linux-kernel - Re: [PATCH v2] tpm: Rework open/close/shutdown to avoid races

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Date:   Mon, 07 Mar 2022 11:22:27 -0500
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>
Cc:     Sergey Temerkhanov <s.temerkhanov@...il.com>,
        Peter Huewe <peterhuewe@....de>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jerry Snitselaar <jsnitsel@...hat.com>,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] tpm: Rework open/close/shutdown to avoid races

On Mon, 2022-03-07 at 17:41 +0200, Jarkko Sakkinen wrote:
[...]
> James, would it possible for you to construct a proper patch from
> this and send it so we could include it to the next PR (use my tree
> as baseline)?

Sure,  There's not really any identifiable patch for a fixes tag, since
the race really always existed.

How about the below.

James

-------------------8>8>8><8<8<8------------------

>From 3e0e640e47e5728b68693c5d45bf3cdd0bff48ca Mon Sep 17 00:00:00 2001
From: James Bottomley <James.Bottomley@...senPartnership.com>
Date: Mon, 7 Mar 2022 11:16:55 -0500
Subject: [PATCH] tpm: use try_get_ops() in tpm-space.c

As part of the series conversion to remove nested TPM operations:

https://lore.kernel.org/all/20190205224723.19671-1-jarkko.sakkinen@linux.intel.com/

exposure of the chip->tpm_mutex was removed from much of the upper
level code.  In this conversion, tpm2_del_space() was missed.  This
didn't matter much because it's usually called closely after a
converted operation, so there's only a very tiny race window where the
chip can be removed before the space flushing is done which causes a
NULL deref on the mutex.  However, there are reports of this window
being hit in practice, so fix this by converting tpm2_del_space() to
use tpm_try_get_ops(), which performs all the teardown checks before
acquring the mutex.

Signed-off-by: James Bottomley <James.Bottomley@...senPartnership.com>
---
 drivers/char/tpm/tpm2-space.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/char/tpm/tpm2-space.c b/drivers/char/tpm/tpm2-space.c
index 265ec72b1d81..ffb35f0154c1 100644
--- a/drivers/char/tpm/tpm2-space.c
+++ b/drivers/char/tpm/tpm2-space.c
@@ -58,12 +58,12 @@ int tpm2_init_space(struct tpm_space *space, unsigned int buf_size)
 
 void tpm2_del_space(struct tpm_chip *chip, struct tpm_space *space)
 {
-	mutex_lock(&chip->tpm_mutex);
-	if (!tpm_chip_start(chip)) {
+
+	if (tpm_try_get_ops(chip) == 0) {
 		tpm2_flush_sessions(chip, space);
-		tpm_chip_stop(chip);
+		tpm_put_ops(chip);
 	}
-	mutex_unlock(&chip->tpm_mutex);
+
 	kfree(space->context_buf);
 	kfree(space->session_buf);
 }
-- 
2.34.1