fork: Define usercopy region in thread_stack slab caches

dwindsor · kees · commit f9d29946c567 · 2018-01-15T12:08:03.000-08:00
In support of usercopy hardening, this patch defines a region in the
thread_stack slab caches in which userspace copy operations are allowed.
Since the entire thread_stack needs to be available to userspace, the
entire slab contents are whitelisted. Note that the slab-based thread
stack is only present on systems with THREAD_SIZE &lt; PAGE_SIZE and
!CONFIG_VMAP_STACK.

cache object allocation:
    kernel/fork.c:
        alloc_thread_stack_node(...):
            return kmem_cache_alloc_node(thread_stack_cache, ...)

        dup_task_struct(...):
            ...
            stack = alloc_thread_stack_node(...)
            ...
            tsk-&gt;stack = stack;

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

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

This region is known as the slab cache's usercopy region. Slab caches
can now check that each dynamically sized copy operation involving
cache-managed memory falls entirely within the slab's usercopy region.

This patch is modified from Brad Spengler/PaX Team's PAX_USERCOPY
whitelisting code in the last public patch of grsecurity/PaX based on my
understanding of the code. Changes or omissions from the original code are
mine and don't reflect the original grsecurity/PaX code.

Signed-off-by: David Windsor &lt;dave@nullcore.net&gt;
[kees: adjust commit log, split patch, provide usage trace]
Cc: Ingo Molnar &lt;mingo@kernel.org&gt;
Cc: Andrew Morton &lt;akpm@linux-foundation.org&gt;
Cc: Thomas Gleixner &lt;tglx@linutronix.de&gt;
Cc: Andy Lutomirski &lt;luto@kernel.org&gt;
Signed-off-by: Kees Cook &lt;keescook@chromium.org&gt;
Acked-by: Rik van Riel &lt;riel@redhat.com&gt;
diff --git a/kernel/fork.c b/kernel/fork.c
@@ -282,8 +282,9 @@ static void free_thread_stack(struct task_struct *tsk)
 
 void thread_stack_cache_init(void)
 {
-	thread_stack_cache = kmem_cache_create("thread_stack", THREAD_SIZE,
-					      THREAD_SIZE, 0, NULL);
+	thread_stack_cache = kmem_cache_create_usercopy("thread_stack",
+					THREAD_SIZE, THREAD_SIZE, 0, 0,
+					THREAD_SIZE, NULL);
 	BUG_ON(thread_stack_cache == NULL);
 }
 # endif

Original file line number	Diff line number	Diff line change
`@@ -282,8 +282,9 @@ static void free_thread_stack(struct task_struct *tsk)`
`282`	`282`
`283`	`283`	`void thread_stack_cache_init(void)`
`284`	`284`	`{`
`285`		`- thread_stack_cache = kmem_cache_create("thread_stack", THREAD_SIZE,`
`286`		`- THREAD_SIZE, 0, NULL);`
	`285`	`+ thread_stack_cache = kmem_cache_create_usercopy("thread_stack",`
	`286`	`+ THREAD_SIZE, THREAD_SIZE, 0, 0,`
	`287`	`+ THREAD_SIZE, NULL);`
`287`	`288`	`BUG_ON(thread_stack_cache == NULL);`
`288`	`289`	`}`
`289`	`290`	`# endif`