[kernel-hardening] [PATCH 31/38] fork: Provide usercopy whitelisting for task_struct

Kees Cook
2018-01-11T02:03:03Z

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 | 11 +++++++++++
include/linux/sched/task.h | 14 ++++++++++++++
kernel/fork.c | 22 ++++++++++++++++++++--
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 */
--
2.7.4