[RFC PATCH 3/8] arm64: head: take KASLR seed passed via x1 into account

[Ard Biesheuvel]

If the kernel was entered with the MMU on, use the value passed via
register x1 as the seed for virtual randomization rather than the one
passed via the /chosen/kaslr-seed DT property. This removes the need to
map and unmap the kernel just to access the DT.

Note that kaslr_early_init() still needs to be called exactly once in
this case, to set the other KASLR related state, so ensure that this
happens when called with the MMU on.

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
 arch/arm64/kernel/head.S | 28 ++++++++++++++++++--
 1 file changed, 26 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index bec9c1483584..0726ce0d6fd4 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -91,10 +91,9 @@
 	 */
 SYM_CODE_START(primary_entry)
 	bl	record_mmu_state
+	bl	record_kaslr_offset
 	bl	preserve_boot_args
 	bl	init_kernel_el			// w0=cpu_boot_mode
-	adrp	x23, __PHYS_OFFSET
-	and	x23, x23, MIN_KIMG_ALIGN - 1	// KASLR offset, defaults to 0
 	bl	set_cpu_boot_mode_flag
 	bl	__create_page_tables
 	/*
@@ -117,6 +116,29 @@ SYM_CODE_START_LOCAL(record_mmu_state)
 	ret
 SYM_CODE_END(record_mmu_state)
 
+SYM_CODE_START_LOCAL(record_kaslr_offset)
+	adrp	x23, __PHYS_OFFSET
+	and	x23, x23, MIN_KIMG_ALIGN - 1	// KASLR offset, defaults to 0
+	cbz	x25, 0f				// skip if MMU is disabled
+#ifdef CONFIG_RANDOMIZE_BASE
+	/*
+	 * Calculate the virtual randomization that will be applied based on
+	 * the KASLR seed passed via x1, if provided.
+	 */
+	and	x4, x1, ~(MIN_KIMG_ALIGN - 1)
+	and	x4, x4, #(1 << (VA_BITS_MIN - 2)) - 1
+	mov	x5, #1 << (VA_BITS_MIN - 3)
+	add	x4, x4, x5
+#if !defined(CONFIG_KASAN_VMALLOC) && \
+    (defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS))
+	and	x4, x4, #(1 << 31) - 1		// limit to 2G
+#endif
+	orr	x23, x23, x4
+	mov	x1, xzr				// wipe the seed from x1
+#endif
+0:	ret
+SYM_CODE_END(record_kaslr_offset)
+
 /*
  * Preserve the arguments passed by the bootloader in x0 .. x3
  */
@@ -467,8 +489,10 @@ SYM_FUNC_START_LOCAL(__primary_switched)
 	bl	init_feature_override		// Parse cpu feature overrides
 #ifdef CONFIG_RANDOMIZE_BASE
 	tst	x23, ~(MIN_KIMG_ALIGN - 1)	// already running randomized?
+	ccmp	x25, #1, #4, ne			// and we booted with the MMU off?
 	b.ne	0f
 	bl	kaslr_early_init		// parse FDT for KASLR options
+	cbnz	x25, 0f				// MMU was on?
 	cbz	x0, 0f				// KASLR disabled? just proceed
 	orr	x23, x23, x0			// record KASLR offset
 	ldp	x29, x30, [sp], #16		// we must enable KASLR, return
-- 
2.30.2