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>

diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 38b6442dc5695433012df91b752073f039c3b846..efe79c1cdd4765335e28efb3cb58b9a1b8b06f18 100644 (file)
--- a/include/linux/uaccess.h
+++ b/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);

diff --git a/mm/slab.c b/mm/slab.c
index 47acfe54e1ae00cbcd26d2bbe73f4345c001e47c..1c02f6e942357f1c89edad77916612b5bcac569c 100644 (file)
--- a/mm/slab.c
+++ b/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 */

diff --git a/mm/slub.c b/mm/slub.c
index f40a57164dd61f95626f13973393082f68c8c174..6d9b1e7d3226bac24e48573e776864468814bf5d 100644 (file)
--- a/mm/slub.c
+++ b/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 */

diff --git a/mm/usercopy.c b/mm/usercopy.c
index a562dd094ace2886f848281acd27034148061c22..e9e9325f7638db115151e953fd66e7ae063a8f2f 100644 (file)
--- a/mm/usercopy.c
+++ b/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)