video: fbdev: fix potential OOB read in vga_8planes_imageblit()

Since transl_l[16] is accessed via cdat[y] >> 4, cdat[y] needs to be
evaluated as an "unsigned char" value in order to fit 0...15 range.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 drivers/video/fbdev/vga16fb.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/vga16fb.c b/drivers/video/fbdev/vga16fb.c
index a20eeb8308ff..578d3541e3d6 100644
--- a/drivers/video/fbdev/vga16fb.c
+++ b/drivers/video/fbdev/vga16fb.c

@@ -1121,7 +1121,7 @@ static void vga_8planes_imageblit(struct fb_info *info, const struct fb_image *i
         char oldop = setop(0);
         char oldsr = setsr(0);
         char oldmask = selectmask();
-        const char *cdat = image->data;
+	const unsigned char *cdat = image->data;
 	u32 dx = image->dx;
         char __iomem *where;
         int y;

-- 
2.18.4

syzbot is reporting OOB read at vga_8planes_imageblit() [1], for
"cdat[y] >> 4" can become a negative value due to "const char *cdat".

[1] https://syzkaller.appspot.com/bug?id
7a0da1557dcd1989e00cb3692b26d4173b4132

Reported-by: syzbot <redacted>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 drivers/video/fbdev/vga16fb.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/vga16fb.c b/drivers/video/fbdev/vga16fb.c
index a20eeb8308ff..578d3541e3d6 100644
--- a/drivers/video/fbdev/vga16fb.c
+++ b/drivers/video/fbdev/vga16fb.c

@@ -1121,7 +1121,7 @@ static void vga_8planes_imageblit(struct fb_info *info, const struct fb_image *i
         char oldop = setop(0);
         char oldsr = setsr(0);
         char oldmask = selectmask();
-        const char *cdat = image->data;
+	const unsigned char *cdat = image->data;
 	u32 dx = image->dx;
         char __iomem *where;
         int y;

-- 
2.18.4

syzbot is reporting OOB read at vga_8planes_imageblit() [1], for
"cdat[y] >> 4" can become a negative value due to "const char *cdat".

[1] https://syzkaller.appspot.com/bug?id
7a0da1557dcd1989e00cb3692b26d4173b4132

Reported-by: syzbot <redacted>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 drivers/video/fbdev/vga16fb.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/vga16fb.c b/drivers/video/fbdev/vga16fb.c
index a20eeb8308ff..578d3541e3d6 100644
--- a/drivers/video/fbdev/vga16fb.c
+++ b/drivers/video/fbdev/vga16fb.c

@@ -1121,7 +1121,7 @@ static void vga_8planes_imageblit(struct fb_info *info, const struct fb_image *i
         char oldop = setop(0);
         char oldsr = setsr(0);
         char oldmask = selectmask();
-        const char *cdat = image->data;
+	const unsigned char *cdat = image->data;
 	u32 dx = image->dx;
         char __iomem *where;
         int y;

-- 
2.18.4

On Fri, Sep 04, 2020 at 11:53:16PM +0900, Tetsuo Handa wrote:

Hello, Greg.

Since nobody is interested in this bug, can you directly pick up
this obvious patch without waiting for maintainer's response?

I'll pick it up now.

thanks,

greg k-h

Hello, Greg.

Since nobody is interested in this bug, can you directly pick up
this obvious patch without waiting for maintainer's response?

On 2020/08/31 19:37, Tetsuo Handa wrote:

syzbot is reporting OOB read at vga_8planes_imageblit() [1], for
"cdat[y] >> 4" can become a negative value due to "const char *cdat".

[1] https://syzkaller.appspot.com/bug?id

7a0da1557dcd1989e00cb3692b26d4173b4132