[PATCH 7.1 025/228] fbdev: core: Fix pointer desynchronization in fb_io_read()

7.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Mingyu Wang <[email protected]>

commit 81cc73be40c6f028f1ee3f438ace46afe666dbae upstream.

In fb_io_read(), if copy_to_user() performs a partial copy (e.g., due to
a faulty user buffer), the loop adjusts the chunk size 'c' and updates
the remaining 'count'. However, the hardware 'src' pointer has already
been eagerly advanced by the original chunk size.

If the loop is allowed to continue, the read will resume from an
incorrect, over-advanced offset. Since the remaining 'count' was only
decremented by the successful bytes, this desynchronization causes the
next iterations to execute more hardware reads than originally bounded,
eventually leading to out-of-bounds I/O reads.

Fix this by breaking out of the loop immediately upon a partial
copy_to_user(). A partial copy indicates a faulty user buffer, making
subsequent read attempts futile. Breaking out ensures we return the
number of successfully read bytes without risking out-of-bounds hardware
accesses in subsequent mismatched iterations.

Fixes: 6121cd9ef911 ("fbdev: Move I/O read and write code into helper functions")
Cc: [email protected]
Signed-off-by: Mingyu Wang <[email protected]>
Signed-off-by: Helge Deller <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
 drivers/video/fbdev/core/fb_io_fops.c |    8 ++++++++
 1 file changed, 8 insertions(+)

--- a/drivers/video/fbdev/core/fb_io_fops.c
+++ b/drivers/video/fbdev/core/fb_io_fops.c
@@ -61,6 +61,14 @@ ssize_t fb_io_read(struct fb_info *info,
 		buf += c;
 		cnt += c;
 		count -= c;
+
+		/*
+		 * If there was a partial copy, the user buffer is faulty.
+		 * Break out to avoid over-advancing the src pointer and
+		 * reading out of bounds in the next iteration.
+		 */
+		if (trailing)
+			break;
 	}
 
 	kfree(buffer);