Re: [PATCH v4 1/3] arm64: Improve kernel address detection of __is_lm_address()

[Mark Rutland]

Hi Vincenzo,

On Fri, Jan 22, 2021 at 03:56:40PM +0000, Vincenzo Frascino wrote:
> Currently, the __is_lm_address() check just masks out the top 12 bits
> of the address, but if they are 0, it still yields a true result.
> This has as a side effect that virt_addr_valid() returns true even for
> invalid virtual addresses (e.g. 0x0).
> 
> Improve the detection checking that it's actually a kernel address
> starting at PAGE_OFFSET.
> 
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Suggested-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>

Looking around, it seems that there are some existing uses of
virt_addr_valid() that expect it to reject addresses outside of the
TTBR1 range. For example, check_mem_type() in drivers/tee/optee/call.c.

Given that, I think we need something that's easy to backport to stable.

This patch itself looks fine, but it's not going to backport very far,
so I suspect we might need to write a preparatory patch that adds an
explicit range check to virt_addr_valid() which can be trivially
backported.

For this patch:

Acked-by: Mark Rutland <mark.rutland@xxxxxxx>

Thanks,
Mark.

> ---
>  arch/arm64/include/asm/memory.h | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> index 18fce223b67b..99d7e1494aaa 100644
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -247,9 +247,11 @@ static inline const void *__tag_set(const void *addr, u8 tag)
>  
>  
>  /*
> - * The linear kernel range starts at the bottom of the virtual address space.
> + * Check whether an arbitrary address is within the linear map, which
> + * lives in the [PAGE_OFFSET, PAGE_END) interval at the bottom of the
> + * kernel's TTBR1 address range.
>   */
> -#define __is_lm_address(addr)	(((u64)(addr) & ~PAGE_OFFSET) < (PAGE_END - PAGE_OFFSET))
> +#define __is_lm_address(addr)	(((u64)(addr) ^ PAGE_OFFSET) < (PAGE_END - PAGE_OFFSET))
>  
>  #define __lm_to_phys(addr)	(((addr) & ~PAGE_OFFSET) + PHYS_OFFSET)
>  #define __kimg_to_phys(addr)	((addr) - kimage_voffset)
> -- 
> 2.30.0
>