Re: [PATCH] arm64: mte: initialize RGSR_EL1.SEED in __cpu_setup

From: Peter Collingbourne <pcc@google.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Will Deacon <will@kernel.org>,
	Evgenii Stepanov <eugenis@google.com>,
	Andrey Konovalov <andreyknvl@gmail.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	stable@vger.kernel.org
Subject: Re: [PATCH] arm64: mte: initialize RGSR_EL1.SEED in __cpu_setup
Date: Fri, 7 May 2021 11:59:41 -0700	[thread overview]
Message-ID: <CAMn1gO6Jyt046KBxO_3tKeQL2Kx2-Aq59R+R5Pb9DhKV1_zD1A@mail.gmail.com> (raw)
In-Reply-To: <20210507101115.GB52849@C02TD0UTHF1T.local>

On Fri, May 7, 2021 at 3:11 AM Mark Rutland <mark.rutland@arm.com> wrote:
>
> Hi Peter,
>
> On Thu, May 06, 2021 at 08:37:25PM -0700, Peter Collingbourne wrote:
> > A valid implementation choice for the ChooseRandomNonExcludedTag()
> > pseudocode function used by IRG is to behave in the same way as with
> > GCR_EL1.RRND=0. This would mean that RGSR_EL1.SEED is used as an LFSR
> > which must have a non-zero value in order for IRG to properly produce
> > pseudorandom numbers. However, RGSR_EL1 is reset to an UNKNOWN value
> > on soft reset and thus may reset to 0. Therefore we must initialize
> > RGSR_EL1.SEED to a non-zero value in order to ensure that IRG behaves
> > as expected.
> >
> > Signed-off-by: Peter Collingbourne <pcc@google.com>
> > Cc: stable@vger.kernel.org
> > Link: https://linux-review.googlesource.com/id/I2b089b6c7d6f17ee37e2f0db7df5ad5bcc04526c
> > ---
> >  arch/arm64/mm/proc.S | 17 +++++++++++++++--
> >  1 file changed, 15 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
> > index 0a48191534ff..e8e1eaee4b3f 100644
> > --- a/arch/arm64/mm/proc.S
> > +++ b/arch/arm64/mm/proc.S
> > @@ -437,7 +437,7 @@ SYM_FUNC_START(__cpu_setup)
> >       mrs     x10, ID_AA64PFR1_EL1
> >       ubfx    x10, x10, #ID_AA64PFR1_MTE_SHIFT, #4
> >       cmp     x10, #ID_AA64PFR1_MTE
> > -     b.lt    1f
> > +     b.lt    2f
> >
> >       /* Normal Tagged memory type at the corresponding MAIR index */
> >       mov     x10, #MAIR_ATTR_NORMAL_TAGGED
> > @@ -447,6 +447,19 @@ SYM_FUNC_START(__cpu_setup)
> >       mov     x10, #(SYS_GCR_EL1_RRND | SYS_GCR_EL1_EXCL_MASK)
> >       msr_s   SYS_GCR_EL1, x10
> >
> > +     /*
> > +      * Initialize RGSR_EL1.SEED to a non-zero value. If the implementation
> > +      * chooses to implement GCR_EL1.RRND=1 in the same way as RRND=0 then
> > +      * the seed will be used as an LFSR, so it should be put onto the MLS.
> > +      */
>
> For those of us not familiar with LFSRs, could we crib a bit from the
> commit message to describe why, e.g.
>
>         /*
>          * If GCR_EL1.RRND=1 is implemented the same way as RRND=0, then
>          * RGSR_EL1.SEED must be non-zero for IRG to produce
>          * pseudorandom numbers. As RGSR_EL1 is UNKNOWN out of reset, we
>          * must initialize it.
>          */

Done in v2.

> > +     mrs     x10, CNTVCT_EL0
> > +     and     x10, x10, #SYS_RGSR_EL1_SEED_MASK
> > +     cbnz    x10, 1f
> > +     mov     x10, #1
> > +1:
>
> To minimize the diff and make this more locally contained, we could
> avoid the branch and relabelling by using ANDS and CSET:
>
>         mrs     x10, CNTVCT_EL0
>         ands    x10, x10, #SYS_RGSR_EL1_SEED_MASK
>         cset    x10, ne
>
> ... or we could unconditionally ORR in 1:
>
>         mrs     x10, CNTVCT_EL0
>         orr     x10, x10, #1
>         and     x10, x10, #SYS_RGSR_EL1_SEED_MASK

Let's go with the first option to preserve as much entropy as
possible. But I think the last instruction needs to be:

csinc   x10, x10, xzr, ne

Otherwise we will end up setting x10 to either 0 or 1.

Peter

>
> Thanks,
> Mark.
>
> > +     lsl     x10, x10, #SYS_RGSR_EL1_SEED_SHIFT
> > +     msr_s   SYS_RGSR_EL1, x10
> > +
> >       /* clear any pending tag check faults in TFSR*_EL1 */
> >       msr_s   SYS_TFSR_EL1, xzr
> >       msr_s   SYS_TFSRE0_EL1, xzr
> > @@ -454,7 +467,7 @@ SYM_FUNC_START(__cpu_setup)
> >       /* set the TCR_EL1 bits */
> >       mov_q   x10, TCR_KASAN_HW_FLAGS
> >       orr     tcr, tcr, x10
> > -1:
> > +2:
> >  #endif
> >       tcr_clear_errata_bits tcr, x9, x5
> >
> > --
> > 2.31.1.607.g51e8a6a459-goog
> >
> >
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