On Fri, May 7, 2021 at 3:11 AM Mark Rutland <mark.rutland@xxxxxxx> 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@xxxxxxxxxx> > > Cc: stable@xxxxxxxxxxxxxxx > > 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 > > > > > > _______________________________________________ > > linux-arm-kernel mailing list > > linux-arm-kernel@xxxxxxxxxxxxxxxxxxx > > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
