usercopy: WARN() on slab cache usercopy region violations

This patch adds checking of usercopy cache whitelisting, and is modified
from Brad Spengler/PaX Team's PAX_USERCOPY whitelisting code in the
last public patch of grsecurity/PaX based on my understanding of the
code. Changes or omissions from the original code are mine and don't
reflect the original grsecurity/PaX code.

The SLAB and SLUB allocators are modified to WARN() on all copy operations
in which the kernel heap memory being modified falls outside of the cache's
defined usercopy region.

Based on an earlier patch from David Windsor.

Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Laura Abbott <labbott@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-mm@kvack.org
Cc: linux-xfs@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

include/linux/uaccess.h

@@ -274,6 +274,8 @@ extern long strncpy_from_unsafe(char *dst, const void *unsafe_addr, long count);
 #endif
 
 #ifdef CONFIG_HARDENED_USERCOPY
+void usercopy_warn(const char *name, const char *detail, bool to_user,
+		   unsigned long offset, unsigned long len);
 void __noreturn usercopy_abort(const char *name, const char *detail,
 			       bool to_user, unsigned long offset,
 			       unsigned long len);

mm/slab.c

@@ -4392,7 +4392,9 @@ module_init(slab_proc_init);
 
 #ifdef CONFIG_HARDENED_USERCOPY
 /*
- * Rejects objects that are incorrectly sized.
+ * Rejects incorrectly sized objects and objects that are to be copied
+ * to/from userspace but do not fall entirely within the containing slab
+ * cache's usercopy region.
  *
  * Returns NULL if check passes, otherwise const char * to name of cache
  * to indicate an error.
@@ -4412,10 +4414,24 @@ void __check_heap_object(const void *ptr, unsigned long n, struct page *page,
 	/* Find offset within object. */
 	offset = ptr - index_to_obj(cachep, page, objnr) - obj_offset(cachep);
 
-	/* Allow address range falling entirely within object size. */
-	if (offset <= cachep->object_size && n <= cachep->object_size - offset)
+	/* Allow address range falling entirely within usercopy region. */
+	if (offset >= cachep->useroffset &&
+	    offset - cachep->useroffset <= cachep->usersize &&
+	    n <= cachep->useroffset - offset + cachep->usersize)
 		return;
 
+	/*
+	 * If the copy is still within the allocated object, produce
+	 * a warning instead of rejecting the copy. This is intended
+	 * to be a temporary method to find any missing usercopy
+	 * whitelists.
+	 */
+	if (offset <= cachep->object_size &&
+	    n <= cachep->object_size - offset) {
+		usercopy_warn("SLAB object", cachep->name, to_user, offset, n);
+		return;
+	}
+
 	usercopy_abort("SLAB object", cachep->name, to_user, offset, n);
 }
 #endif /* CONFIG_HARDENED_USERCOPY */

mm/slub.c

@@ -3813,7 +3813,9 @@ EXPORT_SYMBOL(__kmalloc_node);
 
 #ifdef CONFIG_HARDENED_USERCOPY
 /*
- * Rejects objects that are incorrectly sized.
+ * Rejects incorrectly sized objects and objects that are to be copied
+ * to/from userspace but do not fall entirely within the containing slab
+ * cache's usercopy region.
  *
  * Returns NULL if check passes, otherwise const char * to name of cache
  * to indicate an error.
@@ -3827,7 +3829,6 @@ void __check_heap_object(const void *ptr, unsigned long n, struct page *page,
 
 	/* Find object and usable object size. */
 	s = page->slab_cache;
-	object_size = slab_ksize(s);
 
 	/* Reject impossible pointers. */
 	if (ptr < page_address(page))
@@ -3845,10 +3846,24 @@ void __check_heap_object(const void *ptr, unsigned long n, struct page *page,
 		offset -= s->red_left_pad;
 	}
 
-	/* Allow address range falling entirely within object size. */
-	if (offset <= object_size && n <= object_size - offset)
+	/* Allow address range falling entirely within usercopy region. */
+	if (offset >= s->useroffset &&
+	    offset - s->useroffset <= s->usersize &&
+	    n <= s->useroffset - offset + s->usersize)
 		return;
 
+	/*
+	 * If the copy is still within the allocated object, produce
+	 * a warning instead of rejecting the copy. This is intended
+	 * to be a temporary method to find any missing usercopy
+	 * whitelists.
+	 */
+	object_size = slab_ksize(s);
+	if (offset <= object_size && n <= object_size - offset) {
+		usercopy_warn("SLUB object", s->name, to_user, offset, n);
+		return;
+	}
+
 	usercopy_abort("SLUB object", s->name, to_user, offset, n);
 }
 #endif /* CONFIG_HARDENED_USERCOPY */

mm/usercopy.c

@@ -59,13 +59,28 @@ static noinline int check_stack_object(const void *obj, unsigned long len)
 }
 
 /*
- * If this function is reached, then CONFIG_HARDENED_USERCOPY has found an
- * unexpected state during a copy_from_user() or copy_to_user() call.
+ * If these functions are reached, then CONFIG_HARDENED_USERCOPY has found
+ * an unexpected state during a copy_from_user() or copy_to_user() call.
  * There are several checks being performed on the buffer by the
  * __check_object_size() function. Normal stack buffer usage should never
  * trip the checks, and kernel text addressing will always trip the check.
- * For cache objects, copies must be within the object size.
+ * For cache objects, it is checking that only the whitelisted range of
+ * bytes for a given cache is being accessed (via the cache's usersize and
+ * useroffset fields). To adjust a cache whitelist, use the usercopy-aware
+ * kmem_cache_create_usercopy() function to create the cache (and
+ * carefully audit the whitelist range).
  */
+void usercopy_warn(const char *name, const char *detail, bool to_user,
+		   unsigned long offset, unsigned long len)
+{
+	WARN_ONCE(1, "Bad or missing usercopy whitelist? Kernel memory %s attempt detected %s %s%s%s%s (offset %lu, size %lu)!\n",
+		 to_user ? "exposure" : "overwrite",
+		 to_user ? "from" : "to",
+		 name ? : "unknown?!",
+		 detail ? " '" : "", detail ? : "", detail ? "'" : "",
+		 offset, len);
+}
+
 void __noreturn usercopy_abort(const char *name, const char *detail,
 			       bool to_user, unsigned long offset,
 			       unsigned long len)