Re: [PATCH v2] riscv: don't allow write but not read page mapping request in mmap

[Conor.Dooley]

On 13/06/2022 06:20, Celeste Liu wrote:
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> 
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> 
> When xctan tries to run one of libaio's tests
> (https://pagure.io/libaio/blob/1b18bfafc6a2f7b9fa2c6be77a95afed8b7be448/f/harness/cases/5.t),
> it encounters a strange behavior: for the same PROT_WRITE only mapping,
> there was a discrepancy in whether it could be read before and after writing
> (readable before writing, unreadable after writing). After some investigation,
> I found that mmap allows write only mapping, an undefined behavior, on RISC-V.
> 
> As mentioned in Table 4.5 in RISC-V spec Volume 2 Section 4.3 version
> "20211203 Privileged Architecture v1.12, Ratified"[1], the PTE permission
> bit combination of "write+!read" is "Reserved for future use.". Hence, don't
> allow such mapping request in mmap call. In the current code[2], write+exec
> only is marked as invalid, but write only is not marked as invalid.
> 
> This patch refines that judgment.
> 
> [1]: https://github.com/riscv/riscv-isa-manual/releases/download/Priv-v1.12/riscv-privileged-20211203.pdf
> [2]: modified in commit e0d17c842c0f824fd4df9f4688709fc6907201e1
>       (https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e0d17c842c0f824fd4df9f4688709fc6907201e1)
> 
> Reported-by: xctan <xc-tan@xxxxxxxxxxx>
> Co-developed-by: dram <dramforever@xxxxxxxx>
> Signed-off-by: dram <dramforever@xxxxxxxx>

Those don't look like "real" names?
https://www.kernel.org/doc/html/latest/process/submitting-patches.html#sign-your-work-the-developer-s-certificate-of-origin

> Co-developed-by: Ruizhe Pan <c141028@xxxxxxxxx>
> Signed-off-by: Ruizhe Pan <c141028@xxxxxxxxx>
> Signed-off-by: Celeste Liu <coelacanthus@xxxxxxxxxxx>
> Cc: linux-riscv@xxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: Guo Ren <guoren@xxxxxxxxxx>
> Cc: Yash Shah <yash.shah@xxxxxxxxxx>
> ---
> v2: This version adds a link to the referenced spec, and reference of the
> previous related modification.
> 
>   arch/riscv/kernel/sys_riscv.c | 5 ++---
>   1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
> index 9c0194f176fc..571556bb9261 100644
> --- a/arch/riscv/kernel/sys_riscv.c
> +++ b/arch/riscv/kernel/sys_riscv.c
> @@ -18,9 +18,8 @@ static long riscv_sys_mmap(unsigned long addr, unsigned long len,
>          if (unlikely(offset & (~PAGE_MASK >> page_shift_offset)))
>                  return -EINVAL;
> 
> -       if ((prot & PROT_WRITE) && (prot & PROT_EXEC))
> -               if (unlikely(!(prot & PROT_READ)))
> -                       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));
> --
> 2.36.1
> 
> 
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