linux-kernel - Re: [PATCH v2 1/2] arm64: Fix kernel address detection of __is_lm

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Date:   Thu, 21 Jan 2021 15:49:38 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Vincenzo Frascino <vincenzo.frascino@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Leon Romanovsky <leonro@...lanox.com>,
        Alexander Potapenko <glider@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Will Deacon <will@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH v2 1/2] arm64: Fix kernel address detection of
 __is_lm_address()

On Thu, Jan 21, 2021 at 03:30:51PM +0000, Vincenzo Frascino wrote:
> On 1/21/21 3:12 PM, Mark Rutland wrote:
> > On Thu, Jan 21, 2021 at 01:19:55PM +0000, Vincenzo Frascino wrote:
> >> Currently, the __is_lm_address() check just masks out the top 12 bits
> >> of the address, but if they are 0, it still yields a true result.
> >> This has as a side effect that virt_addr_valid() returns true even for
> >> invalid virtual addresses (e.g. 0x0).
> > 
> > When it was added, __is_lm_address() was intended to distinguish valid
> > kernel virtual addresses (i.e. those in the TTBR1 address range), and
> > wasn't intended to do anything for addresses outside of this range. See
> > commit:
> > 
> >   ec6d06efb0bac6cd ("arm64: Add support for CONFIG_DEBUG_VIRTUAL")
> > 
> > ... where it simply tests a bit.
> > 
> > So I believe that it's working as intended (though this is poorly
> > documented), but I think you're saying that usage isn't aligned with
> > that intent. Given that, I'm not sure the fixes tag is right; I think it
> > has never had the semantic you're after.
> >
> I did not do much thinking on the intended semantics. I based my interpretation
> on what you are saying (the usage is not aligned with the intent). Based on what
> you are are saying, I will change the patch description removing the "Fix" term.

Thanks! I assume that also means removing the fixes tag.

> > I had thought the same was true for virt_addr_valid(), and that wasn't
> > expected to be called for VAs outside of the kernel VA range. Is it
> > actually safe to call that with NULL on other architectures?
> 
> I am not sure on this, did not do any testing outside of arm64.

I think it'd be worth checking, if we're going to use this in common
code.

> > I wonder if it's worth virt_addr_valid() having an explicit check for
> > the kernel VA range, instead.
> 
> I have no strong opinion either way even if personally I feel that modifying
> __is_lm_address() is more clear. Feel free to propose something.

Sure; I'm happy for it to live within __is_lm_address() if that's
simpler overall, given it doesn't look like it's making that more
complex or expensive.

> >> Fix the detection checking that it's actually a kernel address starting
> >> at PAGE_OFFSET.
> >>
> >> Fixes: f4693c2716b35 ("arm64: mm: extend linear region for 52-bit VA configurations")
> >> Cc: Catalin Marinas <catalin.marinas@....com>
> >> Cc: Will Deacon <will@...nel.org>
> >> Suggested-by: Catalin Marinas <catalin.marinas@....com>
> >> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@....com>
> >> ---
> >>  arch/arm64/include/asm/memory.h | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> >> index 18fce223b67b..e04ac898ffe4 100644
> >> --- a/arch/arm64/include/asm/memory.h
> >> +++ b/arch/arm64/include/asm/memory.h
> >> @@ -249,7 +249,7 @@ static inline const void *__tag_set(const void *addr, u8 tag)
> >>  /*
> >>   * The linear kernel range starts at the bottom of the virtual address space.
> >>   */
> >> -#define __is_lm_address(addr)	(((u64)(addr) & ~PAGE_OFFSET) < (PAGE_END - PAGE_OFFSET))
> >> +#define __is_lm_address(addr)	(((u64)(addr) ^ PAGE_OFFSET) < (PAGE_END - PAGE_OFFSET))
> > 
> > If we're going to make this stronger, can we please expand the comment
> > with the intended semantic? Otherwise we're liable to break this in
> > future.
> 
> Based on your reply on the above matter, if you agree, I am happy to extend the
> comment.

Works for me; how about:

/*
 * Check whether an arbitrary address is within the linear map, which
 * lives in the [PAGE_OFFSET, PAGE_END) interval at the bottom of the
 * kernel's TTBR1 address range.
 */

... with "arbitrary" being the key word.

Thanks,
Mark.