[PATCH v2] arm64: mte: initialize RGSR_EL1.SEED in __cpu_setup

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 <redacted>
Fixes: 3b714d24ef17 ("arm64: mte: CPU feature detection and initial sysreg configuration")
Cc: <redacted> # 5.10
Link: https://linux-review.googlesource.com/id/I2b089b6c7d6f17ee37e2f0db7df5ad5bcc04526c
---
 arch/arm64/mm/proc.S | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
index 0a48191534ff..97d7bcd8d4f2 100644
--- a/arch/arm64/mm/proc.S
+++ b/arch/arm64/mm/proc.S

@@ -447,6 +447,18 @@ SYM_FUNC_START(__cpu_setup)
 	mov	x10, #(SYS_GCR_EL1_RRND | SYS_GCR_EL1_EXCL_MASK)
 	msr_s	SYS_GCR_EL1, x10
 
+	/*
+	 * 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.
+	 */
+	mrs	x10, CNTVCT_EL0
+	ands	x10, x10, #SYS_RGSR_EL1_SEED_MASK
+	csinc	x10, x10, xzr, ne
+	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

-- 
2.31.1.607.g51e8a6a459-goog


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On Fri, May 07, 2021 at 11:59:05AM -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 <redacted>
Fixes: 3b714d24ef17 ("arm64: mte: CPU feature detection and initial sysreg configuration")
Cc: <redacted> # 5.10
Link: https://linux-review.googlesource.com/id/I2b089b6c7d6f17ee37e2f0db7df5ad5bcc04526c

FWIW:

Acked-by: Mark Rutland <mark.rutland@arm.com>

Mark.

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On Fri, 7 May 2021 11:59:05 -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.

Applied to arm64 (for-next/fixes), thanks!

[1/1] arm64: mte: initialize RGSR_EL1.SEED in __cpu_setup
      https://git.kernel.org/arm64/c/37a8024d2655

-- 
Catalin


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