[PATCH 3.16 147/328] arm64: mm: check for upper PAGE_SHIFT bits in pfn_valid()

Ben Hutchings
2018-12-09T22:09:12Z

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