[PATCH 5.10.y] fbdev: udlfb: check for fb_deferred_io_init() error

[Sasha Levin <sashal@kernel.org>]

From: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>

[ Upstream commit eae055f33e41e1d3e1a7e905bdcb1e9f196ed31d ]

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

Currently dlfb_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 dlfb_ops_open() returns.

Additionally, if the allocation of info->fbdefio itself fails,
dlfb_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 5bea1fbf9423 ("staging: udlfb: fix incorrect fb_defio
implementation for multiple framebuffers"), so target that for the fix.

Fixes: 5bea1fbf9423 ("staging: udlfb: fix incorrect fb_defio implementation for multiple framebuffers")
Cc: stable@vger.kernel.org
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Signed-off-by: Helge Deller <deller@gmx.de>
[ sashal: Omitted fb_deferred_io_init() error handling because the older API returns void and cannot fail. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/video/fbdev/udlfb.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/video/fbdev/udlfb.c b/drivers/video/fbdev/udlfb.c
index 23097d190e0b1..d3e7c8f6ae7eb 100644
--- a/drivers/video/fbdev/udlfb.c
+++ b/drivers/video/fbdev/udlfb.c
@@ -1006,10 +1006,10 @@ static int dlfb_ops_open(struct fb_info *info, int user)
 		if (fbdefio) {
 			fbdefio->delay = DL_DEFIO_WRITE_DELAY;
 			fbdefio->deferred_io = dlfb_dpy_deferred_io;
-		}
 
-		info->fbdefio = fbdefio;
-		fb_deferred_io_init(info);
+			info->fbdefio = fbdefio;
+			fb_deferred_io_init(info);
+		}
 	}
 
 	dev_dbg(info->dev, "open, user=%d fb_info=%p count=%d\n",
-- 
2.53.0