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    Subject[PATCH 3.16 147/328] arm64: mm: check for upper PAGE_SHIFT bits in pfn_valid()
    3.16.62-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Greg Hackmann <ghackmann@android.com>

    commit 5ad356eabc47d26a92140a0c4b20eba471c10de3 upstream.

    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.

    Fixes: c1cc1552616d ("arm64: MMU initialisation")
    Signed-off-by: Greg Hackmann <ghackmann@google.com>
    Signed-off-by: Will Deacon <will.deacon@arm.com>
    [bwh: Backported to 3.16: Keep using memblock_is_memory()]
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    arch/arm64/mm/init.c | 6 +++++-
    1 file changed, 5 insertions(+), 1 deletion(-)

    --- a/arch/arm64/mm/init.c
    +++ b/arch/arm64/mm/init.c
    @@ -113,7 +113,11 @@ static void __init zone_sizes_init(unsig
    #ifdef CONFIG_HAVE_ARCH_PFN_VALID
    int pfn_valid(unsigned long pfn)
    {
    - return memblock_is_memory(pfn << PAGE_SHIFT);
    + phys_addr_t addr = pfn << PAGE_SHIFT;
    +
    + if ((addr >> PAGE_SHIFT) != pfn)
    + return 0;
    + return memblock_is_memory(addr);
    }
    EXPORT_SYMBOL(pfn_valid);
    #endif
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