aio: fix rcu ioctx lookup

aio-dio-invalidate-failure GPFs in aio_put_req from io_submit.

lookup_ioctx doesn't implement the rcu lookup pattern properly.
rcu_read_lock does not prevent refcount going to zero, so we might take
a refcount on a zero count ioctx.

Fix the bug by atomically testing for zero refcount before incrementing.

[jack@suse.cz: added comment into the code]
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Nick Piggin <npiggin@kernel.dk>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 24 insertions, 11 deletions

diff --git a/fs/aio.c b/fs/aio.c
index fc557a3be0a9..b4dd668fbccc 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -239,15 +239,23 @@ static void __put_ioctx(struct kioctx *ctx)
         call_rcu(&ctx->rcu_head, ctx_rcu_free);
 }
 
-#define get_ioctx(kioctx) do {                                          \
-        BUG_ON(atomic_read(&(kioctx)->users) <= 0);                     \
-        atomic_inc(&(kioctx)->users);                                   \
-} while (0)
-#define put_ioctx(kioctx) do {                                          \
-        BUG_ON(atomic_read(&(kioctx)->users) <= 0);                     \
-        if (unlikely(atomic_dec_and_test(&(kioctx)->users)))            \
-                __put_ioctx(kioctx);                                    \
-} while (0)
+static inline void get_ioctx(struct kioctx *kioctx)
+{
+        BUG_ON(atomic_read(&kioctx->users) <= 0);
+        atomic_inc(&kioctx->users);
+}
+
+static inline int try_get_ioctx(struct kioctx *kioctx)
+{
+        return atomic_inc_not_zero(&kioctx->users);
+}
+
+static inline void put_ioctx(struct kioctx *kioctx)
+{
+        BUG_ON(atomic_read(&kioctx->users) <= 0);
+        if (unlikely(atomic_dec_and_test(&kioctx->users)))
+                __put_ioctx(kioctx);
+}
 
 /* ioctx_alloc
  *      Allocates and initializes an ioctx.  Returns an ERR_PTR if it failed.
@@ -601,8 +609,13 @@ static struct kioctx *lookup_ioctx(unsigned long ctx_id)
         rcu_read_lock();
 
         hlist_for_each_entry_rcu(ctx, n, &mm->ioctx_list, list) {
-                if (ctx->user_id == ctx_id && !ctx->dead) {
-                        get_ioctx(ctx);
+                /*
+                 * RCU protects us against accessing freed memory but
+                 * we have to be careful not to get a reference when the
+                 * reference count already dropped to 0 (ctx->dead test
+                 * is unreliable because of races).
+                 */
+                if (ctx->user_id == ctx_id && !ctx->dead && try_get_ioctx(ctx)){
                         ret = ctx;
                         break;
                 }