fbdev: Add bounds checking in bit_putcs to fix vmalloc-out-of-bounds

Add bounds checking to prevent writes past framebuffer boundaries when
rendering text near screen edges. Return early if the Y position is off-screen
and clip image height to screen boundary. Break from the rendering loop if the
X position is off-screen. When clipping image width to fit the screen, update
the character count to match the clipped width to prevent buffer size
mismatches.

Without the character count update, bit_putcs_aligned and bit_putcs_unaligned
receive mismatched parameters where the buffer is allocated for the clipped
width but cnt reflects the original larger count, causing out-of-bounds writes.

Reported-by: syzbot+48b0652a95834717f190@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=48b0652a95834717f190
Suggested-by: Helge Deller <deller@gmx.de>
Tested-by: syzbot+48b0652a95834717f190@syzkaller.appspotmail.com
Signed-off-by: Albin Babu Varghese <albinbabuvarghese20@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>

diff --git a/drivers/video/fbdev/core/bitblit.c b/drivers/video/fbdev/core/bitblit.c
index f9475c14f7339b074205efe5a23109eae1d6b011..a9ec7f488522c6a6ba2ff63df1c3c85fe4ba80ea 100644 (file)
--- a/drivers/video/fbdev/core/bitblit.c
+++ b/drivers/video/fbdev/core/bitblit.c
@@ -160,6 +160,11 @@ static void bit_putcs(struct vc_data *vc, struct fb_info *info,
        image.height = vc->vc_font.height;
        image.depth = 1;
 
+       if (image.dy >= info->var.yres)
+               return;
+
+       image.height = min(image.height, info->var.yres - image.dy);
+
        if (attribute) {
                buf = kmalloc(cellsize, GFP_ATOMIC);
                if (!buf)
@@ -173,6 +178,18 @@ static void bit_putcs(struct vc_data *vc, struct fb_info *info,
                        cnt = count;
 
                image.width = vc->vc_font.width * cnt;
+
+               if (image.dx >= info->var.xres)
+                       break;
+
+               if (image.dx + image.width > info->var.xres) {
+                       image.width = info->var.xres - image.dx;
+                       cnt = image.width / vc->vc_font.width;
+                       if (cnt == 0)
+                               break;
+                       image.width = cnt * vc->vc_font.width;
+               }
+
                pitch = DIV_ROUND_UP(image.width, 8) + scan_align;
                pitch &= ~scan_align;
                size = pitch * image.height + buf_align;