Re: dm crypt: initialize tasklet in crypt_io_init()

[Mike Snitzer <snitzer@kernel.org>]

On Wed, Mar 08 2023 at  2:19P -0500,
Mike Snitzer <snitzer@kernel.org> wrote:

> On Wed, Mar 08 2023 at  8:55P -0500,
> Ignat Korchagin <ignat@cloudflare.com> wrote:
> 
> > Perhaps instead we can just pass an additional flag from
> > tasklet_schedule to indicate to the function that we're running in a
> > tasklet. I originally have chosen the tasklet_trylock/unlock hack to
> > avoid passing an extra flag. But unitialized memory makes sense as
> > well as the desire to avoid calling tasklet_init unconditionally. So
> > an extra member in dm_crypt_io might be the most straightforward here.
> 
> ... I think we should certainly evaluate the use of an extra flag.
> 
> Ignat: I'll have a look at implementing it but if you have a patch
> already developed please do share.

I've staged the following in linux-next for 6.3 via the linux-dm.git,
but if you see anything wrong with it I can obviously fix:

From: Mike Snitzer <snitzer@kernel.org>
Date: Wed, 8 Mar 2023 14:39:54 -0500
Subject: [PATCH] dm crypt: avoid accessing uninitialized tasklet

When neither "no_read_workqueue" nor "no_write_workqueue" are enabled,
tasklet_trylock() in crypt_dec_pending() may still return false due to
an uninitialized state, and dm-crypt will unnecessarily do io completion
in io_queue workqueue instead of current context.

Fix this by adding an 'in_tasklet' flag to dm_crypt_io struct and
initialize it to false in crypt_io_init(). Set this flag to true in
kcryptd_queue_crypt() before calling tasklet_schedule(). If set
crypt_dec_pending() will punt io completion to a workqueue.

This also nicely avoids the tasklet_trylock/unlock hack when tasklets
aren't in use.

Fixes: 8e14f610159d ("dm crypt: do not call bio_endio() from the dm-crypt tasklet")
Cc: stable@vger.kernel.org
Reported-by: Hou Tao <houtao1@huawei.com>
Suggested-by: Ignat Korchagin <ignat@cloudflare.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
---
 drivers/md/dm-crypt.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
index faba1be572f9..de08ff4f7c98 100644
--- a/drivers/md/dm-crypt.c
+++ b/drivers/md/dm-crypt.c
@@ -72,7 +72,9 @@ struct dm_crypt_io {
 	struct crypt_config *cc;
 	struct bio *base_bio;
 	u8 *integrity_metadata;
-	bool integrity_metadata_from_pool;
+	bool integrity_metadata_from_pool:1;
+	bool in_tasklet:1;
+
 	struct work_struct work;
 	struct tasklet_struct tasklet;
 
@@ -1731,6 +1733,7 @@ static void crypt_io_init(struct dm_crypt_io *io, struct crypt_config *cc,
 	io->ctx.r.req = NULL;
 	io->integrity_metadata = NULL;
 	io->integrity_metadata_from_pool = false;
+	io->in_tasklet = false;
 	atomic_set(&io->io_pending, 0);
 }
 
@@ -1777,8 +1780,7 @@ static void crypt_dec_pending(struct dm_crypt_io *io)
 	 * our tasklet. In this case we need to delay bio_endio()
 	 * execution to after the tasklet is done and dequeued.
 	 */
-	if (tasklet_trylock(&io->tasklet)) {
-		tasklet_unlock(&io->tasklet);
+	if (!io->in_tasklet) {
 		bio_endio(base_bio);
 		return;
 	}
@@ -2233,6 +2235,7 @@ static void kcryptd_queue_crypt(struct dm_crypt_io *io)
 		 * it is being executed with irqs disabled.
 		 */
 		if (in_hardirq() || irqs_disabled()) {
+			io->in_tasklet = true;
 			tasklet_init(&io->tasklet, kcryptd_crypt_tasklet, (unsigned long)&io->work);
 			tasklet_schedule(&io->tasklet);
 			return;
-- 
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