Re: [PATCH v12 06/28] riscv/mm : ensure PROT_WRITE leads to VM_READ | VM_WRITE

Radim Krčmář
2025-04-10T10:03:47Z

2025-03-14T14:39:25-07:00, Deepak Gupta <debug@rivosinc.com>:
> diff --git a/arch/riscv/include/asm/mman.h b/arch/riscv/include/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.
> + */

This function also changes PROT_WX to VM_RWX, which is effectively not
changing anything, but I think it deserves an explicit intent.
(At least in the commit message.)

> + if (prot & PROT_WRITE)
> + ret = (VM_READ | VM_WRITE);
> + return ret;
> +}
> diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
> @@ -16,6 +17,15 @@ static long riscv_sys_mmap(unsigned long addr, unsigned long len,
> + /*
> + * 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;

Why isn't the previous hunk be enough? (Or why don't we do just this?)

riscv_sys_mmap() eventually calls arch_calc_vm_prot_bits(), so I'd
rather fix each code path just once.

Thanks.