[PATCH] usercopy: remove page-spanning test for now

[Kees Cook <keescook@chromium.org>]

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>
---
 mm/usercopy.c    |  8 ++++++++
 security/Kconfig | 11 +++++++++++
 2 files changed, 19 insertions(+)

diff --git a/mm/usercopy.c b/mm/usercopy.c
index a3cc3052f830..b3f0c3355dc8 100644
--- a/mm/usercopy.c
+++ b/mm/usercopy.c
@@ -172,6 +172,14 @@ static inline const char *check_heap_object(const void *ptr, unsigned long n,
 		return NULL;
 	}
 
+#ifndef CONFIG_HARDENED_USERCOPY_PAGESPAN
+	/*
+	 * The page-spanning checks are hitting false positives, so
+	 * do not check them for now.
+	 */
+	return NULL;
+#endif
+
 	/* Allow kernel data region (if not marked as Reserved). */
 	if (ptr >= (const void *)_sdata && end <= (const void *)_edata)
 		return NULL;
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
-- 
2.7.4


-- 
Kees Cook
Nexus Security