[RFC PATCH] arm64: kaslr: Set TCR_EL1.NFD1 when CONFIG_RANDOMIZE_BASE=y

[will.deacon@arm.com (Will Deacon)]

TCR_EL1.NFD1 was allocated by SVE and ensures that fault-surpressing SVE
memory accesses (e.g. speculative accesses from a first-fault gather load)
which translate via TTBR1_EL1 result in a translation fault if they
miss in the TLB when executed from EL0. This mitigates some timing attacks
against KASLR, where the kernel address space could otherwise be probed
efficiently using the FFR in conjunction with suppressed faults on SVE
loads.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
---

Sending as RFC because this doesn't make any difference if kpti is enabled,
which is the default with KASLR. It helps if kpti=off is being passed and
shouldn't have an impact on performance.

 arch/arm64/include/asm/pgtable-hwdef.h | 1 +
 arch/arm64/mm/proc.S                   | 9 ++++++++-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h
index cdfe3e657a9e..fd208eac9f2a 100644
--- a/arch/arm64/include/asm/pgtable-hwdef.h
+++ b/arch/arm64/include/asm/pgtable-hwdef.h
@@ -291,6 +291,7 @@
 #define TCR_TBI0		(UL(1) << 37)
 #define TCR_HA			(UL(1) << 39)
 #define TCR_HD			(UL(1) << 40)
+#define TCR_NFD1		(UL(1) << 54)
 
 /*
  * TTBR.
diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
index c0af47617299..8f074d64b760 100644
--- a/arch/arm64/mm/proc.S
+++ b/arch/arm64/mm/proc.S
@@ -36,6 +36,12 @@
 #define TCR_TG_FLAGS	TCR_TG0_4K | TCR_TG1_4K
 #endif
 
+#ifdef CONFIG_RANDOMIZE_BASE
+#define TCR_KASLR_FLAGS	TCR_NFD1
+#else
+#define TCR_KASLR_FLAGS	0
+#endif
+
 #define TCR_SMP_FLAGS	TCR_SHARED
 
 /* PTWs cacheable, inner/outer WBWA */
@@ -432,7 +438,8 @@ ENTRY(__cpu_setup)
 	 * both user and kernel.
 	 */
 	ldr	x10, =TCR_TxSZ(VA_BITS) | TCR_CACHE_FLAGS | TCR_SMP_FLAGS | \
-			TCR_TG_FLAGS | TCR_ASID16 | TCR_TBI0 | TCR_A1
+			TCR_TG_FLAGS | TCR_KASLR_FLAGS | TCR_ASID16 | \
+			TCR_TBI0 | TCR_A1
 	tcr_set_idmap_t0sz	x10, x9
 
 	/*
-- 
2.1.4