[PATCH v4 1/2] i2c: imx: Fix slave registration race and error handling

[Liem <liem16213@gmail.com>]

In i2c_imx_reg_slave(), the slave pointer was assigned before
pm_runtime_resume_and_get().  If pm_runtime_resume_and_get() failed,
the error path returned without clearing i2c_imx->slave, leaving it
non-NULL and causing all subsequent registration attempts to fail
with -EBUSY.

Additionally, because this driver uses a shared IRQ, the interrupt
handler i2c_imx_isr() can execute concurrently and, after acquiring
slave_lock, dereference i2c_imx->slave.  The previous fix attempt
added a lockless i2c_imx->slave = NULL on the error path, but that
could race with the ISR under the lock and still cause a NULL pointer
dereference.

Fix both issues by deferring the assignment of i2c_imx->slave and
i2c_imx->last_slave_event to after a successful resume, and by
performing the assignment inside the slave_lock critical section.
This guarantees that the slave pointer is never left stale on the
error path and is always valid when observed by the interrupt handler.

Fixes: f7414cd6923f ("i2c: imx: support slave mode for imx I2C driver")
Cc: stable@vger.kernel.org
Signed-off-by: Liem <liem16213@gmail.com>
---
v3 -> v4:
  - Instead of clearing the slave pointer on error, defer the
    assignment until after pm_runtime_resume_and_get() succeeds,
    and take slave_lock to avoid racing with the shared IRQ handler.
    Suggested by Sashiko and Carlos Song
---
 drivers/i2c/busses/i2c-imx.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/i2c/busses/i2c-imx.c b/drivers/i2c/busses/i2c-imx.c
index 28313d0fad37..2398c406e913 100644
--- a/drivers/i2c/busses/i2c-imx.c
+++ b/drivers/i2c/busses/i2c-imx.c
@@ -930,9 +930,6 @@ static int i2c_imx_reg_slave(struct i2c_client *client)
 	if (i2c_imx->slave)
 		return -EBUSY;
 
-	i2c_imx->slave = client;
-	i2c_imx->last_slave_event = I2C_SLAVE_STOP;
-
 	/* Resume */
 	ret = pm_runtime_resume_and_get(i2c_imx->adapter.dev.parent);
 	if (ret < 0) {
@@ -940,6 +937,11 @@ static int i2c_imx_reg_slave(struct i2c_client *client)
 		return ret;
 	}
 
+	scoped_guard(spinlock_irqsave, &i2c_imx->slave_lock) {
+		i2c_imx->slave = client;
+		i2c_imx->last_slave_event = I2C_SLAVE_STOP;
+	}
+
 	i2c_imx_slave_init(i2c_imx);
 
 	return 0;
-- 
2.34.1