[PATCH v3 2/2] riscv: Allow PROT_WRITE-only mmap()

[Andrew Bresticker]

On Thu, Sep 15, 2022 at 12:56 PM <Conor.Dooley at microchip.com> wrote:
>
> On 09/09/2022 22:27, Andrew Bresticker wrote:
> > EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> >
> > Commit 2139619bcad7 ("riscv: mmap with PROT_WRITE but no PROT_READ is
> > invalid") made mmap() return EINVAL if PROT_WRITE was set wihtout
> > PROT_READ with the justification that a write-only PTE is considered a
> > reserved PTE permission bit pattern in the privileged spec. This check
> > is unnecessary since we let VM_WRITE imply VM_READ on RISC-V, and it is
> > inconsistent with other architectures that don't support write-only PTEs,
> > creating a potential software portability issue. Just remove the check
> > altogether and let PROT_WRITE imply PROT_READ as is the case on other
> > architectures.
> >
> > Note that this also allows PROT_WRITE|PROT_EXEC mappings which were
> > disallowed prior to the aforementioned commit; PROT_READ is implied in
> > such mappings as well.
> >
> > Fixes: 2139619bcad7 ("riscv: mmap with PROT_WRITE but no PROT_READ is invalid")
>
> For the naive members of the audience such as myself, this patch
> came after a non-fixes patch in the series. What is the dependence
> of this patch on the other one (if any)?

This patch is dependent on the first. Happy to re-spin with a "Fixes"
tag on the first patch (or maybe Palmer can add when applying).

-Andrew

> Thanks,
> Conor.
>
> > Reviewed-by: Atish Patra <atishp at rivosinc.com>
> > Signed-off-by: Andrew Bresticker <abrestic at rivosinc.com>
> > ---
> > v1 -> v2: Update access_error() to account for write-implies-read
> > v2 -> v3: Separate into two commits
> > ---
> >  arch/riscv/kernel/sys_riscv.c | 3 ---
> >  1 file changed, 3 deletions(-)
> >
> > diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
> > index 571556bb9261..5d3f2fbeb33c 100644
> > --- a/arch/riscv/kernel/sys_riscv.c
> > +++ b/arch/riscv/kernel/sys_riscv.c
> > @@ -18,9 +18,6 @@ static long riscv_sys_mmap(unsigned long addr, unsigned long len,
> >         if (unlikely(offset & (~PAGE_MASK >> page_shift_offset)))
> >                 return -EINVAL;
> >
> > -       if (unlikely((prot & PROT_WRITE) && !(prot & PROT_READ)))
> > -               return -EINVAL;
> > -
> >         return ksys_mmap_pgoff(addr, len, prot, flags, fd,
> >                                offset >> (PAGE_SHIFT - page_shift_offset));
> >  }
> > --
> > 2.25.1
> >
> >
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>