[PATCH] usercopy: Remove HARDENED_USERCOPY_PAGESPAN

Kees Cook
2019-05-10T20:43:42Z

This feature continues to cause more problems than it solves[1]. Its
intention was to check the bounds of page-allocator allocations by using
__GFP_COMP, for which we would need to find all missing __GFP_COMP
markings. This work has been on hold and there is an argument[2]
that such markings are not even the correct signal for checking for
same-allocation pages. Instead of depending on BROKEN, this just removes
it entirely. It can be trivially reverted if/when a better solution for
tracking page allocator sizes is found.

[1] https://www.mail-archive.com/linux-crypto@vger.kernel.org/msg37479.html
[2] https://lkml.kernel.org/r/20190415022412.GA29714@bombadil.infradead.org

Suggested-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
mm/usercopy.c | 67 ------------------------------------------------
security/Kconfig | 11 --------
2 files changed, 78 deletions(-)

diff --git a/mm/usercopy.c b/mm/usercopy.c
index 14faadcedd06..15dc1bf03303 100644
--- a/mm/usercopy.c
+++ b/mm/usercopy.c
@@ -159,70 +159,6 @@ static inline void check_bogus_address(const unsigned long ptr, unsigned long n,
usercopy_abort("null address", NULL, to_user, ptr, n);
}

-/* Checks for allocs that are marked in some way as spanning multiple pages. */
-static inline void check_page_span(const void *ptr, unsigned long n,
- struct page *page, bool to_user)
-{
-#ifdef CONFIG_HARDENED_USERCOPY_PAGESPAN
- const void *end = ptr + n - 1;
- struct page *endpage;
- bool is_reserved, is_cma;
-
- /*
- * Sometimes the kernel data regions are not marked Reserved (see
- * check below). And sometimes [_sdata,_edata) does not cover
- * rodata and/or bss, so check each range explicitly.
- */
-
- /* Allow reads of kernel rodata region (if not marked as Reserved). */
- if (ptr >= (const void *)__start_rodata &&
- end <= (const void *)__end_rodata) {
- if (!to_user)
- usercopy_abort("rodata", NULL, to_user, 0, n);
- return;
- }
-
- /* Allow kernel data region (if not marked as Reserved). */
- if (ptr >= (const void *)_sdata && end <= (const void *)_edata)
- return;
-
- /* Allow kernel bss region (if not marked as Reserved). */
- if (ptr >= (const void *)__bss_start &&
- end <= (const void *)__bss_stop)
- return;
-
- /* Is the object wholly within one base page? */
- if (likely(((unsigned long)ptr & (unsigned long)PAGE_MASK) ==
- ((unsigned long)end & (unsigned long)PAGE_MASK)))
- return;
-
- /* Allow if fully inside the same compound (__GFP_COMP) page. */
- endpage = virt_to_head_page(end);
- if (likely(endpage == page))
- return;
-
- /*
- * Reject if range is entirely either Reserved (i.e. special or
- * device memory), or CMA. Otherwise, reject since the object spans
- * several independently allocated pages.
- */
- is_reserved = PageReserved(page);
- is_cma = is_migrate_cma_page(page);
- if (!is_reserved && !is_cma)
- usercopy_abort("spans multiple pages", NULL, to_user, 0, n);
-
- for (ptr += PAGE_SIZE; ptr <= end; ptr += PAGE_SIZE) {
- page = virt_to_head_page(ptr);
- if (is_reserved && !PageReserved(page))
- usercopy_abort("spans Reserved and non-Reserved pages",
- NULL, to_user, 0, n);
- if (is_cma && !is_migrate_cma_page(page))
- usercopy_abort("spans CMA and non-CMA pages", NULL,
- to_user, 0, n);
- }
-#endif
-}
-
static inline void check_heap_object(const void *ptr, unsigned long n,
bool to_user)
{
@@ -236,9 +172,6 @@ static inline void check_heap_object(const void *ptr, unsigned long n,
if (PageSlab(page)) {
/* Check slab allocator for flags and size. */
__check_heap_object(ptr, n, page, to_user);
- } else {
- /* Verify object does not incorrectly span multiple pages. */
- check_page_span(ptr, n, page, to_user);
}
}

diff --git a/security/Kconfig b/security/Kconfig
index 353cfef71d4e..8392647f5a4c 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -176,17 +176,6 @@ config HARDENED_USERCOPY_FALLBACK
Booting with "slab_common.usercopy_fallback=Y/N" can change
this setting.

-config HARDENED_USERCOPY_PAGESPAN
- bool "Refuse to copy allocations that span multiple pages"
- depends on HARDENED_USERCOPY
- depends on EXPERT
- 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.
-
config FORTIFY_SOURCE
bool "Harden common str/mem functions against buffer overflows"
depends on ARCH_HAS_FORTIFY_SOURCE
--
2.17.1

--
Kees Cook