fork: unconditionally clear stack on fork

BugLink: http://bugs.launchpad.net/bugs/1815234
commit e01e80634ecdde1dd113ac43b3adad21b47f3957 upstream.

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

I did some more with perf and cycle counts on running 100,000 execs of
/bin/true.

before:
Cycles: 218858861551 218853036130 214727610969 227656844122 224980542841
Mean:  221015379122.60
Std Dev: 4662486552.47

after:
Cycles: 213868945060 213119275204 211820169456 224426673259 225489986348
Mean:  217745009865.40
Std Dev: 5935559279.99

It continues to look like it's faster, though the deviation is rather
wide, but I'm not sure what I could do that would be less noisy.  I'm
open to ideas!

Link: http://lkml.kernel.org/r/20180221021659.GA37073@beast
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Laura Abbott <labbott@redhat.com>
Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
index 34f053a150a969bf03805cd56ee7f5487e041642..cf2862bd134a400b99136aaf2ec1357bd79ba3d2 100644 (file)
--- 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 778a1f688d1a197d983c8cbf8aede6146a49308d..caaeaea5af5e853c526778cbdec3bb4e8ce0fbf2 100644 (file)
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -220,10 +220,9 @@ 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;
        }