On 2025-08-25 at 15:59:46 -0500, Samuel Holland wrote:
>Hi Maciej,
>
>On 2025-08-25 3:24 PM, Maciej Wieczor-Retman wrote:
>> For an address to be canonical it has to have its top bits equal to each
>> other. The number of bits depends on the paging level and whether
>> they're supposed to be ones or zeroes depends on whether the address
>> points to kernel or user space.
>>
>> With Linear Address Masking (LAM) enabled, the definition of linear
>> address canonicality is modified. Not all of the previously required
>> bits need to be equal, only the first and last from the previously equal
>> bitmask. So for example a 5-level paging kernel address needs to have
>> bits [63] and [56] set.
>>
>> Add separate __canonical_address() implementation for
>> CONFIG_KASAN_SW_TAGS since it's the only thing right now that enables
>> LAM for kernel addresses (LAM_SUP bit in CR4).
>>
>> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@xxxxxxxxx>
>> ---
>> Changelog v4:
>> - Add patch to the series.
>>
>> arch/x86/include/asm/page.h | 10 ++++++++++
>> 1 file changed, 10 insertions(+)
>>
>> diff --git a/arch/x86/include/asm/page.h b/arch/x86/include/asm/page.h
>> index bcf5cad3da36..a83f23a71f35 100644
>> --- a/arch/x86/include/asm/page.h
>> +++ b/arch/x86/include/asm/page.h
>> @@ -82,10 +82,20 @@ static __always_inline void *pfn_to_kaddr(unsigned long pfn)
>> return __va(pfn << PAGE_SHIFT);
>> }
>>
>> +/*
>> + * CONFIG_KASAN_SW_TAGS requires LAM which changes the canonicality checks.
>> + */
>> +#ifdef CONFIG_KASAN_SW_TAGS
>> +static __always_inline u64 __canonical_address(u64 vaddr, u8 vaddr_bits)
>> +{
>> + return (vaddr | BIT_ULL(63) | BIT_ULL(vaddr_bits - 1));
>> +}
>> +#else
>> static __always_inline u64 __canonical_address(u64 vaddr, u8 vaddr_bits)
>> {
>> return ((s64)vaddr << (64 - vaddr_bits)) >> (64 - vaddr_bits);
>> }
>> +#endif
>
>These two implementations have different semantics. The new function works only
>on kernel addresses, whereas the existing one works on user addresses as well.
>It looks like at least KVM's use of __is_canonical_address() expects the
>function to work with user addresses.
Thanks for noticing that, I'll think of a way to make it work for user addresses
too :)
>
>Regards,
>Samuel
>
>>
>> static __always_inline u64 __is_canonical_address(u64 vaddr, u8 vaddr_bits)
>> {
>
--
Kind regards
Maciej Wieczór-Retman
