usercopy: remove page-spanning test for now

A custom allocator without __GFP_COMP that copies to userspace has been
found in vmw_execbuf_process[1], so this disables the page-span checker
by placing it behind a CONFIG for future work where such things can be
tracked down later.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1373326

Reported-by: Vinson Lee <vlee@freedesktop.org>
Fixes: f5509cc18daa ("mm: Hardened usercopy")
Signed-off-by: Kees Cook <keescook@chromium.org>

1 files changed, 11 insertions, 0 deletions

diff --git a/security/Kconfig b/security/Kconfig
index da10d9b573a4..2dfc0ce4083e 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -147,6 +147,17 @@ config HARDENED_USERCOPY
           or are part of the kernel text. This kills entire classes
           of heap overflow exploits and similar kernel memory exposures.
 
+config HARDENED_USERCOPY_PAGESPAN
+        bool "Refuse to copy allocations that span multiple pages"
+        depends on HARDENED_USERCOPY
+        depends on !COMPILE_TEST
+        help
+          When a multi-page allocation is done without __GFP_COMP,
+          hardened usercopy will reject attempts to copy it. There are,
+          however, several cases of this in the kernel that have not all
+          been removed. This config is intended to be used only while
+          trying to find such users.
+
 source security/selinux/Kconfig
 source security/smack/Kconfig
 source security/tomoyo/Kconfig