Re: [PATCHv3 6/6] arm64: Add support for CONFIG_DEBUG_VIRTUAL

[Laura Abbott]

On 11/18/2016 09:53 AM, Mark Rutland wrote:
> Hi,
> 
> On Thu, Nov 17, 2016 at 05:16:56PM -0800, Laura Abbott wrote:
>>
>> x86 has an option CONFIG_DEBUG_VIRTUAL to do additional checks
>> on virt_to_phys calls. The goal is to catch users who are calling
>> virt_to_phys on non-linear addresses immediately. This inclues callers
>> using virt_to_phys on image addresses instead of __pa_symbol. As features
>> such as CONFIG_VMAP_STACK get enabled for arm64, this becomes increasingly
>> important. Add checks to catch bad virt_to_phys usage.
>>
>> Signed-off-by: Laura Abbott <labbott@xxxxxxxxxx>
>> ---
>> v3: Make use of __pa_symbol required via debug checks. It's a WARN for now but
>> it can become a BUG after wider testing. __virt_to_phys is
>> now for linear addresses only. Dropped the VM_BUG_ON from Catalin's suggested
>> version from the nodebug version since having that in the nodebug version
>> essentially made them the debug version. Changed to KERNEL_START/KERNEL_END
>> for bounds checking. More comments.
> 
> I gave this a go with DEBUG_VIRTUAL && KASAN_INLINE selected, and the
> kernel dies somewhere before bringing up earlycon. :(
> 
> I mentioned some possible reasons in a reply to pastch 5, and I have
> some more comments below.
> 
> [...]
> 
>> -#define __virt_to_phys(x) ({						\
>> +
>> +
>> +/*
>> + * This is for translation from the standard linear map to physical addresses.
>> + * It is not to be used for kernel symbols.
>> + */
>> +#define __virt_to_phys_nodebug(x) ({					\
>>  	phys_addr_t __x = (phys_addr_t)(x);				\
>> -	__x & BIT(VA_BITS - 1) ? (__x & ~PAGE_OFFSET) + PHYS_OFFSET :	\
>> -				 (__x - kimage_voffset); })
>> +	((__x & ~PAGE_OFFSET) + PHYS_OFFSET);				\
>> +})
> 
> Given the KASAN failure, and the strong possibility that there's even
> more stuff lurking in common code, I think we should retain the logic to
> handle kernel image addresses for the timebeing (as x86 does). Once
> we've merged DEBUG_VIRTUAL, it will be easier to track those down.
> 

Agreed. I might see about adding another option DEBUG_STRICT_VIRTUAL
for catching bad __pa vs __pa_symbol usage and keep DEBUG_VIRTUAL for
catching addresses that will work in neither case.

> Catalin, I think you suggested removing that logic; are you happy for it
> to be restored?
> 
> See below for a refactoring that retains this logic.
> 
> [...]
> 
>> +/*
>> + * This is for translation from a kernel image/symbol address to a
>> + * physical address.
>> + */
>> +#define __pa_symbol_nodebug(x) ({					\
>> +	phys_addr_t __x = (phys_addr_t)(x);				\
>> +	(__x - kimage_voffset);						\
>> +})
> 
> We can avoid duplication here (and in physaddr.c) if we factor the logic
> into helpers, e.g.
> 
> /*
>  * The linear kernel range starts in the middle of the virtual adddress
>  * space. Testing the top bit for the start of the region is a
>  * sufficient check.
>  */
> #define __is_lm_address(addr)	(!!((addr) & BIT(VA_BITS - 1)))
> 
> #define __lm_to_phys(addr)	(((addr) & ~PAGE_OFFSET) + PHYS_OFFSET)
> #define __kimg_to_phys(addr)	((addr) - kimage_voffset)
> 
> #define __virt_to_phys_nodebug(x) ({					\
> 	phys_addr_t __x = (phys_addr_t)(x);				\
> 	__is_lm_address(__x) ? __lm_to_phys(__x) :			\
> 			       __kimg_to_phys(__x);			\
> })
> 
> #define __pa_symbol_nodebug(x)	__kimg_to_phys((phys_addr_t)(x))
> 

Yes, this is much cleaner

>> +#ifdef CONFIG_DEBUG_VIRTUAL
>> +extern unsigned long __virt_to_phys(unsigned long x);
>> +extern unsigned long __phys_addr_symbol(unsigned long x);
> 
> It would be better for both of these to return phys_addr_t.
> 
> [...]
> 

I was worried this would turn into another warning project but
at first pass this works fine and the type will hopefully catch
some bad uses elsewhere.

>> diff --git a/arch/arm64/mm/physaddr.c b/arch/arm64/mm/physaddr.c
>> new file mode 100644
>> index 0000000..f8eb781
>> --- /dev/null
>> +++ b/arch/arm64/mm/physaddr.c
>> @@ -0,0 +1,39 @@
>> +#include <linux/mm.h>
>> +
>> +#include <asm/memory.h>
> 
> We also need:
> 
> #include <linux/bug.h>
> #include <linux/export.h>
> #include <linux/types.h>
> #include <linux/mmdebug.h>
> 
>> +unsigned long __virt_to_phys(unsigned long x)
>> +{
>> +	phys_addr_t __x = (phys_addr_t)x;
>> +
>> +	if (__x & BIT(VA_BITS - 1)) {
>> +		/*
>> +		 * The linear kernel range starts in the middle of the virtual
>> +		 * adddress space. Testing the top bit for the start of the
>> +		 * region is a sufficient check.
>> +		 */
>> +		return (__x & ~PAGE_OFFSET) + PHYS_OFFSET;
>> +	} else {
>> +		/*
>> +		 * __virt_to_phys should not be used on symbol addresses.
>> +		 * This should be changed to a BUG once all basic bad uses have
>> +		 * been cleaned up.
>> +		 */
>> +		WARN(1, "Do not use virt_to_phys on symbol addresses");
>> +		return __phys_addr_symbol(x);
>> +	}
>> +}
>> +EXPORT_SYMBOL(__virt_to_phys);
> 
> I think this would be better something like:
> 
> phys_addr_t __virt_to_phys(unsigned long x)
> {
> 	WARN(!__is_lm_address(x),
> 	     "virt_to_phys() used for non-linear address: %pK\n",
> 	     (void*)x);
> 	
> 	return __virt_to_phys_nodebug(x);
> }
> EXPORT_SYMBOL(__virt_to_phys);
> 
>> +
>> +unsigned long __phys_addr_symbol(unsigned long x)
>> +{
>> +	phys_addr_t __x = (phys_addr_t)x;
>> +
>> +	/*
>> +	 * This is bounds checking against the kernel image only.
>> +	 * __pa_symbol should only be used on kernel symbol addresses.
>> +	 */
>> +	VIRTUAL_BUG_ON(x < (unsigned long) KERNEL_START || x > (unsigned long) KERNEL_END);
>> +	return (__x - kimage_voffset);
>> +}
>> +EXPORT_SYMBOL(__phys_addr_symbol);
> 
> Similarly:
> 
> phys_addr_t __phys_addr_symbol(unsigned long x)
> {
> 	/*
> 	 * This is bounds checking against the kernel image only.
> 	 * __pa_symbol should only be used on kernel symbol addresses.
> 	 */
> 	VIRTUAL_BUG_ON(x < (unsigned long) KERNEL_START ||
> 		       x > (unsigned long) KERNEL_END);
> 
> 	return __pa_symbol_nodebug(x);
> }
> EXPORT_SYMBOL(__phys_addr_symbol);
> 
> Thanks,
> Mark.
> 

Thanks,
Laura