Re: [PATCH] arm64: trap illegal translations in __virt_to_phys()

From: Mark Rutland
Date: Mon Jan 07 2019 - 10:00:26 EST

On Mon, Jan 07, 2019 at 07:21:20PM +0800, Miles Chen wrote:
> Current __virt_to_phys() only print warning messages for non-linear
> addresses. It's hard to catch all warnings by those messages.

Why? Are you seeing a large number of warnings somewhere?

> So add a VIRTUAL_BUG_ON() to trap all non-linear and non-symbol
> addresses (e.g., stack addresses)
> Tested by pass stack addresses and symbol addresses to __pa(). Result:
> stack addresses: kernel BUG()


* Stacks are vmap'd, and __is_lm_address(stack_addr) is false. We'll
produce a WARNING() today (and return a junk physical address).

* Stacks are linear mapped, and cannot be distinguished from other
linear mapped addresses. The physical address will be valid.

... so I don't understand why you need to change this.

> symbol addresses: kernel warning message

That should already be the case today, since the kernel image is mapped
separately from the linear map, so __is_lm_address(symbol_addr) should
be false.

> Maybe we should trap all non-linear address translations in the future.
> Signed-off-by: Miles Chen <miles.chen@xxxxxxxxxxxx>
> ---
> arch/arm64/mm/physaddr.c | 5 +++++
> 1 file changed, 5 insertions(+)
> diff --git a/arch/arm64/mm/physaddr.c b/arch/arm64/mm/physaddr.c
> index 67a9ba9eaa96..f6b935dad19c 100644
> --- a/arch/arm64/mm/physaddr.c
> +++ b/arch/arm64/mm/physaddr.c
> @@ -14,6 +14,11 @@ phys_addr_t __virt_to_phys(unsigned long x)
> (void *)x,
> (void *)x);
> + /* trap all non-linear and non-symbol addresses */
> + VIRTUAL_BUG_ON(!__is_lm_address(x) &&
> + (x < (unsigned long)KERNEL_START ||
> + x > (unsigned long)KERNEL_END));

The KERNEL_START and KERNEL_END definitions refer to the kernel image,
not the linear map, so it doesn't make any sense to permit those here.

It is *not* valid to call __virt_to_phys() with a symbol address. We
only support those in __virt_to_phys_nodebug() so that broken code has a
chance of stumbling on.

If you want the kernel to die immediately when it hits a warning here,
please set panic_on_warn.