[PATCH] fbdev: Fix slab-out-of-bounds read in fb_pad_unaligned_buffer

[Osama Abdelkader <osama.abdelkader@gmail.com>]

The function fb_pad_unaligned_buffer() was reading idx+1 bytes per row
from the source buffer, but when mod == 0 (font width is a multiple of
8 bits), the source buffer only has idx bytes per row. This caused a
slab-out-of-bounds read when accessing src[idx] after the inner loop.

Fix this by only reading the extra byte when mod != 0, ensuring we
never read beyond the source buffer boundaries.

This fixes the KASAN slab-out-of-bounds read reported by syzkaller:
https://syzkaller.appspot.com/bug?extid=55e03490a0175b8dd81d

Reported-by: syzbot+55e03490a0175b8dd81d@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=55e03490a0175b8dd81d
Signed-off-by: Osama Abdelkader <osama.abdelkader@gmail.com>
---
 drivers/video/fbdev/core/fbmem.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index eff757ebbed1..a0c4932a6758 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -113,15 +113,17 @@ void fb_pad_unaligned_buffer(u8 *dst, u32 d_pitch, u8 *src, u32 idx, u32 height,
 			dst[j+1] = tmp;
 			src++;
 		}
-		tmp = dst[idx];
-		tmp &= mask;
-		tmp |= *src >> shift_low;
-		dst[idx] = tmp;
-		if (shift_high < mod) {
-			tmp = *src << shift_high;
-			dst[idx+1] = tmp;
+		if (mod) {
+			tmp = dst[idx];
+			tmp &= mask;
+			tmp |= *src >> shift_low;
+			dst[idx] = tmp;
+			if (shift_high < mod) {
+				tmp = *src << shift_high;
+				dst[idx+1] = tmp;
+			}
+			src++;
 		}
-		src++;
 		dst += d_pitch;
 	}
 }
-- 
2.43.0