Re: [PATCH v6 11/33] riscv/mm : ensure PROT_WRITE leads to VM_READ | VM_WRITE

[Deepak Gupta <debug@rivosinc.com>]

On Wed, Oct 09, 2024 at 02:36:12PM +0100, Lorenzo Stoakes wrote:
>On Tue, Oct 08, 2024 at 03:36:53PM -0700, Deepak Gupta wrote:
>> `arch_calc_vm_prot_bits` is implemented on risc-v to return VM_READ |
>> VM_WRITE if PROT_WRITE is specified. Similarly `riscv_sys_mmap` is
>> updated to convert all incoming PROT_WRITE to (PROT_WRITE | PROT_READ).
>> This is to make sure that any existing apps using PROT_WRITE still work.
>>
>> Earlier `protection_map[VM_WRITE]` used to pick read-write PTE encodings.
>> Now `protection_map[VM_WRITE]` will always pick PAGE_SHADOWSTACK PTE
>> encodings for shadow stack. Above changes ensure that existing apps
>> continue to work because underneath kernel will be picking
>> `protection_map[VM_WRITE|VM_READ]` PTE encodings.
>>
>> Signed-off-by: Deepak Gupta <debug@rivosinc.com>
>> ---
>>  arch/riscv/include/asm/mman.h    | 24 ++++++++++++++++++++++++
>>  arch/riscv/include/asm/pgtable.h |  1 +
>>  arch/riscv/kernel/sys_riscv.c    | 10 ++++++++++
>>  arch/riscv/mm/init.c             |  2 +-
>>  mm/mmap.c                        |  1 +
>>  5 files changed, 37 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/riscv/include/asm/mman.h b/arch/riscv/include/asm/mman.h
>> new file mode 100644
>> index 000000000000..ef9fedf32546
>> --- /dev/null
>> +++ b/arch/riscv/include/asm/mman.h
>> @@ -0,0 +1,24 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +#ifndef __ASM_MMAN_H__
>> +#define __ASM_MMAN_H__
>> +
>> +#include <linux/compiler.h>
>> +#include <linux/types.h>
>> +#include <uapi/asm/mman.h>
>> +
>> +static inline unsigned long arch_calc_vm_prot_bits(unsigned long prot,
>> +	unsigned long pkey __always_unused)
>> +{
>> +	unsigned long ret = 0;
>> +
>> +	/*
>> +	 * If PROT_WRITE was specified, force it to VM_READ | VM_WRITE.
>> +	 * Only VM_WRITE means shadow stack.
>> +	 */
>> +	if (prot & PROT_WRITE)
>> +		ret = (VM_READ | VM_WRITE);
>> +	return ret;
>> +}
>> +#define arch_calc_vm_prot_bits(prot, pkey) arch_calc_vm_prot_bits(prot, pkey)
>> +
>> +#endif /* ! __ASM_MMAN_H__ */
>> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
>> index e79f15293492..4948a1f18ae8 100644
>> --- a/arch/riscv/include/asm/pgtable.h
>> +++ b/arch/riscv/include/asm/pgtable.h
>> @@ -177,6 +177,7 @@ extern struct pt_alloc_ops pt_ops __meminitdata;
>>  #define PAGE_READ_EXEC		__pgprot(_PAGE_BASE | _PAGE_READ | _PAGE_EXEC)
>>  #define PAGE_WRITE_EXEC		__pgprot(_PAGE_BASE | _PAGE_READ |	\
>>  					 _PAGE_EXEC | _PAGE_WRITE)
>> +#define PAGE_SHADOWSTACK       __pgprot(_PAGE_BASE | _PAGE_WRITE)
>>
>>  #define PAGE_COPY		PAGE_READ
>>  #define PAGE_COPY_EXEC		PAGE_READ_EXEC
>> diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
>> index d77afe05578f..43a448bf254b 100644
>> --- a/arch/riscv/kernel/sys_riscv.c
>> +++ b/arch/riscv/kernel/sys_riscv.c
>> @@ -7,6 +7,7 @@
>>
>>  #include <linux/syscalls.h>
>>  #include <asm/cacheflush.h>
>> +#include <asm-generic/mman-common.h>
>>
>>  static long riscv_sys_mmap(unsigned long addr, unsigned long len,
>>  			   unsigned long prot, unsigned long flags,
>> @@ -16,6 +17,15 @@ static long riscv_sys_mmap(unsigned long addr, unsigned long len,
>>  	if (unlikely(offset & (~PAGE_MASK >> page_shift_offset)))
>>  		return -EINVAL;
>>
>> +	/*
>> +	 * If PROT_WRITE is specified then extend that to PROT_READ
>> +	 * protection_map[VM_WRITE] is now going to select shadow stack encodings.
>> +	 * So specifying PROT_WRITE actually should select protection_map [VM_WRITE | VM_READ]
>> +	 * If user wants to create shadow stack then they should use `map_shadow_stack` syscall.
>> +	 */
>> +	if (unlikely((prot & PROT_WRITE) && !(prot & PROT_READ)))
>> +		prot |= PROT_READ;
>> +
>>  	return ksys_mmap_pgoff(addr, len, prot, flags, fd,
>>  			       offset >> (PAGE_SHIFT - page_shift_offset));
>>  }
>> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
>> index 0e8c20adcd98..964810aeb405 100644
>> --- a/arch/riscv/mm/init.c
>> +++ b/arch/riscv/mm/init.c
>> @@ -326,7 +326,7 @@ pgd_t early_pg_dir[PTRS_PER_PGD] __initdata __aligned(PAGE_SIZE);
>>  static const pgprot_t protection_map[16] = {
>>  	[VM_NONE]					= PAGE_NONE,
>>  	[VM_READ]					= PAGE_READ,
>> -	[VM_WRITE]					= PAGE_COPY,
>> +	[VM_WRITE]					= PAGE_SHADOWSTACK,
>>  	[VM_WRITE | VM_READ]				= PAGE_COPY,
>>  	[VM_EXEC]					= PAGE_EXEC,
>>  	[VM_EXEC | VM_READ]				= PAGE_READ_EXEC,
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index dd4b35a25aeb..b56f1e8cbfc6 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -47,6 +47,7 @@
>>  #include <linux/oom.h>
>>  #include <linux/sched/mm.h>
>>  #include <linux/ksm.h>
>> +#include <linux/processor.h>
>
>This seems benign enough, just wonder why you need it?

I think leftover from previous versions. Will remove it.
Don't think its needed here anymore.

>
>>
>>  #include <linux/uaccess.h>
>>  #include <asm/cacheflush.h>
>>
>> --
>> 2.45.0
>>