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

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Date:   Tue, 28 Jun 2022 16:40:48 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, Kostya Serebryany <kcc@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv3 5/8] x86/uaccess: Provide untagged_addr() and remove
 tags before address check

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