[PATCH v2 4/4] slub: Force on no_hash_pointers when slub_debug is enabled

[Stephen Boyd]

Obscuring the pointers that slub shows when debugging makes for some
confusing slub debug messages:

 Padding overwritten. 0x0000000079f0674a-0x000000000d4dce17

Those addresses are hashed for kernel security reasons. If we're trying
to be secure with slub_debug on the commandline we have some big
problems given that we dump whole chunks of kernel memory to the kernel
logs. Let's force on the no_hash_pointers commandline flag when
slub_debug is on the commandline. This makes slub debug messages more
meaningful and if by chance a kernel address is in some slub debug
object dump we will have a better chance of figuring out what went
wrong.

Note that we don't use %px in the slub code because we want to reduce
the number of places that %px is used in the kernel. This also nicely
prints a big fat warning at kernel boot if slub_debug is on the
commandline so that we know that this kernel shouldn't be used on
production systems.

Signed-off-by: Stephen Boyd <swboyd@xxxxxxxxxxxx>
---

I opted for extern because I guess we don't want to advertise
no_hash_pointers_enable() in some sort of header file? It can be put in
a header file but I see that the no_hash_pointers variable is also not 
in a header file but exported as symbol.

 lib/vsprintf.c | 2 +-
 mm/slub.c      | 6 ++++++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index f0c35d9b65bf..cc281f5895f9 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -2186,7 +2186,7 @@ char *fwnode_string(char *buf, char *end, struct fwnode_handle *fwnode,
 bool no_hash_pointers __ro_after_init;
 EXPORT_SYMBOL_GPL(no_hash_pointers);
 
-static int __init no_hash_pointers_enable(char *str)
+int __init no_hash_pointers_enable(char *str)
 {
 	if (no_hash_pointers)
 		return 0;
diff --git a/mm/slub.c b/mm/slub.c
index bf4949115412..1c30436d3e6c 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -4451,6 +4451,8 @@ static struct kmem_cache * __init bootstrap(struct kmem_cache *static_cache)
 	return s;
 }
 
+extern int no_hash_pointers_enable(char *str);
+
 void __init kmem_cache_init(void)
 {
 	static __initdata struct kmem_cache boot_kmem_cache,
@@ -4470,6 +4472,10 @@ void __init kmem_cache_init(void)
 	for_each_node_state(node, N_NORMAL_MEMORY)
 		node_set(node, slab_nodes);
 
+	/* Print slub debugging pointers without hashing */
+	if (static_branch_unlikely(&slub_debug_enabled))
+		no_hash_pointers_enable(NULL);
+
 	create_boot_cache(kmem_cache_node, "kmem_cache_node",
 		sizeof(struct kmem_cache_node), SLAB_HWCACHE_ALIGN, 0, 0);
 
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