[PATCH bpf-next v3 07/10] bpf: lsm: Make the allocated callback RO+X

[KP Singh <kpsingh@chromium.org>]

From: KP Singh <kpsingh@google.com>

This patch is not needed after arch_bpf_prepare_trampoline
moves to using text_poke.

The two IPI TLB flushes can be further optimized if a new API to handle
W^X in the kernel emerges as an outcome of:

  https://lore.kernel.org/bpf/20200103234725.22846-1-kpsingh@chromium.org/

Signed-off-by: KP Singh <kpsingh@google.com>
Reviewed-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Florent Revest <revest@google.com>
Reviewed-by: Thomas Garnier <thgarnie@google.com>
---
 security/bpf/hooks.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/security/bpf/hooks.c b/security/bpf/hooks.c
index f1d4fdcdb20e..beeeb8c1f9c2 100644
--- a/security/bpf/hooks.c
+++ b/security/bpf/hooks.c
@@ -190,6 +190,15 @@ static struct bpf_lsm_hook *bpf_lsm_hook_alloc(struct bpf_lsm_list *list,
 		goto error;
 	}
 
+	/* First make the page read-only, and only then make it executable to
+	 * prevent it from being W+X in between.
+	 */
+	set_memory_ro((unsigned long)image, 1);
+	/* More checks can be done here to ensure that nothing was changed
+	 * between arch_prepare_bpf_trampoline and set_memory_ro.
+	 */
+	set_memory_x((unsigned long)image, 1);
+
 	hook = kzalloc(sizeof(struct bpf_lsm_hook), GFP_KERNEL);
 	if (!hook) {
 		ret = -ENOMEM;
-- 
2.20.1