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
> 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
>
