linux-kernel - Re: [PATCH v3 10/29] riscv/mm : ensure PROT_WRITE leads to VM_READ

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Date: Sun, 12 May 2024 18:24:45 +0200
From: Alexandre Ghiti <alex@...ti.fr>
To: Deepak Gupta <debug@...osinc.com>, paul.walmsley@...ive.com,
 rick.p.edgecombe@...el.com, broonie@...nel.org, Szabolcs.Nagy@....com,
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Cc: linux-doc@...r.kernel.org, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
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Subject: Re: [PATCH v3 10/29] riscv/mm : ensure PROT_WRITE leads to VM_READ |
 VM_WRITE

Hi Deepak,

On 04/04/2024 01:34, 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@...osinc.com>
> ---
>   arch/riscv/include/asm/mman.h    | 24 ++++++++++++++++++++++++
>   arch/riscv/include/asm/pgtable.h |  1 +
>   arch/riscv/kernel/sys_riscv.c    | 11 +++++++++++
>   arch/riscv/mm/init.c             |  2 +-
>   mm/mmap.c                        |  1 +
>   5 files changed, 38 insertions(+), 1 deletion(-)
>   create mode 100644 arch/riscv/include/asm/mman.h
>
> 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 6066822e7396..4d5983bc6766 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -184,6 +184,7 @@ extern struct pt_alloc_ops pt_ops __initdata;
>   #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 f1c1416a9f1e..846c36b1b3d5 100644
> --- a/arch/riscv/kernel/sys_riscv.c
> +++ b/arch/riscv/kernel/sys_riscv.c
> @@ -8,6 +8,8 @@
>   #include <linux/syscalls.h>
>   #include <asm/cacheflush.h>
>   #include <asm-generic/mman-common.h>
> +#include <vdso/vsyscall.h>
> +#include <asm/mman.h>
>   
>   static long riscv_sys_mmap(unsigned long addr, unsigned long len,
>   			   unsigned long prot, unsigned long flags,
> @@ -17,6 +19,15 @@ static long riscv_sys_mmap(unsigned long addr, unsigned long len,
>   	if (unlikely(offset & (~PAGE_MASK >> page_shift_offset)))
>   		return -EINVAL;
>   
> +	/*
> +	 * If only 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 fa34cf55037b..98e5ece4052a 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -299,7 +299,7 @@ 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 d89770eaab6b..57a974f49b00 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>
>   
>   #include <linux/uaccess.h>
>   #include <asm/cacheflush.h>


What happens if someone restricts the permission to PROT_WRITE using 
mprotect()? I would say this is an issue since it would turn the pages 
into shadow stack pages.