[PATCH v2] fbdev: avoid out-of-bounds read in fb_pad_unaligned_buffer()

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

fb_pad_unaligned_buffer() unconditionally reads and advances the source
pointer for the final byte of each row, even when no bits from that byte
are actually consumed.

When shift_high >= mod, the remaining bits do not cross a byte boundary,
but the code still accesses the next source byte. This can lead to
out-of-bounds reads under malformed geometry, as reported by syzbot.

Fix this by only accessing and consuming the final source byte when it
contributes bits (shift_high < mod).

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>
---
v2: address the real issue (shift_high >= mod) condition.
---
 drivers/video/fbdev/core/fbmem.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index eff757ebbed1..d125c3db37a1 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -100,7 +100,7 @@ EXPORT_SYMBOL(fb_pad_aligned_buffer);
 void fb_pad_unaligned_buffer(u8 *dst, u32 d_pitch, u8 *src, u32 idx, u32 height,
 				u32 shift_high, u32 shift_low, u32 mod)
 {
-	u8 mask = (u8) (0xfff << shift_high), tmp;
+	u8 mask = (u8) (0xff << shift_high), tmp;
 	int i, j;
 
 	for (i = height; i--; ) {
@@ -113,15 +113,18 @@ 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;
+
+		/* Only consume another source byte if it contributes bits */
 		if (shift_high < mod) {
+			tmp = dst[idx];
+			tmp &= mask;
+			tmp |= *src >> shift_low;
+			dst[idx] = tmp;
 			tmp = *src << shift_high;
 			dst[idx+1] = tmp;
+			src++;
 		}
-		src++;
+
 		dst += d_pitch;
 	}
 }
-- 
2.43.0