net: possible use after free in dst_release

commit 07a5d38453599052aff0877b16bb9c1585f08609 upstream.

dst_release should not access dst->flags after decrementing
__refcnt to 0. The dst_entry may be in dst_busy_list and
dst_gc_task may dst_destroy it before dst_release gets a chance
to access dst->flags.

Fixes: d69bbf88c8d0 ("net: fix a race in dst_release()")
Fixes: 27b75c95f10d ("net: avoid RCU for NOCACHE dst")
Signed-off-by: Francesco Ruggeri <fruggeri@arista.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[lizf: Backported to 3.4: adjust context]
Signed-off-by: Zefan Li <lizefan@huawei.com>

diff --git a/net/core/dst.c b/net/core/dst.c
index 54ba1ebba30ef92933c583451483b392cc9908f0..48cff898b5a4a564d58484b3d4a38b1cc98567c5 100644 (file)
--- a/net/core/dst.c
+++ b/net/core/dst.c
@@ -269,10 +269,11 @@ void dst_release(struct dst_entry *dst)
 {
        if (dst) {
                int newrefcnt;
+               unsigned short nocache = dst->flags & DST_NOCACHE;
 
                newrefcnt = atomic_dec_return(&dst->__refcnt);
                WARN_ON(newrefcnt < 0);
-               if (!newrefcnt && unlikely(dst->flags & DST_NOCACHE)) {
+               if (!newrefcnt && unlikely(nocache)) {
                        dst = dst_destroy(dst);
                        if (dst)
                                __dst_free(dst);