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>

3 files changed, 45 insertions, 2 deletions

diff --git a/arch/Kconfig b/arch/Kconfig
index 400b9e1b2f27..8911ff37335a 100644
--- a/arch/Kconfig
+++ b/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
diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 05b8650f06f5..5be31eb7b266 100644
--- a/include/linux/sched/task.h
+++ b/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)
 {
diff --git a/kernel/fork.c b/kernel/fork.c
index 0e086af148f2..5977e691c754 100644
--- a/kernel/fork.c
+++ b/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 */