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Date: Wed, 20 Sep 2017 13:45:31 -0700
From: Kees Cook <>
Cc: Kees Cook <>,
	David Windsor <>,
	Ingo Molnar <>,
	Andrew Morton <>,
	Thomas Gleixner <>,
	Andy Lutomirski <>,,,,
Subject: [PATCH v3 25/31] fork: Define usercopy region in thread_stack slab caches

From: David Windsor <>

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 < PAGE_SIZE and

cache object allocation:
            return kmem_cache_alloc_node(thread_stack_cache, ...)

            stack = alloc_thread_stack_node(...)
            tsk->stack = stack;



This region is known as the slab cache's usercopy region. Slab caches
can now check that each 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 <>
[kees: adjust commit log, split patch, provide usage trace]
Cc: Ingo Molnar <>
Cc: Andrew Morton <>
Cc: Thomas Gleixner <>
Cc: Andy Lutomirski <>
Signed-off-by: Kees Cook <>
Acked-by: Rik van Riel <>
I wasn't able to test this, so anyone with a system that can try running
with a large PAGE_SIZE and without VMAP_STACK would be appreciated.
 kernel/fork.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index dc1437f8b702..720109dc723a 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -278,8 +278,9 @@ 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",
 	BUG_ON(thread_stack_cache == NULL);
 # endif

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