linux-kernel - [PATCH 4.19 19/50] mm/usercopy.c: no check page span for stack objects

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Date:   Tue, 15 Jan 2019 17:35:55 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Qian Cai <cai@....pw>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 4.19 19/50] mm/usercopy.c: no check page span for stack objects

4.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Qian Cai <cai@....pw>

commit 7bff3c06997374fb9b9991536a547b840549a813 upstream.

It is easy to trigger this with CONFIG_HARDENED_USERCOPY_PAGESPAN=y,

  usercopy: Kernel memory overwrite attempt detected to spans multiple pages (offset 0, size 23)!
  kernel BUG at mm/usercopy.c:102!

For example,

print_worker_info
char name[WQ_NAME_LEN] = { };
char desc[WORKER_DESC_LEN] = { };
  probe_kernel_read(name, wq->name, sizeof(name) - 1);
  probe_kernel_read(desc, worker->desc, sizeof(desc) - 1);
    __copy_from_user_inatomic
      check_object_size
        check_heap_object
          check_page_span

This is because on-stack variables could cross PAGE_SIZE boundary, and
failed this check,

if (likely(((unsigned long)ptr & (unsigned long)PAGE_MASK) ==
	   ((unsigned long)end & (unsigned long)PAGE_MASK)))

ptr = FFFF889007D7EFF8
end = FFFF889007D7F00E

Hence, fix it by checking if it is a stack object first.

[keescook@...omium.org: improve comments after reorder]
  Link: http://lkml.kernel.org/r/20190103165151.GA32845@beast
Link: http://lkml.kernel.org/r/20181231030254.99441-1-cai@lca.pw
Signed-off-by: Qian Cai <cai@....pw>
Signed-off-by: Kees Cook <keescook@...omium.org>
Acked-by: Kees Cook <keescook@...omium.org>
Cc: <stable@...r.kernel.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 mm/usercopy.c |    9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

--- a/mm/usercopy.c
+++ b/mm/usercopy.c
@@ -247,7 +247,8 @@ static DEFINE_STATIC_KEY_FALSE_RO(bypass
 /*
  * Validates that the given object is:
  * - not bogus address
- * - known-safe heap or stack object
+ * - fully contained by stack (or stack frame, when available)
+ * - fully within SLAB object (or object whitelist area, when available)
  * - not in kernel text
  */
 void __check_object_size(const void *ptr, unsigned long n, bool to_user)
@@ -262,9 +263,6 @@ void __check_object_size(const void *ptr
 	/* Check for invalid addresses. */
 	check_bogus_address((const unsigned long)ptr, n, to_user);
 
-	/* Check for bad heap object. */
-	check_heap_object(ptr, n, to_user);
-
 	/* Check for bad stack object. */
 	switch (check_stack_object(ptr, n)) {
 	case NOT_STACK:
@@ -282,6 +280,9 @@ void __check_object_size(const void *ptr
 		usercopy_abort("process stack", NULL, to_user, 0, n);
 	}
 
+	/* Check for bad heap object. */
+	check_heap_object(ptr, n, to_user);
+
 	/* Check for object in kernel to avoid text exposure. */
 	check_kernel_text_object((const unsigned long)ptr, n, to_user);
 }