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

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Message-Id: <20180813193013.236362-1-ghackmann@google.com>
Date:   Mon, 13 Aug 2018 12:30:11 -0700
From:   Greg Hackmann <ghackmann@...roid.com>
To:     linux-arm-kernel@...ts.infradead.org
Cc:     kernel-team@...roid.com, Greg Hackmann <ghackmann@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....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: [PATCH] arm64: mm: check for upper PAGE_SHIFT bits in pfn_valid()

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(-)

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 9abf8a1e7b25..787e27964ab9 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -287,7 +287,11 @@ static void __init zone_sizes_init(unsigned long min, unsigned long max)
 #ifdef CONFIG_HAVE_ARCH_PFN_VALID
 int pfn_valid(unsigned long pfn)
 {
-	return memblock_is_map_memory(pfn << PAGE_SHIFT);
+	phys_addr_t addr = pfn << PAGE_SHIFT;
+
+	if ((addr >> PAGE_SHIFT) != pfn)
+		return 0;
+	return memblock_is_map_memory(addr);
 }
 EXPORT_SYMBOL(pfn_valid);
 #endif
-- 
2.18.0.597.ga71716f1ad-goog