[PATCH] EDAC/ghes: Do not warn when incrementing refcount on 0

[Robert Richter]

Following warning from the refcount framework is seen during ghes
initialization:

 EDAC MC0: Giving out device to module ghes_edac.c controller ghes_edac: DEV ghes (INTERRUPT)
 ------------[ cut here ]------------
 refcount_t: increment on 0; use-after-free.
 WARNING: CPU: 36 PID: 1 at lib/refcount.c:156 refcount_inc_checked+0x44/0x50
[...]
 Call trace:
  refcount_inc_checked+0x44/0x50
  ghes_edac_register+0x258/0x388
  ghes_probe+0x28c/0x5f0

It warns if the refcount is incremented from zero. This warning is
reasonable as a kernel object is typically created with a refcount of
one and freed once the refcount is zero. Afterwards the object would
be "used-after-free".

For ghes the refcount is initialized with zero, and that is why this
message is seen when initializing the first instance. However,
whenever the refcount is zero, the device will be allocated and
registered. Since the ghes_reg_mutex protects the refcount and
serializes allocation and freeing of ghes devices, a use-after-free
cannot happen here.

Instead of using refcount_inc() for the first instance, use
refcount_set(). This can be used here because the refcount is zero at
this point and can not change due to its protection by the mutex.

Reported-by: John Garry <john.garry@xxxxxxxxxx>
Tested-by: John Garry <john.garry@xxxxxxxxxx>
Signed-off-by: Robert Richter <rrichter@xxxxxxxxxxx>
---
 drivers/edac/ghes_edac.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/edac/ghes_edac.c b/drivers/edac/ghes_edac.c
index 47f4e7f90ef0..b99080d8a10c 100644
--- a/drivers/edac/ghes_edac.c
+++ b/drivers/edac/ghes_edac.c
@@ -556,8 +556,8 @@ int ghes_edac_register(struct ghes *ghes, struct device *dev)
 	ghes_pvt = pvt;
 	spin_unlock_irqrestore(&ghes_lock, flags);
 
-	/* only increment on success */
-	refcount_inc(&ghes_refcount);
+	/* only set on success */
+	refcount_set(&ghes_refcount, 1);
 
 unlock:
 	mutex_unlock(&ghes_reg_mutex);
-- 
2.20.1