From 42bc07b4e5a3c8a02a433388f562a8f46d093e11 Mon Sep 17 00:00:00 2001 From: Mingyu Wang <25181214217@stu.xidian.edu.cn> Date: Tue, 21 Jul 2026 16:19:42 +0800 Subject: [PATCH] fbdev: core: Fix pointer desynchronization in fb_io_read() 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: stable@vger.kernel.org Signed-off-by: Mingyu Wang <25181214217@stu.xidian.edu.cn> Signed-off-by: Helge Deller Signed-off-by: Greg Kroah-Hartman --- drivers/video/fbdev/core/fb_io_fops.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/drivers/video/fbdev/core/fb_io_fops.c b/drivers/video/fbdev/core/fb_io_fops.c index 871b829521af..15cfef88efbd 100644 --- a/drivers/video/fbdev/core/fb_io_fops.c +++ b/drivers/video/fbdev/core/fb_io_fops.c @@ -57,6 +57,14 @@ ssize_t fb_io_read(struct fb_info *info, char __user *buf, size_t count, loff_t 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);