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Date: Tue, 24 Dec 2019 17:41:25 +1100
From: Russell Currey <ruscur@...sell.cc>
To: linuxppc-dev@...ts.ozlabs.org
Cc: ajd@...ux.ibm.com,
	mpe@...erman.id.au,
	christophe.leroy@....fr,
	npiggin@...il.com,
	kernel-hardening@...ts.openwall.com,
	Russell Currey <ruscur@...sell.cc>
Subject: [PATCH 1/2] powerpc/book3s64/hash: Disable 16M linear mapping size if not aligned

With STRICT_KERNEL_RWX on in a relocatable kernel under the hash MMU, if
the position the kernel is loaded at is not 16M aligned, the kernel
miscalculates its ALIGN*()s and things go horribly wrong.

We can easily avoid this when selecting the linear mapping size, so do
so and print a warning.  I tested this for various alignments and as
long as the position is 64K aligned it's fine (the base requirement for
powerpc).

Signed-off-by: Russell Currey <ruscur@...sell.cc>
---
 arch/powerpc/mm/book3s64/hash_utils.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/mm/book3s64/hash_utils.c b/arch/powerpc/mm/book3s64/hash_utils.c
index b30435c7d804..523d4d39d11e 100644
--- a/arch/powerpc/mm/book3s64/hash_utils.c
+++ b/arch/powerpc/mm/book3s64/hash_utils.c
@@ -652,6 +652,7 @@ static void init_hpte_page_sizes(void)
 
 static void __init htab_init_page_sizes(void)
 {
+	bool aligned = true;
 	init_hpte_page_sizes();
 
 	if (!debug_pagealloc_enabled()) {
@@ -659,7 +660,15 @@ static void __init htab_init_page_sizes(void)
 		 * Pick a size for the linear mapping. Currently, we only
 		 * support 16M, 1M and 4K which is the default
 		 */
-		if (mmu_psize_defs[MMU_PAGE_16M].shift)
+		if (IS_ENABLED(STRICT_KERNEL_RWX) &&
+		    (unsigned long)_stext % 0x1000000) {
+			if (mmu_psize_defs[MMU_PAGE_16M].shift)
+				pr_warn("Kernel not 16M aligned, "
+					"disabling 16M linear map alignment");
+			aligned = false;
+		}
+
+		if (mmu_psize_defs[MMU_PAGE_16M].shift && aligned)
 			mmu_linear_psize = MMU_PAGE_16M;
 		else if (mmu_psize_defs[MMU_PAGE_1M].shift)
 			mmu_linear_psize = MMU_PAGE_1M;
-- 
2.24.1

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