Re: [PATCH RFC] riscv: Map the kernel with correct permissions the first time

Alex Ghiti
2021-05-20T05:11:43Z

Hi Jisheng,

On 19/05/2021 18:29, Jisheng Zhang wrote:
> On Tue, 18 May 2021 17:21:34 +0200
> Alexandre Ghiti <alex@ghiti.fr> wrote:
>
>> For 64b kernels, we map all the kernel with write and execute permissions
>> and afterwards remove writability from text and executability from data.
>>
>> For 32b kernels, the kernel mapping resides in the linear mapping, so we
>> map all the linear mapping as writable and executable and afterwards we
>> remove those properties for unused memory and kernel mapping as
>> described above.
>>
>> Change this behavior to directly map the kernel with correct permissions
>> and avoid going through the whole mapping to fix the permissions.
>>
>> At the same time, this fixes an issue introduced by commit 2bfc6cd81bd1
>> ("riscv: Move kernel mapping outside of linear mapping") as reported
>> here https://github.com/starfive-tech/linux/issues/17.
>>
>> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
>> ---
>>
>> This patchset was tested on:
>>
>> * kernel:
>> - rv32 with CONFIG_STRICT_KERNEL_RWX: OK
>> - rv32 without CONFIG_STRICT_KERNEL_RWX: OK
>> - rv64 with CONFIG_STRICT_KERNEL_RWX: OK
>> - rv64 without CONFIG_STRICT_KERNEL_RWX: OK
>>
>> * xipkernel:
>> - rv32: build only
>> - rv64: OK
>>
>> arch/riscv/include/asm/set_memory.h | 2 -
>> arch/riscv/kernel/setup.c | 1 -
>> arch/riscv/mm/init.c | 80 ++++++++++++++---------------
>> 3 files changed, 38 insertions(+), 45 deletions(-)
>>
>> diff --git a/arch/riscv/include/asm/set_memory.h b/arch/riscv/include/asm/set_memory.h
>> index 086f757e8ba3..70154f012791 100644
>> --- a/arch/riscv/include/asm/set_memory.h
>> +++ b/arch/riscv/include/asm/set_memory.h
>> @@ -16,13 +16,11 @@ int set_memory_rw(unsigned long addr, int numpages);
>> int set_memory_x(unsigned long addr, int numpages);
>> int set_memory_nx(unsigned long addr, int numpages);
>> int set_memory_rw_nx(unsigned long addr, int numpages);
>> -void protect_kernel_text_data(void);
>> #else
>> static inline int set_memory_ro(unsigned long addr, int numpages) { return 0; }
>> static inline int set_memory_rw(unsigned long addr, int numpages) { return 0; }
>> static inline int set_memory_x(unsigned long addr, int numpages) { return 0; }
>> static inline int set_memory_nx(unsigned long addr, int numpages) { return 0; }
>> -static inline void protect_kernel_text_data(void) {}
>> static inline int set_memory_rw_nx(unsigned long addr, int numpages) { return 0; }
>> #endif
>>
>> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
>> index 03901d3a8b02..1eb50e512056 100644
>> --- a/arch/riscv/kernel/setup.c
>> +++ b/arch/riscv/kernel/setup.c
>> @@ -292,7 +292,6 @@ void __init setup_arch(char **cmdline_p)
>> sbi_init();
>>
>> if (IS_ENABLED(CONFIG_STRICT_KERNEL_RWX)) {
>> - protect_kernel_text_data();
>> protect_kernel_linear_mapping_text_rodata();
>
> If we extend the idea a bit, I.E create the correct permission for alias line
> mapping at the first time, we can remove protect_kernel_linear_mapping_text_rodata()
>

Yes, I agree, I will do that.

> PS: No matter whether it's fine to extend, the set_memory_nx() calling in
> protect_kernel_linear_mapping_text_rodata() is not necessary since the linear
> mapping for 64bit is NX from the beginning.

You're totally right, that will be fixed when removing
protect_kernel_linear_mapping_text_rodata() entirely.

>
>> }
>>
>> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
>> index 4faf8bd157ea..92b3184420a2 100644
>> --- a/arch/riscv/mm/init.c
>> +++ b/arch/riscv/mm/init.c
>> @@ -436,6 +436,36 @@ asmlinkage void __init __copy_data(void)
>> }
>> #endif
>>
>> +#ifdef CONFIG_STRICT_KERNEL_RWX
>> +#define is_text_va(va) ({ \
>> + unsigned long _va = va; \
>> + (_va < (unsigned long)__init_data_begin && _va >= (unsigned long)_start); \
>> + })
>
> It's better if is_text_va() is a inline function
>

Ok, will do.

>> +
>> +static inline __init pgprot_t pgprot_from_kernel_va(uintptr_t va)
>
> I'm not sure whether it's necessary to put __init marker to inline functions
> There are such issues in current riscv code, I planed to cook one patch
> to remove __init from inline functions.
>
>
>> +{
>> + return is_text_va(va) ? PAGE_KERNEL_READ_EXEC : PAGE_KERNEL;
>
> if we extend the idea, then here we may return PAGE_KERNEL_READ, PAGE_KERNEL
> and PAGE_KERNEL_READ_EXEC correspondingly.

Again, I agree :)

I will come up with an enhanced v2 soon,

Thanks for your comments,

Alex

>
> Thanks
>
>> +}
>> +
>> +void mark_rodata_ro(void)
>> +{
>> + unsigned long rodata_start = (unsigned long)__start_rodata;
>> + unsigned long data_start = (unsigned long)_data;
>> +
>> + set_memory_ro(rodata_start, (data_start - rodata_start) >> PAGE_SHIFT);
>> +
>> + debug_checkwx();
>> +}
>> +#else
>> +static inline __init pgprot_t pgprot_from_kernel_va(uintptr_t va)
>> +{
>> + if (IS_ENABLED(CONFIG_32BIT))
>> + return PAGE_KERNEL_EXEC;
>> +
>> + return (va < kernel_virt_addr) ? PAGE_KERNEL : PAGE_KERNEL_EXEC;
>> +}
>> +#endif
>> +
>> /*
>> * setup_vm() is called from head.S with MMU-off.
>> *
>> @@ -465,7 +495,8 @@ uintptr_t xiprom, xiprom_sz;
>> #define xiprom_sz (*((uintptr_t *)XIP_FIXUP(&xiprom_sz)))
>> #define xiprom (*((uintptr_t *)XIP_FIXUP(&xiprom)))
>>
>> -static void __init create_kernel_page_table(pgd_t *pgdir, uintptr_t map_size)
>> +static void __init create_kernel_page_table(pgd_t *pgdir, uintptr_t map_size,
>> + __always_unused bool early)
>> {
>> uintptr_t va, end_va;
>>
>> @@ -484,7 +515,7 @@ static void __init create_kernel_page_table(pgd_t *pgdir, uintptr_t map_size)
>> map_size, PAGE_KERNEL);
>> }
>> #else
>> -static void __init create_kernel_page_table(pgd_t *pgdir, uintptr_t map_size)
>> +static void __init create_kernel_page_table(pgd_t *pgdir, uintptr_t map_size, bool early)
>> {
>> uintptr_t va, end_va;
>>
>> @@ -492,7 +523,7 @@ static void __init create_kernel_page_table(pgd_t *pgdir, uintptr_t map_size)
>> for (va = kernel_virt_addr; va < end_va; va += map_size)
>> create_pgd_mapping(pgdir, va,
>> load_pa + (va - kernel_virt_addr),
>> - map_size, PAGE_KERNEL_EXEC);
>> + map_size, early ? PAGE_KERNEL_EXEC : pgprot_from_kernel_va(va));
>> }
>> #endif
>>
>> @@ -569,7 +600,7 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>> * us to reach paging_init(). We map all memory banks later
>> * in setup_vm_final() below.
>> */
>> - create_kernel_page_table(early_pg_dir, map_size);
>> + create_kernel_page_table(early_pg_dir, map_size, true);
>>
>> #ifndef __PAGETABLE_PMD_FOLDED
>> /* Setup early PMD for DTB */
>> @@ -693,21 +724,15 @@ static void __init setup_vm_final(void)
>> map_size = best_map_size(start, end - start);
>> for (pa = start; pa < end; pa += map_size) {
>> va = (uintptr_t)__va(pa);
>> - create_pgd_mapping(swapper_pg_dir, va, pa,
>> - map_size,
>> -#ifdef CONFIG_64BIT
>> - PAGE_KERNEL
>> -#else
>> - PAGE_KERNEL_EXEC
>> -#endif
>> - );
>>
>> + create_pgd_mapping(swapper_pg_dir, va, pa, map_size,
>> + pgprot_from_kernel_va(va));
>> }
>> }
>>
>> #ifdef CONFIG_64BIT
>> /* Map the kernel */
>> - create_kernel_page_table(swapper_pg_dir, PMD_SIZE);
>> + create_kernel_page_table(swapper_pg_dir, PMD_SIZE, false);
>> #endif
>>
>> /* Clear fixmap PTE and PMD mappings */
>> @@ -738,35 +763,6 @@ static inline void setup_vm_final(void)
>> }
>> #endif /* CONFIG_MMU */
>>
>> -#ifdef CONFIG_STRICT_KERNEL_RWX
>> -void __init protect_kernel_text_data(void)
>> -{
>> - unsigned long text_start = (unsigned long)_start;
>> - unsigned long init_text_start = (unsigned long)__init_text_begin;
>> - unsigned long init_data_start = (unsigned long)__init_data_begin;
>> - unsigned long rodata_start = (unsigned long)__start_rodata;
>> - unsigned long data_start = (unsigned long)_data;
>> - unsigned long max_low = (unsigned long)(__va(PFN_PHYS(max_low_pfn)));
>> -
>> - set_memory_ro(text_start, (init_text_start - text_start) >> PAGE_SHIFT);
>> - set_memory_ro(init_text_start, (init_data_start - init_text_start) >> PAGE_SHIFT);
>> - set_memory_nx(init_data_start, (rodata_start - init_data_start) >> PAGE_SHIFT);
>> - /* rodata section is marked readonly in mark_rodata_ro */
>> - set_memory_nx(rodata_start, (data_start - rodata_start) >> PAGE_SHIFT);
>> - set_memory_nx(data_start, (max_low - data_start) >> PAGE_SHIFT);
>> -}
>> -
>> -void mark_rodata_ro(void)
>> -{
>> - unsigned long rodata_start = (unsigned long)__start_rodata;
>> - unsigned long data_start = (unsigned long)_data;
>> -
>> - set_memory_ro(rodata_start, (data_start - rodata_start) >> PAGE_SHIFT);
>> -
>> - debug_checkwx();
>> -}
>> -#endif
>> -
>> #ifdef CONFIG_KEXEC_CORE
>> /*
>> * reserve_crashkernel() - reserves memory for crash kernel
>
>
>
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