On 08/14/2018 08:29 AM, Will Deacon wrote:
> 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 <ghackm...@google.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
>
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.
