Messages in this thread | | | Subject | Re: [PATCH] arm64: mm: check for upper PAGE_SHIFT bits in pfn_valid() | From | Greg Hackmann <> | Date | Tue, 14 Aug 2018 08:17:48 -0700 |
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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 <ghackmann@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?
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