fork: Provide usercopy whitelisting for task_struct

While the blocked and saved_sigmask fields of task_struct are copied to
userspace (via sigmask_to_save() and setup_rt_frame()), it is always
copied with a static length (i.e. sizeof(sigset_t)).

The only portion of task_struct that is potentially dynamically sized and
may be copied to userspace is in the architecture-specific thread_struct
at the end of task_struct.

cache object allocation:
    kernel/fork.c:
        alloc_task_struct_node(...):
            return kmem_cache_alloc_node(task_struct_cachep, ...);

        dup_task_struct(...):
            ...
            tsk = alloc_task_struct_node(node);

        copy_process(...):
            ...
            dup_task_struct(...)

        _do_fork(...):
            ...
            copy_process(...)

example usage trace:

    arch/x86/kernel/fpu/signal.c:
        __fpu__restore_sig(...):
            ...
            struct task_struct *tsk = current;
            struct fpu *fpu = &tsk->thread.fpu;
            ...
            __copy_from_user(&fpu->state.xsave, ..., state_size);

        fpu__restore_sig(...):
            ...
            return __fpu__restore_sig(...);

    arch/x86/kernel/signal.c:
        restore_sigcontext(...):
            ...
            fpu__restore_sig(...)

This introduces arch_thread_struct_whitelist() to let an architecture
declare specifically where the whitelist should be within thread_struct.
If undefined, the entire thread_struct field is left whitelisted.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Laura Abbott <labbott@redhat.com>
Cc: "Mickaël Salaün" <mic@digikod.net>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Rik van Riel <riel@redhat.com>

arch/Kconfig

@@ -242,6 +242,17 @@ config ARCH_INIT_TASK
 config ARCH_TASK_STRUCT_ALLOCATOR
 	bool
 
+config HAVE_ARCH_THREAD_STRUCT_WHITELIST
+	bool
+	depends on !ARCH_TASK_STRUCT_ALLOCATOR
+	help
+	  An architecture should select this to provide hardened usercopy
+	  knowledge about what region of the thread_struct should be
+	  whitelisted for copying to userspace. Normally this is only the
+	  FPU registers. Specifically, arch_thread_struct_whitelist()
+	  should be implemented. Without this, the entire thread_struct
+	  field in task_struct will be left whitelisted.
+
 # Select if arch has its private alloc_thread_stack() function
 config ARCH_THREAD_STACK_ALLOCATOR
 	bool

include/linux/sched/task.h

@@ -104,6 +104,20 @@ extern int arch_task_struct_size __read_mostly;
 # define arch_task_struct_size (sizeof(struct task_struct))
 #endif
 
+#ifndef CONFIG_HAVE_ARCH_THREAD_STRUCT_WHITELIST
+/*
+ * If an architecture has not declared a thread_struct whitelist we
+ * must assume something there may need to be copied to userspace.
+ */
+static inline void arch_thread_struct_whitelist(unsigned long *offset,
+						unsigned long *size)
+{
+	*offset = 0;
+	/* Handle dynamically sized thread_struct. */
+	*size = arch_task_struct_size - offsetof(struct task_struct, thread);
+}
+#endif
+
 #ifdef CONFIG_VMAP_STACK
 static inline struct vm_struct *task_stack_vm_area(const struct task_struct *t)
 {

kernel/fork.c

@@ -458,6 +458,21 @@ static void set_max_threads(unsigned int max_threads_suggested)
 int arch_task_struct_size __read_mostly;
 #endif
 
+static void task_struct_whitelist(unsigned long *offset, unsigned long *size)
+{
+	/* Fetch thread_struct whitelist for the architecture. */
+	arch_thread_struct_whitelist(offset, size);
+
+	/*
+	 * Handle zero-sized whitelist or empty thread_struct, otherwise
+	 * adjust offset to position of thread_struct in task_struct.
+	 */
+	if (unlikely(*size == 0))
+		*offset = 0;
+	else
+		*offset += offsetof(struct task_struct, thread);
+}
+
 void __init fork_init(void)
 {
 	int i;
@@ -466,11 +481,14 @@ void __init fork_init(void)
 #define ARCH_MIN_TASKALIGN	0
 #endif
 	int align = max_t(int, L1_CACHE_BYTES, ARCH_MIN_TASKALIGN);
+	unsigned long useroffset, usersize;
 
 	/* create a slab on which task_structs can be allocated */
-	task_struct_cachep = kmem_cache_create("task_struct",
+	task_struct_whitelist(&useroffset, &usersize);
+	task_struct_cachep = kmem_cache_create_usercopy("task_struct",
 			arch_task_struct_size, align,
-			SLAB_PANIC|SLAB_ACCOUNT, NULL);
+			SLAB_PANIC|SLAB_ACCOUNT,
+			useroffset, usersize, NULL);
 #endif
 
 	/* do the arch specific task caches init */