}
+
+static inline bool is_va_init_shadow_stack(uintptr_t va)
+{
+ uintptr_t start = (uintptr_t)(__init_shstk_start);
+ uintptr_t end = (uintptr_t)(__init_shstk_end);
+
+ return va >= start && va < end;
+}
+#endif
+
#endif /* __ASM_SECTIONS_H */
diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
index 5d473343634b..7ae28d627f84 100644
--- a/arch/riscv/include/asm/thread_info.h
+++ b/arch/riscv/include/asm/thread_info.h
@@ -63,12 +63,18 @@ struct thread_info {
};

#ifdef CONFIG_SHADOW_CALL_STACK
+#ifdef CONFIG_DYNAMIC_SCS
#define INIT_SCS \
- .scs_base = init_shadow_call_stack, \
+ .scs_base = init_shadow_call_stack, \
+ .scs_sp = &init_shadow_call_stack[SCS_SIZE / sizeof(long)],
+#else
+#define INIT_SCS \
+ .scs_base = init_shadow_call_stack, \
.scs_sp = init_shadow_call_stack,
+#endif /* CONFIG_DYNAMIC_SCS */
#else
#define INIT_SCS
-#endif
+#endif /* CONFIG_SHADOW_CALL_STACK */

/*
* macros/functions for gaining access to the thread information structure
diff --git a/arch/riscv/kernel/vmlinux.lds.S b/arch/riscv/kernel/vmlinux.lds.S
index 002ca58dd998..cccc51f845ab 100644
--- a/arch/riscv/kernel/vmlinux.lds.S
+++ b/arch/riscv/kernel/vmlinux.lds.S
@@ -126,6 +126,18 @@ SECTIONS
*(.srodata*)
}

+ . = ALIGN(SECTION_ALIGN);
+ __end_srodata = .;
+
+#ifdef CONFIG_DYNAMIC_SCS
+ .shadowstack : AT(ADDR(.shadowstack) - LOAD_OFFSET){
+ __init_shstk_start = .;
+ KEEP(*(.shadowstack..init))
+ . = __init_shstk_start + PAGE_SIZE;
+ __init_shstk_end = .;
+ }
+#endif
+
. = ALIGN(SECTION_ALIGN);
_data = .;

diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index fe8e159394d8..5b6f0cfa5719 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -713,14 +713,22 @@ static __init pgprot_t pgprot_from_va(uintptr_t va)
if (IS_ENABLED(CONFIG_64BIT) && is_va_kernel_lm_alias_text(va))
return PAGE_KERNEL_READ;

+#ifdef CONFIG_DYNAMIC_SCS
+ /* If init task's shadow stack va, return write only page protections */
+ if (IS_ENABLED(CONFIG_64BIT) && is_va_init_shadow_stack(va)) {
+ pr_info("Shadow stack protections are being applied to for init\n");
+ return PAGE_KERNEL_SHADOWSTACK;
+ }
+#endif
+
return PAGE_KERNEL;
}

void mark_rodata_ro(void)
{
- set_kernel_memory(__start_rodata, _data, set_memory_ro);
+ set_kernel_memory(__start_rodata, __end_srodata, set_memory_ro);
if (IS_ENABLED(CONFIG_64BIT))
- set_kernel_memory(lm_alias(__start_rodata), lm_alias(_data),
+ set_kernel_memory(lm_alias(__start_rodata), lm_alias(__end_srodata),
set_memory_ro);
}
#else
@@ -913,14 +921,21 @@ static void __init create_kernel_page_table(pgd_t *pgdir,
static void __init create_kernel_page_table(pgd_t *pgdir, bool early)
{
uintptr_t va, end_va;
+ pgprot_t prot;

end_va = kernel_map.virt_addr + kernel_map.size;
- for (va = kernel_map.virt_addr; va < end_va; va += PMD_SIZE)
+ for (va = kernel_map.virt_addr; va < end_va; va += PMD_SIZE) {
+ prot = PAGE_KERNEL_EXEC;
+#ifdef CONFIG_DYNAMIC_SCS
+ if (early && is_va_init_shadow_stack_early(va))
+ prot = PAGE_KERNEL_SHADOWSTACK;
+#endif
create_pgd_mapping(pgdir, va,
- kernel_map.phys_addr + (va - kernel_map.virt_addr),
- PMD_SIZE,
- early ?
- PAGE_KERNEL_EXEC : pgprot_from_va(va));
+ kernel_map.phys_addr + (va - kernel_map.virt_addr),
+ PMD_SIZE,
+ early ?
+ prot : pgprot_from_va(va));
+ }
}
#endif

--
2.43.2