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Subject[PATCH v3 25/31] fork: Define usercopy region in thread_stack slab caches
Date
From: David Windsor <dave@nullcore.net>

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
!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->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 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 <dave@nullcore.net>
[kees: adjust commit log, split patch, provide usage trace]
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.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>
---
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",
+ THREAD_SIZE, THREAD_SIZE, 0, 0,
+ THREAD_SIZE, NULL);
BUG_ON(thread_stack_cache == NULL);
}
# endif
--
2.7.4
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