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 <[email protected]>
>> ---
>> 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?
