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Subject[PATCH 4.19 19/50] mm/usercopy.c: no check page span for stack objects
Date
4.19-stable review patch.  If anyone has any objections, please let me know.

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

From: Qian Cai <cai@lca.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@chromium.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@lca.pw>
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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);
}

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