Messages in this thread | | | From | Ard Biesheuvel <> | Date | Mon, 7 Aug 2017 22:46:16 +0100 | Subject | Re: [kernel-hardening] [PATCH] lkdtm: Test VMAP_STACK allocates leading/trailing guard pages |
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On 7 August 2017 at 22:44, Kees Cook <keescook@chromium.org> wrote: > On Mon, Aug 7, 2017 at 2:27 PM, Ard Biesheuvel > <ard.biesheuvel@linaro.org> wrote: >> On 7 August 2017 at 21:39, Kees Cook <keescook@chromium.org> wrote: >>> Two new tests STACK_GUARD_PAGE_LEADING and STACK_GUARD_PAGE_TRAILING >>> attempt to read the byte before and after, respectively, of the current >>> stack frame, which should fault under VMAP_STACK. >>> >>> Signed-off-by: Kees Cook <keescook@chromium.org> >>> --- >>> Do these tests both trip with the new arm64 VMAP_STACK code? >> >> Probably not. On arm64, the registers are stacked by software at >> exception entry, and so we decrement the sp first by the size of the >> register file, and if the resulting value overflows the stack, the >> situation is handled as if the exception was caused by a faulting >> stack access while it may be caused by something else in reality. >> Since the act of handling the exception is guaranteed to overflow the >> stack anyway, this does not really make a huge difference, and it >> prevents the recursive fault from wiping the context that we need to >> produce the diagnostics. >> >> This means an illegal access right above the stack will go undetected. > > I thought vmap entries provided guard pages around allocations? > Shouldn't that catch it? >
Ah yes, so we will fault. We should probably double check whether we will not misidentify the fault because of the subtraction we do first, but that should be trivial to add.
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