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    Subject[PATCH v1 RFC Zisslpcfi 06/20] riscv: Implementing "PROT_SHADOWSTACK" on riscv
    Date
    This patchimplements new mmap protection flag "PROT_SHADOWSTACK" on riscv

    Zisslpcfi extension on riscv uses R=0, W=1, X=0 as shadow stack PTE
    encoding. This encoding is reserved if Zisslpcfi is not implemented or
    backward cfi is not enabled for the respective mode.

    Signed-off-by: Deepak Gupta <debug@rivosinc.com>
    ---
    arch/riscv/include/asm/mman.h | 19 +++++++++++++++++++
    arch/riscv/include/asm/pgtable.h | 1 +
    arch/riscv/kernel/sys_riscv.c | 22 ++++++++++++++++++++++
    arch/riscv/mm/init.c | 2 +-
    4 files changed, 43 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..9c8499294a60
    --- /dev/null
    +++ b/arch/riscv/include/asm/mman.h
    @@ -0,0 +1,19 @@
    +/* 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;
    +
    + ret = (prot & PROT_SHADOWSTACK)?VM_WRITE:0;
    + 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 4eba9a98d0e3..74dbe122f2fa 100644
    --- a/arch/riscv/include/asm/pgtable.h
    +++ b/arch/riscv/include/asm/pgtable.h
    @@ -159,6 +159,7 @@ 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_EXEC
    diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
    index 5d3f2fbeb33c..c3cf6b94c710 100644
    --- a/arch/riscv/kernel/sys_riscv.c
    +++ b/arch/riscv/kernel/sys_riscv.c
    @@ -18,6 +18,28 @@ 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 specify PROT_SHADOWSTACK
    + * protection
    + */
    + if (unlikely((prot & PROT_WRITE) && !(prot & PROT_READ)))
    + prot |= PROT_READ;
    +
    + /*
    + * PROT_SHADOWSTACK is new protection flag. If specified with other like PROT_WRITE or
    + * PROT_READ PROT_SHADOWSTACK takes precedence. We can do either of following
    + * - ensure no other protection flags are specified along with it and return EINVAL
    + * OR
    + * - ensure we clear other protection flags.
    + * Choosing to follow former, if any other bit is set in prot, we return EINVAL
    + * Other architectures can treat different combinations for PROT_SHADOWSTACK
    + */
    + if (unlikely((prot & PROT_SHADOWSTACK) && (prot & ~PROT_SHADOWSTACK)))
    + return -EINVAL;
    +
    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 478d6763a01a..ba8138c90450 100644
    --- a/arch/riscv/mm/init.c
    +++ b/arch/riscv/mm/init.c
    @@ -294,7 +294,7 @@ static pmd_t __maybe_unused early_dtb_pmd[PTRS_PER_PMD] __initdata __aligned(PAG
    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,
    --
    2.25.1
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