[PATCH v5 8/8] powerpc/mm: Disable set_memory() routines when strict RWX isn't enabled

[Russell Currey]

There are a couple of reasons that the set_memory() functions are
problematic when STRICT_KERNEL_RWX isn't enabled:

 - The linear mapping is a different size and apply_to_page_range()
	may modify a giant section, breaking everything
 - patch_instruction() doesn't know to work around a page being marked
 	RO, and will subsequently crash

The latter can be replicated by building a kernel with the set_memory()
patches but with STRICT_KERNEL_RWX off and running ftracetest.

Reported-by: Jordan Niethe <jniethe5 at gmail.com>
Signed-off-by: Russell Currey <ruscur at russell.cc>
---
v5: Apply to both set_memory_attr() and change_memory_attr()
v4: New

 arch/powerpc/mm/pageattr.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/arch/powerpc/mm/pageattr.c b/arch/powerpc/mm/pageattr.c
index ee6b5e3b7604..49b8e2e0581d 100644
--- a/arch/powerpc/mm/pageattr.c
+++ b/arch/powerpc/mm/pageattr.c
@@ -64,13 +64,18 @@ static int change_page_attr(pte_t *ptep, unsigned long addr, void *data)
 
 int change_memory_attr(unsigned long addr, int numpages, long action)
 {
-	unsigned long start = ALIGN_DOWN(addr, PAGE_SIZE);
-	unsigned long sz = numpages * PAGE_SIZE;
+	unsigned long start, size;
+
+	if (!IS_ENABLED(CONFIG_STRICT_KERNEL_RWX))
+		return 0;
 
 	if (!numpages)
 		return 0;
 
-	return apply_to_page_range(&init_mm, start, sz, change_page_attr, (void *)action);
+	start = ALIGN_DOWN(addr, PAGE_SIZE);
+	size = numpages * PAGE_SIZE;
+
+	return apply_to_page_range(&init_mm, start, size, change_page_attr, (void *)action);
 }
 
 /*
@@ -96,12 +101,17 @@ static int set_page_attr(pte_t *ptep, unsigned long addr, void *data)
 
 int set_memory_attr(unsigned long addr, int numpages, pgprot_t prot)
 {
-	unsigned long start = ALIGN_DOWN(addr, PAGE_SIZE);
-	unsigned long sz = numpages * PAGE_SIZE;
+	unsigned long start, size;
+
+	if (!IS_ENABLED(CONFIG_STRICT_KERNEL_RWX))
+		return 0;
 
 	if (!numpages)
 		return 0;
 
-	return apply_to_page_range(&init_mm, start, sz, set_page_attr,
+	start = ALIGN_DOWN(addr, PAGE_SIZE);
+	size = numpages * PAGE_SIZE;
+
+	return apply_to_page_range(&init_mm, start, size, set_page_attr,
 				   (void *)pgprot_val(prot));
 }
-- 
2.25.1