powerpc/mm: Warn if W+X pages found on boot

Implement code to walk all pages and warn if any are found to be both
writable and executable.  Depends on STRICT_KERNEL_RWX enabled, and is
behind the DEBUG_WX config option.

This only runs on boot and has no runtime performance implications.

Very heavily influenced (and in some cases copied verbatim) from the
ARM64 code written by Laura Abbott (thanks!), since our ptdump
infrastructure is similar.

Signed-off-by: Russell Currey <ruscur@russell.cc>
[mpe: Fixup build error when disabled]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 42 insertions, 1 deletions

diff --git a/arch/powerpc/mm/ptdump/ptdump.c b/arch/powerpc/mm/ptdump/ptdump.c
index e249a56c07bf..646876d9da64 100644
--- a/arch/powerpc/mm/ptdump/ptdump.c
+++ b/arch/powerpc/mm/ptdump/ptdump.c
@@ -31,7 +31,7 @@
 #include "ptdump.h"
 
 #ifdef CONFIG_PPC32
-#define KERN_VIRT_START 0
+#define KERN_VIRT_START PAGE_OFFSET
 #endif
 
 /*
@@ -68,6 +68,8 @@ struct pg_state {
         unsigned long last_pa;
         unsigned int level;
         u64 current_flags;
+        bool check_wx;
+        unsigned long wx_pages;
 };
 
 struct addr_marker {
@@ -181,6 +183,20 @@ static void dump_addr(struct pg_state *st, unsigned long addr)
 
 }
 
+static void note_prot_wx(struct pg_state *st, unsigned long addr)
+{
+        if (!st->check_wx)
+                return;
+
+        if (!((st->current_flags & pgprot_val(PAGE_KERNEL_X)) == pgprot_val(PAGE_KERNEL_X)))
+                return;
+
+        WARN_ONCE(1, "powerpc/mm: Found insecure W+X mapping at address %p/%pS\n",
+                  (void *)st->start_address, (void *)st->start_address);
+
+        st->wx_pages += (addr - st->start_address) / PAGE_SIZE;
+}
+
 static void note_page(struct pg_state *st, unsigned long addr,
                unsigned int level, u64 val)
 {
@@ -210,6 +226,7 @@ static void note_page(struct pg_state *st, unsigned long addr,
 
                 /* Check the PTE flags */
                 if (st->current_flags) {
+                        note_prot_wx(st, addr);
                         dump_addr(st, addr);
 
                         /* Dump all the flags */
@@ -387,6 +404,30 @@ static void build_pgtable_complete_mask(void)
                                 pg_level[i].mask |= pg_level[i].flag[j].mask;
 }
 
+#ifdef CONFIG_PPC_DEBUG_WX
+void ptdump_check_wx(void)
+{
+        struct pg_state st = {
+                .seq = NULL,
+                .marker = address_markers,
+                .check_wx = true,
+        };
+
+        if (radix_enabled())
+                st.start_address = PAGE_OFFSET;
+        else
+                st.start_address = KERN_VIRT_START;
+
+        walk_pagetables(&st);
+
+        if (st.wx_pages)
+                pr_warn("Checked W+X mappings: FAILED, %lu W+X pages found\n",
+                        st.wx_pages);
+        else
+                pr_info("Checked W+X mappings: passed, no W+X pages found\n");
+}
+#endif
+
 static int ptdump_init(void)
 {
         struct dentry *debugfs_file;