Re: [PATCHv3 5/8] x86/uaccess: Provide untagged_addr() and remove tags before address check

[Andy Lutomirski]

On 6/10/22 07:35, Kirill A. Shutemov wrote:
> untagged_addr() is a helper used by the core-mm to strip tag bits and
> get the address to the canonical shape. In only handles userspace
> addresses. The untagging mask is stored in mmu_context and will be set
> on enabling LAM for the process.
>
> The tags must not be included into check whether it's okay to access the
> userspace address.
>
> Strip tags in access_ok().

What is the intended behavior for an access that spans a tag boundary?

Also, at the risk of a potentially silly question, why do we need to 
strip the tag before access_ok()?  With LAM, every valid tagged user 
address is also a valid untagged address, right?  (There is no 
particular need to enforce the actual value of TASK_SIZE_MAX on 
*access*, just on mmap.)

IOW, wouldn't it be sufficient, and probably better than what we have 
now, to just check that the entire range has the high bit clear?

--Andy