tpm: Provide strong locking for device removal

Add a read/write semaphore around the ops function pointers so
ops can be set to null when the driver un-registers.

Previously the tpm core expected module locking to be enough to
ensure that tpm_unregister could not be called during certain times,
however that hasn't been sufficient for a long time.

Introduce a read/write semaphore around 'ops' so the core can set
it to null when unregistering. This provides a strong fence around
the driver callbacks, guaranteeing to the driver that no callbacks
are running or will run again.

For now the ops_lock is placed very high in the call stack, it could
be pushed down and made more granular in future if necessary.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Reviewed-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/drivers/char/tpm/tpm-sysfs.c b/drivers/char/tpm/tpm-sysfs.c
index d93736aa2703..34e7fc7e590c 100644
--- a/drivers/char/tpm/tpm-sysfs.c
+++ b/drivers/char/tpm/tpm-sysfs.c
@@ -295,5 +295,10 @@ int tpm_sysfs_add_device(struct tpm_chip *chip)
 
 void tpm_sysfs_del_device(struct tpm_chip *chip)
 {
+	/* The sysfs routines rely on an implicit tpm_try_get_ops, this
+	 * function is called before ops is null'd and the sysfs core
+	 * synchronizes this removal so that no callbacks are running or can
+	 * run again
+	 */
 	sysfs_remove_group(&chip->dev.parent->kobj, &tpm_dev_group);
 }