From: Kees Cook <keescook@xxxxxxxxxxxx> Subject: 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 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@xxxxxxxxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> Reviewed-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxx> Cc: Laura Abbott <labbott@xxxxxxxxxx> Cc: Rasmus Villemoes <rasmus.villemoes@xxxxxxxxx> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- include/linux/thread_info.h | 6 +----- kernel/fork.c | 3 +-- 2 files changed, 2 insertions(+), 7 deletions(-) diff -puN include/linux/thread_info.h~fork-unconditionally-clear-stack-on-fork include/linux/thread_info.h --- a/include/linux/thread_info.h~fork-unconditionally-clear-stack-on-fork +++ a/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 -puN kernel/fork.c~fork-unconditionally-clear-stack-on-fork kernel/fork.c --- a/kernel/fork.c~fork-unconditionally-clear-stack-on-fork +++ a/kernel/fork.c @@ -216,10 +216,9 @@ static unsigned long *alloc_thread_stack 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; } _ -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
