linux-kernel - Re: [PATCHv11 05/16] x86/uaccess: Provide untagged_addr() and remove tags before address check

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Message-ID: <20221107173352.nx5zu67umwhuccir@box.shutemov.name>
Date:   Mon, 7 Nov 2022 20:33:52 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>, 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>,
        Taras Madan <tarasmadan@...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>,
        Bharata B Rao <bharata@....com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv11 05/16] x86/uaccess: Provide untagged_addr() and remove
 tags before address check

On Mon, Nov 07, 2022 at 06:50:51AM -0800, Andy Lutomirski wrote:
> > @@ -21,6 +22,30 @@ static inline bool pagefault_disabled(void);
> >   # define WARN_ON_IN_IRQ()
> >   #endif
> > +#ifdef CONFIG_X86_64
> > +/*
> > + * Mask out tag bits from the address.
> > + *
> > + * Magic with the 'sign' allows to untag userspace pointer without any branches
> > + * while leaving kernel addresses intact.
> > + */
> > +#define untagged_addr(mm, addr)	({					\
> > +	u64 __addr = (__force u64)(addr);				\
> > +	s64 sign = (s64)__addr >> 63;					\
> > +	__addr &= (mm)->context.untag_mask | sign;			\
> > +	(__force __typeof__(addr))__addr;				\
> > +})
> > +
> 
> I think this implementation is correct, but I'm wondering if there are any
> callers of untagged_addr that actually need to preserve kernel addresses.
> Are there?  (There certainly *were* back when we had set_fs().)

I don't think there's any.

CONFIG_KASAN_SW_TAGS uses untagged_addr() on kernel addresses, but it is
only enabled on arm64. On x86, it will use CR4.LAM_SUP and the enabling
would require a new helper for untagging kernel addresses.

That said, I would rather stay on the safe side.

> I'm also mildly uneasy about a potential edge case.  Naively, one would
> expect:
> 
> untagged_addr(current->mm, addr) + size ==
> untagged_addr(current->mm, addr + size)
> 
> at least for an address that is valid enough to be potentially dereferenced.
> This isn't true any more for size that overflows into the tag bit range.

That's definitely a new edge case.

>From quick grep, the only CONFIG_KASAN_SW_TAGS code obviously does
arithmetics on address before untagging.

> I *think* we're okay though -- __access_ok requires that addr <= limit -
> size, so any range that overflows into tag bits will be rejected even if the
> entire range consists of valid (tagged) user addresses.

True.

> So:
> 
> Acked-by: Andy Lutomirski <luto@...nel.org>

Thanks!

-- 
  Kiryl Shutsemau / Kirill A. Shutemov