[RFC PATCH 1/8] arm64: kaslr: deal with init called with VA randomization enabled

[Ard Biesheuvel]

We will pass the KASLR seed via x1 as well as via the device tree, so
that the boot code does not need to create the kernel mapping twice.

This means we may enter kaslr_early_init() fully randomized, and the
offset calculated by this function should be identical to the offset
that has already been taken into account.

This means that taking the address of _end or _etext and adding offset
to it produces the wrong value, given that _end and _etext references
will have been fixed up already, and therefore already incorporate
offset.

So instead of referring to these symbols directly, use their offsets
relative to _text, which should produce values that depend on the size
and layout of the Image only. Then, add KIMAGE_VADDR to obtain the
unrandomized values.

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
 arch/arm64/kernel/kaslr.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/kaslr.c b/arch/arm64/kernel/kaslr.c
index 418b2bba1521..00cec96003fd 100644
--- a/arch/arm64/kernel/kaslr.c
+++ b/arch/arm64/kernel/kaslr.c
@@ -145,6 +145,8 @@ u64 __init kaslr_early_init(void)
 		return offset % SZ_2G;
 
 	if (IS_ENABLED(CONFIG_RANDOMIZE_MODULE_REGION_FULL)) {
+		u64 end = (u64)_end - (u64)_text + KIMAGE_VADDR;
+
 		/*
 		 * Randomize the module region over a 2 GB window covering the
 		 * kernel. This reduces the risk of modules leaking information
@@ -154,9 +156,11 @@ u64 __init kaslr_early_init(void)
 		 * resolved normally.)
 		 */
 		module_range = SZ_2G - (u64)(_end - _stext);
-		module_alloc_base = max((u64)_end + offset - SZ_2G,
+		module_alloc_base = max(end + offset - SZ_2G,
 					(u64)MODULES_VADDR);
 	} else {
+		u64 end = (u64)_etext - (u64)_text + KIMAGE_VADDR;
+
 		/*
 		 * Randomize the module region by setting module_alloc_base to
 		 * a PAGE_SIZE multiple in the range [_etext - MODULES_VSIZE,
@@ -167,7 +171,7 @@ u64 __init kaslr_early_init(void)
 		 * when ARM64_MODULE_PLTS is enabled.
 		 */
 		module_range = MODULES_VSIZE - (u64)(_etext - _stext);
-		module_alloc_base = (u64)_etext + offset - MODULES_VSIZE;
+		module_alloc_base = end + offset - MODULES_VSIZE;
 	}
 
 	/* use the lower 21 bits to randomize the base of the module region */
-- 
2.30.2