linux-kernel - Re: [PATCH] arm64: mm: check for upper PAGE_SHIFT bits in pfn

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Message-ID: <20180814152958.GD567@arm.com>
Date:   Tue, 14 Aug 2018 16:29:59 +0100
From:   Will Deacon <will.deacon@....com>
To:     Greg Hackmann <ghackmann@...gle.com>
Cc:     Greg Hackmann <ghackmann@...roid.com>,
        linux-arm-kernel@...ts.infradead.org, kernel-team@...roid.com,
        Catalin Marinas <catalin.marinas@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Robin Murphy <robin.murphy@....com>,
        Laura Abbott <labbott@...hat.com>,
        Steve Capper <steve.capper@....com>,
        Kristina Martsenko <kristina.martsenko@....com>,
        Stefan Agner <stefan@...er.ch>,
        CHANDAN VN <chandan.vn@...sung.com>,
        Johannes Weiner <hannes@...xchg.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: mm: check for upper PAGE_SHIFT bits in pfn_valid()

On Tue, Aug 14, 2018 at 08:17:48AM -0700, Greg Hackmann wrote:
> On 08/14/2018 03:40 AM, Will Deacon wrote:
> > Hi Greg,
> > 
> > On Mon, Aug 13, 2018 at 12:30:11PM -0700, Greg Hackmann wrote:
> >> ARM64's pfn_valid() shifts away the upper PAGE_SHIFT bits of the input
> >> before seeing if the PFN is valid.  This leads to false positives when
> >> some of the upper bits are set, but the lower bits match a valid PFN.
> >>
> >> For example, the following userspace code looks up a bogus entry in
> >> /proc/kpageflags:
> >>
> >>     int pagemap = open("/proc/self/pagemap", O_RDONLY);
> >>     int pageflags = open("/proc/kpageflags", O_RDONLY);
> >>     uint64_t pfn, val;
> >>
> >>     lseek64(pagemap, [...], SEEK_SET);
> >>     read(pagemap, &pfn, sizeof(pfn));
> >>     if (pfn & (1UL << 63)) {        /* valid PFN */
> >>         pfn &= ((1UL << 55) - 1);   /* clear flag bits */
> >>         pfn |= (1UL << 55);
> >>         lseek64(pageflags, pfn * sizeof(uint64_t), SEEK_SET);
> >>         read(pageflags, &val, sizeof(val));
> >>     }
> >>
> >> On ARM64 this causes the userspace process to crash with SIGSEGV rather
> >> than reading (1 << KPF_NOPAGE).  kpageflags_read() treats the offset as
> >> valid, and stable_page_flags() will try to access an address between the
> >> user and kernel address ranges.
> >>
> >> Signed-off-by: Greg Hackmann <ghackmann@...gle.com>
> >> ---
> >>  arch/arm64/mm/init.c | 6 +++++-
> >>  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > Thanks, this looks like a sensible fix to me. Do you think it warrants a
> > CC stable?
> > 
> > Will
> 
> Yes, I think so.  Should I resend with a "Fixes" field?

Could do, but I think this goes all the way back to day 1! Doesn't arch/arm/
also suffer from the same issue?

Will