[RFC PATCH 3/6] arm64: mm: fix restoring linear map permissions on execmem cache clean

["Adrian Barnaś" <abarnas@google.com>]

Strip the read-only attribute from the selected memory range when
restoring the linear map after an execmem cache clean.

An execmem cache clean is performed when a cache block becomes empty
after unloading a module. When making the memory valid again, the linear
memory alias must also have its read-only attribute cleared.

Without this change, the linear memory alias remains read-only even
after the execmem cache block itself is freed, which prevents subsequent
allocations from writing to that memory.

Signed-off-by: Adrian Barnaś <abarnas@google.com>
---
 arch/arm64/mm/pageattr.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
index 88720bbba892..eaefdf90b0d5 100644
--- a/arch/arm64/mm/pageattr.c
+++ b/arch/arm64/mm/pageattr.c
@@ -239,6 +239,13 @@ int set_memory_x(unsigned long addr, int numpages)
 					__pgprot(PTE_PXN));
 }
 
+static int set_memory_default(unsigned long addr, int numpages)
+{
+	return __change_memory_common(addr, PAGE_SIZE * numpages,
+				      __pgprot(PTE_VALID),
+				      __pgprot(PTE_RDONLY));
+}
+
 int set_memory_valid(unsigned long addr, int numpages, int enable)
 {
 	if (enable)
@@ -362,7 +369,15 @@ int set_direct_map_valid_noflush(struct page *page, unsigned nr, bool valid)
 	if (!can_set_direct_map())
 		return 0;
 
-	return set_memory_valid(addr, nr, valid);
+	/*
+	 * Execmem cache uses this function to reset permissions on linear mapping
+	 * when freeing unused cache block. On x86 it makes memory RW which is
+	 * desirable. On ARM64 set_memory_valid() just change valid bit which
+	 * leave direct mapping read-only so use set_memory_default instead.
+	 */
+
+	return valid ? set_memory_default(addr, nr) :
+		       set_memory_valid(addr, nr, false);
 }
 
 #ifdef CONFIG_DEBUG_PAGEALLOC
-- 
2.54.0.1136.gdb2ca164c4-goog