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Subject[PATCH v2] fork: Unconditionally clear stack on fork
One of the classes of kernel stack content leaks[1] is exposing the
contents of prior heap or stack contents when a new process stack is
allocated. Normally, those stacks are not zeroed, and the old contents
remain in place. In the face of stack content exposure flaws, those
contents can leak to userspace.

Fixing this will make the kernel no longer vulnerable to these flaws,
as the stack will be wiped each time a stack is assigned to a new
process. There's not a meaningful change in runtime performance; it
almost looks like it provides a benefit.

Performing back-to-back kernel builds before:
Run times: 157.86 157.09 158.90 160.94 160.80
Mean: 159.12
Std Dev: 1.54

and after:
Run times: 159.31 157.34 156.71 158.15 160.81
Mean: 158.46
Std Dev: 1.46

Instead of making this a build or runtime config, Andy Lutomirski
recommended this just be enabled by default.

[1] A noisy search for many kinds of stack content leaks can be seen here:
https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=linux+kernel+stack+leak

Signed-off-by: Kees Cook <keescook@chromium.org>
---
include/linux/thread_info.h | 6 +-----
kernel/fork.c | 3 +--
2 files changed, 2 insertions(+), 7 deletions(-)

diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
index 34f053a150a9..cf2862bd134a 100644
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h
@@ -43,11 +43,7 @@ enum {
#define THREAD_ALIGN THREAD_SIZE
#endif

-#if IS_ENABLED(CONFIG_DEBUG_STACK_USAGE) || IS_ENABLED(CONFIG_DEBUG_KMEMLEAK)
-# define THREADINFO_GFP (GFP_KERNEL_ACCOUNT | __GFP_ZERO)
-#else
-# define THREADINFO_GFP (GFP_KERNEL_ACCOUNT)
-#endif
+#define THREADINFO_GFP (GFP_KERNEL_ACCOUNT | __GFP_ZERO)

/*
* flag set/clear/test wrappers
diff --git a/kernel/fork.c b/kernel/fork.c
index be8aa5b98666..4f2ee527c7d2 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -216,10 +216,9 @@ static unsigned long *alloc_thread_stack_node(struct task_struct *tsk, int node)
if (!s)
continue;

-#ifdef CONFIG_DEBUG_KMEMLEAK
/* Clear stale pointers from reused stack. */
memset(s->addr, 0, THREAD_SIZE);
-#endif
+
tsk->stack_vm_area = s;
return s->addr;
}
--
2.7.4

--
Kees Cook
Pixel Security

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