[PATCH] usercopy: Remove HARDENED_USERCOPY_PAGESPAN

[Kees Cook]

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@xxxxxxxxxxxxxxx/msg37479.html
[2] https://lkml.kernel.org/r/20190415022412.GA29714@xxxxxxxxxxxxxxxxxxxxxx

Suggested-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 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