Re: [PATCH] kernel: fix data race in put_pid

[Oleg Nesterov <oleg@redhat.com>]

Honestly, I can not see how this can happen. So I do not really
understand the problem and the fix.

And if this can happen I can't understand how this patch can help.
What about "ns = pid->numbers[pid->level].ns" ? It can be reordered
with atomic_read_acquire().

I leave this to other reviewers, but perhaps you can spell the
"For example" part of the changelog.


On 09/17, Dmitry Vyukov wrote:
>
> put_pid checks whether the current thread has the only reference
> to the pid with atomic_read() which does not have any memory
> barriers, and if so proceeds directly to kmem_cache_free().
> As the result memory accesses to the object in kmem_cache_free()
> or user accesses to the object after reallocation (again without
> any memory barriers on fast path) can hoist above the atomic_read()
> check and conflict with memory accesses to the pid object in other
> threads before they released their references.
> 
> There is a control dependency between the atomic_read() check and
> kmem_cache_free(), but control dependencies are disregarded by some
> architectures. Documentation/memory-barriers.txt explicitly states:
> "A load-load control dependency requires a full read memory barrier.
> ... please note that READ_ONCE_CTRL() is not optional! [even for stores]"
> in the CONTROL DEPENDENCIES section.
> 
> For example, if store to the first word of the object to build a freelist
> in kmem_cache_free() hoists above the check, stores to the first word
> in other threads can corrupt the memory allocator freelist.
> 
> Use atomic_read_acquire() for the fast path check to hand off properly
> acquired object to memory allocator.
> 
> The data race was found with KernelThreadSanitizer (KTSAN).
> 
> Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
> ---
>  kernel/pid.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/pid.c b/kernel/pid.c
> index ca36879..3b0b13d 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -242,7 +242,7 @@ void put_pid(struct pid *pid)
>  		return;
>  
>  	ns = pid->numbers[pid->level].ns;
> -	if ((atomic_read(&pid->count) == 1) ||
> +	if ((atomic_read_acquire(&pid->count) == 1) ||
>  	     atomic_dec_and_test(&pid->count)) {
>  		kmem_cache_free(ns->pid_cachep, pid);
>  		put_pid_ns(ns);
> -- 
> 2.6.0.rc0.131.gf624c3d
>