Patch "fs/aio: Use RCU accessors for kioctx_table->table[]" has been added to the 3.18-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    fs/aio: Use RCU accessors for kioctx_table->table[]

to the 3.18-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     fs-aio-use-rcu-accessors-for-kioctx_table-table.patch
and it can be found in the queue-3.18 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From d0264c01e7587001a8c4608a5d1818dba9a4c11a Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Wed, 14 Mar 2018 12:10:17 -0700
Subject: fs/aio: Use RCU accessors for kioctx_table->table[]

From: Tejun Heo <tj@kernel.org>

commit d0264c01e7587001a8c4608a5d1818dba9a4c11a upstream.

While converting ioctx index from a list to a table, db446a08c23d
("aio: convert the ioctx list to table lookup v3") missed tagging
kioctx_table->table[] as an array of RCU pointers and using the
appropriate RCU accessors.  This introduces a small window in the
lookup path where init and access may race.

Mark kioctx_table->table[] with __rcu and use the approriate RCU
accessors when using the field.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Jann Horn <jannh@google.com>
Fixes: db446a08c23d ("aio: convert the ioctx list to table lookup v3")
Cc: Benjamin LaHaise <bcrl@kvack.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: stable@vger.kernel.org # v3.12+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/aio.c |   19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

--- a/fs/aio.c
+++ b/fs/aio.c
@@ -68,9 +68,9 @@ struct aio_ring {
 #define AIO_RING_PAGES	8
 
 struct kioctx_table {
-	struct rcu_head	rcu;
-	unsigned	nr;
-	struct kioctx	*table[];
+	struct rcu_head		rcu;
+	unsigned		nr;
+	struct kioctx __rcu	*table[];
 };
 
 struct kioctx_cpu {
@@ -583,9 +583,9 @@ static int ioctx_add_table(struct kioctx
 	while (1) {
 		if (table)
 			for (i = 0; i < table->nr; i++)
-				if (!table->table[i]) {
+				if (!rcu_access_pointer(table->table[i])) {
 					ctx->id = i;
-					table->table[i] = ctx;
+					rcu_assign_pointer(table->table[i], ctx);
 					spin_unlock(&mm->ioctx_lock);
 
 					/* While kioctx setup is in progress,
@@ -760,8 +760,8 @@ static int kill_ioctx(struct mm_struct *
 
 	spin_lock(&mm->ioctx_lock);
 	table = rcu_dereference_raw(mm->ioctx_table);
-	WARN_ON(ctx != table->table[ctx->id]);
-	table->table[ctx->id] = NULL;
+	WARN_ON(ctx != rcu_access_pointer(table->table[ctx->id]));
+	RCU_INIT_POINTER(table->table[ctx->id], NULL);
 	spin_unlock(&mm->ioctx_lock);
 
 	/* free_ioctx_reqs() will do the necessary RCU synchronization */
@@ -817,7 +817,8 @@ void exit_aio(struct mm_struct *mm)
 		return;
 
 	for (i = 0; i < table->nr; ++i) {
-		struct kioctx *ctx = table->table[i];
+		struct kioctx *ctx =
+			rcu_dereference_protected(table->table[i], true);
 		struct completion requests_done =
 			COMPLETION_INITIALIZER_ONSTACK(requests_done);
 
@@ -1003,7 +1004,7 @@ static struct kioctx *lookup_ioctx(unsig
 	if (!table || id >= table->nr)
 		goto out;
 
-	ctx = table->table[id];
+	ctx = rcu_dereference(table->table[id]);
 	if (ctx && ctx->user_id == ctx_id) {
 		percpu_ref_get(&ctx->users);
 		ret = ctx;


Patches currently in stable-queue which might be from tj@kernel.org are

queue-3.18/fs-aio-add-explicit-rcu-grace-period-when-freeing-kioctx.patch
queue-3.18/fs-aio-use-rcu-accessors-for-kioctx_table-table.patch