On 14 July 2017 at 15:06, Mark Rutland <mark.rutl...@arm.com> wrote: > On Fri, Jul 14, 2017 at 01:27:14PM +0100, Ard Biesheuvel wrote: >> On 14 July 2017 at 11:48, Ard Biesheuvel <ard.biesheu...@linaro.org> wrote: >> > On 14 July 2017 at 11:32, Mark Rutland <mark.rutl...@arm.com> wrote: >> >> On Thu, Jul 13, 2017 at 07:28:48PM +0100, Ard Biesheuvel wrote: > >> >>> OK, so here's a crazy idea: what if we >> >>> a) carve out a dedicated range in the VMALLOC area for stacks >> >>> b) for each stack, allocate a naturally aligned window of 2x the stack >> >>> size, and map the stack inside it, leaving the remaining space >> >>> unmapped > >> >> The logical ops (TST) and conditional branches (TB(N)Z, CB(N)Z) operate >> >> on XZR rather than SP, so to do this we need to get the SP value into a >> >> GPR. >> >> >> >> Previously, I assumed this meant we needed to corrupt a GPR (and hence >> >> stash that GPR in a sysreg), so I started writing code to free sysregs. >> >> >> >> However, I now realise I was being thick, since we can stash the GPR >> >> in the SP: >> >> >> >> sub sp, sp, x0 // sp = orig_sp - x0 >> >> add x0, sp, x0 // x0 = x0 - (orig_sp - x0) == orig_sp > > That comment is off, and should say x0 = x0 + (orig_sp - x0) == orig_sp > >> >> sub x0, x0, #S_FRAME_SIZE >> >> tb(nz) x0, #THREAD_SHIFT, overflow >> >> add x0, x0, #S_FRAME_SIZE >> >> sub x0, sp, x0 >> >> You need a neg x0, x0 here I think > > Oh, whoops. I'd mis-simplified things. > > We can avoid that by storing orig_sp + orig_x0 in sp: > > add sp, sp, x0 // sp = orig_sp + orig_x0 > sub x0, sp, x0 // x0 = orig_sp > < check > > sub x0, sp, x0 // x0 = orig_x0 > sub sp, sp, x0 // sp = orig_sp > > ... which works in a locally-built kernel where I've aligned all the > stacks. >
Yes, that looks correct to me now. >> ... only, this requires a dedicated stack region, and so we'd need to >> check whether sp is inside that window as well. >> >> The easieast way would be to use a window whose start address is base2 >> aligned, but that means the beginning of the kernel VA range (where >> KASAN currently lives, and cannot be moved afaik), or a window at the >> top of the linear region. Neither look very appealing >> >> So that means arbitrary low and high limits to compare against in this >> entry path. That means more GPRs I'm afraid. > > Could you elaborate on that? I'm not sure that I follow. > > My understanding was that the comprimise with this approach is that we > only catch overflow/underflow within THREAD_SIZE of the stack, and can > get false-negatives elsewhere. Otherwise, IIUC this is sufficient > > Are you after a more stringent check (like those from the two existing > proposals that caught all out-of-bounds accesses)? > > Or am I missing something else? > No, not at all. I managed to confuse myself into thinking that we need to validate the value of SP in some way, i.e., as we would when dealing with an arbitrary faulting address.
