[PATCH 3/4] xattr: Use dedicated slab buckets for setxattr()

[Kees Cook <keescook@chromium.org>]

The setxattr() API can be used for exploiting[1][2][3] use-after-free
type confusion flaws in the kernel. Avoid having a user-controlled size
cache share the global kmalloc allocator by using a separate set of
kmalloc buckets.

Link: https://duasynt.com/blog/linux-kernel-heap-spray [1]
Link: https://etenal.me/archives/1336 [2]
Link: https://github.com/a13xp0p0v/kernel-hack-drill/blob/master/drill_exploit_uaf.c [3]
Signed-off-by: Kees Cook <keescook@chromium.org>
---
Cc: Christian Brauner <brauner@kernel.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Jan Kara <jack@suse.cz>
Cc: linux-fsdevel@vger.kernel.org
---
 fs/xattr.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/fs/xattr.c b/fs/xattr.c
index 09d927603433..2b06316f1d1f 100644
--- a/fs/xattr.c
+++ b/fs/xattr.c
@@ -821,6 +821,16 @@ SYSCALL_DEFINE4(fgetxattr, int, fd, const char __user *, name,
 	return error;
 }
 
+static struct kmem_buckets *xattr_buckets;
+static int __init init_xattr_buckets(void)
+{
+	xattr_buckets = kmem_buckets_create("xattr", 0, 0, 0,
+					    XATTR_LIST_MAX, NULL);
+
+	return 0;
+}
+subsys_initcall(init_xattr_buckets);
+
 /*
  * Extended attribute LIST operations
  */
@@ -833,7 +843,7 @@ listxattr(struct dentry *d, char __user *list, size_t size)
 	if (size) {
 		if (size > XATTR_LIST_MAX)
 			size = XATTR_LIST_MAX;
-		klist = kvmalloc(size, GFP_KERNEL);
+		klist = kmem_buckets_alloc(xattr_buckets, size, GFP_KERNEL);
 		if (!klist)
 			return -ENOMEM;
 	}
-- 
2.34.1