[PATCH 5/6] fbdev: smscufx: check for fb_deferred_io_init() error

[Lorenzo Stoakes (ARM)]

fb_deferred_io_init() allocates deferred I/O state, populating
info->fbdefio_state, or leaving it NULL if an error occurs.

Currently ufx_ops_open() ignores its return value.

Therefore if an error arises in fb_deferred_io_init() (for instance, due to
an allocation failure) info->fbdefio_state is left NULL.

fb_open() will then dereference a NULL pointer (calling
fb_deferred_io_open()) as soon as ufx_ops_open() returns.

Additionally, if the allocation of info->fbdefio itself fails,
ufx_ops_open() sets info->fbdefio to NULL and invokes
fb_deferred_io_init() regardless, hitting BUG_ON(!fbdefio).

Fix this by only invoking fb_deferred_io_init() if the allocation
succeeded, and checking for the error.

The driver already supports operating without deferred I/O, so in either
case fall back to that by setting info->fbdefio to NULL.

The ignored return value was introduced in commit 56c134f7f1b5 ("fbdev:
Track deferred-I/O pages in pageref struct").

However, the BUG_ON() issue originates from the earlier
commit 3c8a63e22a08 ("Add support for SMSC UFX6000/7000 USB display
adapters"), so target that for the fix.

Fixes: 3c8a63e22a08 ("Add support for SMSC UFX6000/7000 USB display adapters")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
---
 drivers/video/fbdev/smscufx.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/video/fbdev/smscufx.c b/drivers/video/fbdev/smscufx.c
index 5704f60e1741..34d8abd5b354 100644
--- a/drivers/video/fbdev/smscufx.c
+++ b/drivers/video/fbdev/smscufx.c
@@ -1041,10 +1041,13 @@ static int ufx_ops_open(struct fb_info *info, int user)
 		if (fbdefio) {
 			fbdefio->delay = UFX_DEFIO_WRITE_DELAY;
 			fbdefio->deferred_io = ufx_dpy_deferred_io;
-		}
 
-		info->fbdefio = fbdefio;
-		fb_deferred_io_init(info);
+			info->fbdefio = fbdefio;
+			if (fb_deferred_io_init(info)) {
+				kfree(fbdefio);
+				info->fbdefio = NULL;
+			}
+		}
 	}
 
 	pr_debug("open /dev/fb%d user=%d fb_info=%p count=%d",

-- 
2.55.0