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>

diff --git a/mm/usercopy.c b/mm/usercopy.c
index a3cc305..089328f 100644 (file)
--- a/mm/usercopy.c
+++ b/mm/usercopy.c
@@ -134,30 +134,15 @@ static inline const char *check_bogus_address(const void *ptr, unsigned long n)
        return NULL;
 }
 
-static inline const char *check_heap_object(const void *ptr, unsigned long n,
-                                           bool to_user)
+/* Checks for allocs that are marked in some way as spanning multiple pages. */
+static inline const char *check_page_span(const void *ptr, unsigned long n,
+                                         struct page *page, bool to_user)
 {
-       struct page *page, *endpage;
+#ifdef CONFIG_HARDENED_USERCOPY_PAGESPAN
        const void *end = ptr + n - 1;
+       struct page *endpage;
        bool is_reserved, is_cma;
 
-       /*
-        * Some architectures (arm64) return true for virt_addr_valid() on
-        * vmalloced addresses. Work around this by checking for vmalloc
-        * first.
-        */
-       if (is_vmalloc_addr(ptr))
-               return NULL;
-
-       if (!virt_addr_valid(ptr))
-               return NULL;
-
-       page = virt_to_head_page(ptr);
-
-       /* Check slab allocator for flags and size. */
-       if (PageSlab(page))
-               return __check_heap_object(ptr, n, page);
-
        /*
         * Sometimes the kernel data regions are not marked Reserved (see
         * check below). And sometimes [_sdata,_edata) does not cover
@@ -186,7 +171,7 @@ static inline const char *check_heap_object(const void *ptr, unsigned long n,
                   ((unsigned long)end & (unsigned long)PAGE_MASK)))
                return NULL;
 
-       /* Allow if start and end are inside the same compound page. */
+       /* Allow if fully inside the same compound (__GFP_COMP) page. */
        endpage = virt_to_head_page(end);
        if (likely(endpage == page))
                return NULL;
@@ -199,20 +184,44 @@ static inline const char *check_heap_object(const void *ptr, unsigned long n,
        is_reserved = PageReserved(page);
        is_cma = is_migrate_cma_page(page);
        if (!is_reserved && !is_cma)
-               goto reject;
+               return "<spans multiple pages>";
 
        for (ptr += PAGE_SIZE; ptr <= end; ptr += PAGE_SIZE) {
                page = virt_to_head_page(ptr);
                if (is_reserved && !PageReserved(page))
-                       goto reject;
+                       return "<spans Reserved and non-Reserved pages>";
                if (is_cma && !is_migrate_cma_page(page))
-                       goto reject;
+                       return "<spans CMA and non-CMA pages>";
        }
+#endif
 
        return NULL;
+}
+
+static inline const char *check_heap_object(const void *ptr, unsigned long n,
+                                           bool to_user)
+{
+       struct page *page;
+
+       /*
+        * Some architectures (arm64) return true for virt_addr_valid() on
+        * vmalloced addresses. Work around this by checking for vmalloc
+        * first.
+        */
+       if (is_vmalloc_addr(ptr))
+               return NULL;
+
+       if (!virt_addr_valid(ptr))
+               return NULL;
+
+       page = virt_to_head_page(ptr);
+
+       /* Check slab allocator for flags and size. */
+       if (PageSlab(page))
+               return __check_heap_object(ptr, n, page);
 
-reject:
-       return "<spans multiple pages>";
+       /* Verify object does not incorrectly span multiple pages. */
+       return check_page_span(ptr, n, page, to_user);
 }
 
 /*

diff --git a/security/Kconfig b/security/Kconfig
index da10d9b..2dfc0ce 100644 (file)
--- 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