Messages in this thread | | | From | Ard Biesheuvel <> | Date | Mon, 17 Jun 2019 13:33:19 +0200 | Subject | Re: [RFC] Disable lockref on arm64 |
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On Sun, 16 Jun 2019 at 23:31, Kees Cook <keescook@chromium.org> wrote: > > On Sat, Jun 15, 2019 at 04:18:21PM +0200, Ard Biesheuvel wrote: > > Yes, I am using the same saturation point as x86. In this example, I > > am not entirely sure I understand why it matters, though: the atomics > > guarantee that the write by CPU2 fails if CPU1 changed the value in > > the mean time, regardless of which value it wrote. > > > > I think the concern is more related to the likelihood of another CPU > > doing something nasty between the moment that the refcount overflows > > and the moment that the handler pins it at INT_MIN/2, e.g., > > > > > CPU 1 CPU 2 > > > inc() > > > load INT_MAX > > > about to overflow? > > > yes > > > > > > set to 0 > > > <insert exploit here> > > > set to INT_MIN/2 > > Ah, gotcha, but the "set to 0" is really "set to INT_MAX+1" (not zero) > if you're using the same saturation. >
Of course. So there is no issue here: whatever manipulations are racing with the overflow handler can never result in the counter to unsaturate.
And actually, moving the checks before the stores is not as trivial as I thought, E.g., for the LSE refcount_add case, we have
" ldadd %w[i], w30, %[cval]\n" \ " adds %w[i], %w[i], w30\n" \ REFCOUNT_PRE_CHECK_ ## pre (w30)) \ REFCOUNT_POST_CHECK_ ## post \
and changing this into load/test/store defeats the purpose of using the LSE atomics in the first place.
On my single core TX2, the comparative performance is as follows
Baseline: REFCOUNT_TIMING test using REFCOUNT_FULL (LSE cmpxchg) 191057942484 cycles # 2.207 GHz 148447589402 instructions # 0.78 insn per cycle
86.568269904 seconds time elapsed
Upper bound: ATOMIC_TIMING 116252672661 cycles # 2.207 GHz 28089216452 instructions # 0.24 insn per cycle
52.689793525 seconds time elapsed
REFCOUNT_TIMING test using LSE atomics 127060259162 cycles # 2.207 GHz 0 instructions # 0.00 insn per cycle
57.243690077 seconds time elapsed
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