[PATCH] arm64: mm: check for upper PAGE_SHIFT bits in pfn_valid()

On 08/14/2018 08:29 AM, Will Deacon wrote:

On Tue, Aug 14, 2018 at 08:17:48AM -0700, Greg Hackmann wrote:

quoted

On 08/14/2018 03:40 AM, Will Deacon wrote:

quoted

Hi Greg,

On Mon, Aug 13, 2018 at 12:30:11PM -0700, Greg Hackmann wrote:

quoted

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 <redacted>
---
 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

Yeah, it looks like this happens on non-LPAE 32-bit kernels too.  LPAE
kernels aren't affected since __pfn_to_phys() promotes to a 64-bit type.

I can submit a fix for that too while I'm at it.