usercopy: Remove HARDENED_USERCOPY_PAGESPAN

There isn't enough information to make this a useful check any more;
the useful parts of it were moved in earlier patches, so remove this
set of checks now.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Kees Cook <keescook@chromium.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220110231530.665970-5-willy@infradead.org

mm/usercopy.c

@@ -158,64 +158,6 @@ 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;
-	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;
-
-	/*
-	 * 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)
 {
@@ -257,9 +199,6 @@ static inline void check_heap_object(const void *ptr, unsigned long n,
 		unsigned long offset = ptr - folio_address(folio);
 		if (offset + n > folio_size(folio))
 			usercopy_abort("page alloc", NULL, to_user, offset, n);
-	} else {
-		/* Verify object does not incorrectly span multiple pages. */
-		check_page_span(ptr, n, folio_page(folio, 0), to_user);
 	}
 }
 

security/Kconfig

@@ -160,20 +160,9 @@ config HARDENED_USERCOPY
 	  copy_from_user() functions) by rejecting memory ranges that
 	  are larger than the specified heap object, span multiple
 	  separately allocated pages, are not on the process stack,
-	  or are part of the kernel text. This kills entire classes
+	  or are part of the kernel text. This prevents 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 BROKEN
-	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