[PATCH] char: applicom: fix NULL pointer dereference in ac_ioctl

[Tianchu Chen <tianchu.chen@linux.dev>]

From: Tianchu Chen <flynnnchen@tencent.com>

Discovered by Atuin - Automated Vulnerability Discovery Engine.

In ac_ioctl, the validation of IndexCard and the check for a valid
RamIO pointer are skipped when cmd is 6. However, the function
unconditionally executes readb(apbs[IndexCard].RamIO + VERS) at the
end.

If cmd is 6, IndexCard may reference a board that does not exist
(where RamIO is NULL), leading to a NULL pointer dereference.

Fix this by skipping the readb access when cmd is 6, as this
command is a global information query and does not target a specific
board context.

Signed-off-by: Tianchu Chen <flynnnchen@tencent.com>
---
 drivers/char/applicom.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/char/applicom.c b/drivers/char/applicom.c
index 9fed9706d..c138c468f 100644
--- a/drivers/char/applicom.c
+++ b/drivers/char/applicom.c
@@ -835,7 +835,10 @@ static long ac_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 		ret = -ENOTTY;
 		break;
 	}
-	Dummy = readb(apbs[IndexCard].RamIO + VERS);
+
+	if (cmd != 6)
+		Dummy = readb(apbs[IndexCard].RamIO + VERS);
+
 	kfree(adgl);
 	mutex_unlock(&ac_mutex);
 	return ret;
-- 
2.39.5