Re: [PATCH v2 7/7] ARM: mm: Fix PXN process with LPAE feature

Russell King (Oracle)
2021-06-02T10:53:02Z

Hi,

On Wed, Jun 02, 2021 at 03:02:46PM +0800, Kefeng Wang wrote:
> When user code execution with privilege mode, it will lead to
> infinite loop in the page fault handler if ARM_LPAE enabled,
>
> The issue could be reproduced with
> "echo EXEC_USERSPACE > /sys/kernel/debug/provoke-crash/DIRECT"
>
> Lets' fix it by adding the check in do_page_fault() and panic
> when ARM_LPAE enabled.
>
> Fixes: 1d4d37159d01 ("ARM: 8235/1: Support for the PXN CPU feature on ARMv7")
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
> arch/arm/mm/fault.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
> index 7cfa9a59d3ec..279bbeb33b48 100644
> --- a/arch/arm/mm/fault.c
> +++ b/arch/arm/mm/fault.c
> @@ -257,8 +257,14 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
> vm_flags = VM_WRITE;
> }
>
> - if (fsr & FSR_LNX_PF)
> + if (fsr & FSR_LNX_PF) {
> vm_flags = VM_EXEC;
> +#ifdef CONFIG_ARM_LPAE
> + if (addr && addr < TASK_SIZE && !user_mode(regs))
> + die_kernel_fault("execution of user memory",
> + addr, fsr, regs);
> +#endif
> + }

Do we need to do this test here?

Also, is this really LPAE specific? We have similar protection on 32-bit
ARM using domains to disable access to userspace except when the user
accessors are being used, so I would expect kernel-mode execution to
also cause a fault there.

--
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