Re: [PATCH] riscv/mm: Prevent kernel module access user-space memory without uaccess routines

[Pekka Enberg]

On Mon, Nov 30, 2020 at 7:33 AM Eric Lin <tesheng@xxxxxxxxxxxxx> wrote:
>
> In the page fault handler, an access to user-space memory
> without get/put_user() or copy_from/to_user() routines is
> not resolved properly. Like arm and other architectures,
> we need to let it die earlier in page fault handler.

Fix looks good to me. Can you elaborate on how you found the issue and
how the bug manifests itself?

>
> Signed-off-by: Eric Lin <tesheng@xxxxxxxxxxxxx>
> Cc: Alan Kao <alankao@xxxxxxxxxxxxx>
> ---
>  arch/riscv/mm/fault.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
> index 3c8b9e433c67..a452cfa266a2 100644
> --- a/arch/riscv/mm/fault.c
> +++ b/arch/riscv/mm/fault.c
> @@ -232,6 +232,9 @@ asmlinkage void do_page_fault(struct pt_regs *regs)
>         if (user_mode(regs))
>                 flags |= FAULT_FLAG_USER;
>
> +       if (!user_mode(regs) && addr < TASK_SIZE && unlikely(!(regs->status & SR_SUM)))
> +               die(regs, "Accessing user space memory without uaccess routines\n");

Let's introduce a die_kernel_fault() helper (similar to arm64, for
example) to ensure same semantics for the different kernel faults. You
can extract the helper from no_context().

> +
>         perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, addr);
>
>         if (cause == EXC_STORE_PAGE_FAULT)
> --
> 2.17.0
>
>
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