[PATCH v6 2/9] arm64: kaslr: don't pretend KASLR is enabled if offset < MIN_KIMG_ALIGN

[Ard Biesheuvel]

Our virtual KASLR displacement consists of a fully randomized multiple
of 2 MiB, combined with an offset that is equal to the physical
placement modulo 2 MiB. This arrangement ensures that we can always use
2 MiB block mappings (or contiguous PTE mappings for 16k or 64k pages)
to map the kernel.

This means that a KASLR offset of less than 2 MiB is simply the product
of this physical displacement, and no randomization has actually taken
place. So let's avoid misreporting this case as 'KASLR enabled'.

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
 arch/arm64/include/asm/memory.h | 11 +++++++++++
 arch/arm64/kernel/cpufeature.c  |  2 +-
 arch/arm64/kernel/kaslr.c       |  2 +-
 3 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
index c751cd9b94f8..498af99d1adc 100644
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h
@@ -172,6 +172,7 @@
 #include <linux/compiler.h>
 #include <linux/mmdebug.h>
 #include <linux/types.h>
+#include <asm/boot.h>
 #include <asm/bug.h>
 
 #if VA_BITS > 48
@@ -195,6 +196,16 @@ static inline unsigned long kaslr_offset(void)
 	return kimage_vaddr - KIMAGE_VADDR;
 }
 
+static inline bool kaslr_enabled(void)
+{
+	/*
+	 * The KASLR offset modulo MIN_KIMG_ALIGN is taken from the physical
+	 * placement of the image rather than from the seed, so a displacement
+	 * of less than MIN_KIMG_ALIGN means that no seed was provided.
+	 */
+	return kaslr_offset() >= MIN_KIMG_ALIGN;
+}
+
 /*
  * Allow all memory at the discovery stage. We will clip it later.
  */
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 98b48d9069a7..22e3604aee02 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -1562,7 +1562,7 @@ bool kaslr_requires_kpti(void)
 			return false;
 	}
 
-	return kaslr_offset() > 0;
+	return kaslr_enabled();
 }
 
 static bool __meltdown_safe = true;
diff --git a/arch/arm64/kernel/kaslr.c b/arch/arm64/kernel/kaslr.c
index bcbcca938da8..d63322fc1d40 100644
--- a/arch/arm64/kernel/kaslr.c
+++ b/arch/arm64/kernel/kaslr.c
@@ -43,7 +43,7 @@ static int __init kaslr_init(void)
 		return 0;
 	}
 
-	if (!kaslr_offset()) {
+	if (!kaslr_enabled()) {
 		pr_warn("KASLR disabled due to lack of seed\n");
 		return 0;
 	}
-- 
2.35.1