Re: [RFC PATCH 1/1] Revert "rseq/selftests: arm: use udf instruction for RSEQ_SIG"

[Will Deacon]

On Mon, Jun 24, 2019 at 02:20:26PM -0400, Mathieu Desnoyers wrote:
> ----- On Jun 24, 2019, at 1:24 PM, Will Deacon will.deacon@xxxxxxx wrote:
> 
> > On Mon, Jun 17, 2019 at 05:23:04PM +0200, Mathieu Desnoyers wrote:
> >> This reverts commit 2b845d4b4acd9422bbb668989db8dc36dfc8f438.
> >> 
> >> That commit introduces build issues for programs compiled in Thumb mode.
> >> Rather than try to be clever and emit a valid trap instruction on arm32,
> >> which requires special care about big/little endian handling on that
> >> architecture, just emit plain data. Data in the instruction stream is
> >> technically expected on arm32: this is how literal pools are
> >> implemented. Reverting to the prior behavior does exactly that.
> >> 
> >> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> >> CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >> CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> >> CC: Joel Fernandes <joelaf@xxxxxxxxxx>
> >> CC: Catalin Marinas <catalin.marinas@xxxxxxx>
> >> CC: Dave Watson <davejwatson@xxxxxx>
> >> CC: Will Deacon <will.deacon@xxxxxxx>
> >> CC: Shuah Khan <shuah@xxxxxxxxxx>
> >> CC: Andi Kleen <andi@xxxxxxxxxxxxxx>
> >> CC: linux-kselftest@xxxxxxxxxxxxxxx
> >> CC: "H . Peter Anvin" <hpa@xxxxxxxxx>
> >> CC: Chris Lameter <cl@xxxxxxxxx>
> >> CC: Russell King <linux@xxxxxxxxxxxxxxxx>
> >> CC: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
> >> CC: "Paul E . McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> >> CC: Paul Turner <pjt@xxxxxxxxxx>
> >> CC: Boqun Feng <boqun.feng@xxxxxxxxx>
> >> CC: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> >> CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
> >> CC: Ben Maurer <bmaurer@xxxxxx>
> >> CC: linux-api@xxxxxxxxxxxxxxx
> >> CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> >> CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> >> CC: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> >> CC: Carlos O'Donell <carlos@xxxxxxxxxx>
> >> CC: Florian Weimer <fweimer@xxxxxxxxxx>
> >> ---
> >>  tools/testing/selftests/rseq/rseq-arm.h | 52 ++-------------------------------
> >>  1 file changed, 2 insertions(+), 50 deletions(-)
> >> 
> >> diff --git a/tools/testing/selftests/rseq/rseq-arm.h
> >> b/tools/testing/selftests/rseq/rseq-arm.h
> >> index 84f28f147fb6..5f262c54364f 100644
> >> --- a/tools/testing/selftests/rseq/rseq-arm.h
> >> +++ b/tools/testing/selftests/rseq/rseq-arm.h
> >> @@ -5,54 +5,7 @@
> >>   * (C) Copyright 2016-2018 - Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> >>   */
> >>  
> >> -/*
> >> - * RSEQ_SIG uses the udf A32 instruction with an uncommon immediate operand
> >> - * value 0x5de3. This traps if user-space reaches this instruction by mistake,
> >> - * and the uncommon operand ensures the kernel does not move the instruction
> >> - * pointer to attacker-controlled code on rseq abort.
> >> - *
> >> - * The instruction pattern in the A32 instruction set is:
> >> - *
> >> - * e7f5def3    udf    #24035    ; 0x5de3
> >> - *
> >> - * This translates to the following instruction pattern in the T16 instruction
> >> - * set:
> >> - *
> >> - * little endian:
> >> - * def3        udf    #243      ; 0xf3
> >> - * e7f5        b.n    <7f5>
> >> - *
> >> - * pre-ARMv6 big endian code:
> >> - * e7f5        b.n    <7f5>
> >> - * def3        udf    #243      ; 0xf3
> >> - *
> >> - * ARMv6+ -mbig-endian generates mixed endianness code vs data: little-endian
> >> - * code and big-endian data. Ensure the RSEQ_SIG data signature matches code
> >> - * endianness. Prior to ARMv6, -mbig-endian generates big-endian code and data
> >> - * (which match), so there is no need to reverse the endianness of the data
> >> - * representation of the signature. However, the choice between BE32 and BE8
> >> - * is done by the linker, so we cannot know whether code and data endianness
> >> - * will be mixed before the linker is invoked.
> >> - */
> >> -
> >> -#define RSEQ_SIG_CODE	0xe7f5def3
> >> -
> >> -#ifndef __ASSEMBLER__
> >> -
> >> -#define RSEQ_SIG_DATA							\
> >> -	({								\
> >> -		int sig;						\
> >> -		asm volatile ("b 2f\n\t"				\
> >> -			      "1: .inst " __rseq_str(RSEQ_SIG_CODE) "\n\t" \
> >> -			      "2:\n\t"					\
> >> -			      "ldr %[sig], 1b\n\t"			\
> >> -			      : [sig] "=r" (sig));			\
> >> -		sig;							\
> >> -	})
> >> -
> >> -#define RSEQ_SIG	RSEQ_SIG_DATA
> >> -
> >> -#endif
> >> +#define RSEQ_SIG	0x53053053
> > 
> > I don't get why you're reverting back to this old signature value, when the
> > one we came up with will work well when interpreted as an instruction in the
> > *vast* majority of scenarios that people care about (A32/T32 little-endian).
> > I think you might be under-estimating just how dead things like BE32 really
> > are.
> 
> My issue is that the current .instr approach is broken for programs or functions
> built in Thumb mode, and I received no feedback on the solutions I proposed for
> those issues, which led me to propose a patch reverting to a simple .word.

I understand why you're moving from .inst to .word, but I don't understand
why that necessitates a change in the value. Why not .word 0xe7f5def3 ? You
could also flip the bytes around in case of big-endian, which would keep the
instruction coding clean for BE8.

> > That said, when you ran into .inst.n/.inst.w issues, did you try something
> > along the lines of the WASM() macro we use in arch/arm/, which adds the ".w"
> > suffix when targetting Thumb?
> 
> AFAIU, the WASM macros depend on CONFIG_THUMB2_KERNEL, which may be fine within
> the kernel, but for user-space things are a bit more complex.
> 
> A compile-unit can be compiled as thumb, which will set a compiler define
> which we could use to detect thumb mode. However, unfortunately, a single
> function can also be compiled with an attribute selecting thumb mode, which
> AFAIU does not influence the preprocessor defines.

Thanks, I hadn't considered that case. I don't know the right way to handle
that in the toolchain, so using .word is probably the best bet in the
absence of any better suggestions from the tools folks.

Will