On Thu, Sep 03, 2020 at 11:12:02AM +0100, Boyan Karatotev wrote: > On 02/09/2020 17:49, Dave Martin wrote: > > On Fri, Aug 28, 2020 at 02:16:03PM +0100, Boyan Karatotev wrote: > >> PAuth signs and verifies return addresses on the stack. It does so by > >> inserting a Pointer Authentication code (PAC) into some of the unused top > >> bits of an address. This is achieved by adding paciasp/autiasp instructions > >> at the beginning and end of a function. > >> > >> This feature is partially backwards compatible with earlier versions of the > >> ARM architecture. To coerce the compiler into emitting fully backwards > >> compatible code the main file is compiled to target an earlier ARM version. > >> This allows the tests to check for the feature and print meaningful error > >> messages instead of crashing. > >> > >> Add a test to verify that corrupting the return address results in a > >> SIGSEGV on return. > >> > >> Cc: Shuah Khan <shuah@xxxxxxxxxx> > >> Cc: Catalin Marinas <catalin.marinas@xxxxxxx> > >> Cc: Will Deacon <will@xxxxxxxxxx> > >> Signed-off-by: Boyan Karatotev <boyan.karatotev@xxxxxxx> > >> --- [...] > >> diff --git a/tools/testing/selftests/arm64/pauth/pac_corruptor.S b/tools/testing/selftests/arm64/pauth/pac_corruptor.S > >> new file mode 100644 > >> index 000000000000..6a34ec23a034 > >> --- /dev/null > >> +++ b/tools/testing/selftests/arm64/pauth/pac_corruptor.S > >> @@ -0,0 +1,36 @@ > >> +/* SPDX-License-Identifier: GPL-2.0 */ > >> +/* Copyright (C) 2020 ARM Limited */ > >> + > >> +.global pac_corruptor > >> + > >> +.text > >> +/* > >> + * Corrupting a single bit of the PAC ensures the authentication will fail. It > >> + * also guarantees no possible collision. TCR_EL1.TBI0 is set by default so no > >> + * top byte PAC is tested > >> + */ > >> + pac_corruptor: > >> + paciasp > >> + > >> + /* make stack frame */ > >> + sub sp, sp, #16 > >> + stp x29, lr, [sp] > > > > Nit: if respinning, you can optimise a few sequences of this sort, e.g. > > > > stp x29, lr, [sp, #-16]! > > > >> + mov x29, sp > >> + > >> + /* prepare mask for bit to be corrupted (bit 54) */ > >> + mov x1, xzr > >> + add x1, x1, #1 > >> + lsl x1, x1, #54 > > > > Nit: > > > > mov x1, #1 << 54 > Thank you for this, didn't know I could do it this way. > > > > but anyway, the logic operations can encode most simple bitmasks > > directly as immediate operands, so you can skip this and just do > > > >> + > >> + /* get saved lr, corrupt selected bit, put it back */ > >> + ldr x0, [sp, #8] > >> + eor x0, x0, x1 > > > > eor x0, x0, #1 << 54 > > > >> + str x0, [sp, #8] > >> + > >> + /* remove stack frame */ > >> + ldp x29, lr, [sp] > >> + add sp, sp, #16 > > > > ldp x29, lr, [sp], #16 > > > > [...] > > > > Actually, since there are no leaf nested function calls and no trap is > > expected until the function returns (so backtracing in the middle of > > this function is unlikely to be needed), could we optimise this whole > > thing down to the following? > > > I suppose you're right. The intent was to emulate a c function but there > really is no point in doing all this extra work. Will change it. It's not critical either way, but this way it's at least less code to maintain / read. > > pac_corruptor: > > paciasp > > eor lr, lr, #1 << 53 > > autiasp > > ret > > > > Cheers > > ---Dave [...] Cheers ---Dave
