arm64: mm: Fix "rodata=on" when CONFIG_RODATA_FULL_DEFAULT_ENABLED=y

When CONFIG_RODATA_FULL_DEFAULT_ENABLED=y, passing "rodata=on" on the
kernel command-line (rather than "rodata=full") should turn off the
"full" behaviour, leaving writable linear aliases of read-only kernel
memory. Unfortunately, the option has no effect in this situation and
the only way to disable the "rodata=full" behaviour is to disable rodata
protection entirely by passing "rodata=off".

Fix this by parsing the "on" and "off" options in the arch code,
additionally enforcing that 'rodata_full' cannot be set without also
setting 'rodata_enabled', allowing us to simplify a couple of checks
in the process.

Fixes: 2e8cff0 ("arm64: fix rodata=full")
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
Reviewed-by: "Russell King (Oracle)" <rmk+kernel@armlinux.org.uk>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20231117131422.29663-1-will@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: willdeacon

arch/arm64/include/asm/setup.h

@@ -21,9 +21,22 @@ static inline bool arch_parse_debug_rodata(char *arg)
 	extern bool rodata_enabled;
 	extern bool rodata_full;
 
-	if (arg && !strcmp(arg, "full")) {
+	if (!arg)
+		return false;
+
+	if (!strcmp(arg, "full")) {
+		rodata_enabled = rodata_full = true;
+		return true;
+	}
+
+	if (!strcmp(arg, "off")) {
+		rodata_enabled = rodata_full = false;
+		return true;
+	}
+
+	if (!strcmp(arg, "on")) {
 		rodata_enabled = true;
-		rodata_full = true;
+		rodata_full = false;
 		return true;
 	}
 

arch/arm64/mm/pageattr.c

@@ -29,8 +29,8 @@ bool can_set_direct_map(void)
 	 *
 	 * KFENCE pool requires page-granular mapping if initialized late.
 	 */
-	return (rodata_enabled && rodata_full) || debug_pagealloc_enabled() ||
-		arm64_kfence_can_set_direct_map();
+	return rodata_full || debug_pagealloc_enabled() ||
+	       arm64_kfence_can_set_direct_map();
 }
 
 static int change_page_range(pte_t *ptep, unsigned long addr, void *data)
@@ -105,8 +105,7 @@ static int change_memory_common(unsigned long addr, int numpages,
 	 * If we are manipulating read-only permissions, apply the same
 	 * change to the linear mapping of the pages that back this VM area.
 	 */
-	if (rodata_enabled &&
-	    rodata_full && (pgprot_val(set_mask) == PTE_RDONLY ||
+	if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY ||
 			    pgprot_val(clear_mask) == PTE_RDONLY)) {
 		for (i = 0; i < area->nr_pages; i++) {
 			__change_memory_common((u64)page_address(area->pages[i]),