[PATCH 5.17 0146/1126] video: fbdev: sm712fb: Fix crash in smtcfb_read()

[Greg Kroah-Hartman]

From: Helge Deller <deller@xxxxxx>

commit bd771cf5c4254511cc4abb88f3dab3bd58bdf8e8 upstream.

Zheyu Ma reported this crash in the sm712fb driver when reading
three bytes from the framebuffer:

 BUG: unable to handle page fault for address: ffffc90001ffffff
 RIP: 0010:smtcfb_read+0x230/0x3e0
 Call Trace:
  vfs_read+0x198/0xa00
  ? do_sys_openat2+0x27d/0x350
  ? __fget_light+0x54/0x340
  ksys_read+0xce/0x190
  do_syscall_64+0x43/0x90

Fix it by removing the open-coded endianess fixup-code and
by moving the pointer post decrement out the fb_readl() function.

Reported-by: Zheyu Ma <zheyuma97@xxxxxxxxx>
Signed-off-by: Helge Deller <deller@xxxxxx>
Tested-by: Zheyu Ma <zheyuma97@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/video/fbdev/sm712fb.c |   25 +++++++------------------
 1 file changed, 7 insertions(+), 18 deletions(-)

--- a/drivers/video/fbdev/sm712fb.c
+++ b/drivers/video/fbdev/sm712fb.c
@@ -1047,7 +1047,7 @@ static ssize_t smtcfb_read(struct fb_inf
 	if (count + p > total_size)
 		count = total_size - p;
 
-	buffer = kmalloc((count > PAGE_SIZE) ? PAGE_SIZE : count, GFP_KERNEL);
+	buffer = kmalloc(PAGE_SIZE, GFP_KERNEL);
 	if (!buffer)
 		return -ENOMEM;
 
@@ -1059,25 +1059,14 @@ static ssize_t smtcfb_read(struct fb_inf
 	while (count) {
 		c = (count > PAGE_SIZE) ? PAGE_SIZE : count;
 		dst = buffer;
-		for (i = c >> 2; i--;) {
-			*dst = fb_readl(src++);
-			*dst = big_swap(*dst);
+		for (i = (c + 3) >> 2; i--;) {
+			u32 val;
+
+			val = fb_readl(src);
+			*dst = big_swap(val);
+			src++;
 			dst++;
 		}
-		if (c & 3) {
-			u8 *dst8 = (u8 *)dst;
-			u8 __iomem *src8 = (u8 __iomem *)src;
-
-			for (i = c & 3; i--;) {
-				if (i & 1) {
-					*dst8++ = fb_readb(++src8);
-				} else {
-					*dst8++ = fb_readb(--src8);
-					src8 += 2;
-				}
-			}
-			src = (u32 __iomem *)src8;
-		}
 
 		if (copy_to_user(buf, buffer, c)) {
 			err = -EFAULT;