video: fbdev: sm712fb: Fix crash in smtcfb_read()

BugLink: https://bugs.launchpad.net/bugs/1969110
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@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>
Tested-by: Zheyu Ma <zheyuma97@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 46cdbff26c88fd75dccbf28df1d07cbe18007eac)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>

diff --git a/drivers/video/fbdev/sm712fb.c b/drivers/video/fbdev/sm712fb.c
index 0dbc6bf8268acf5fa0528f28f047372cbd4fd5e8..b60a2730f0a8a8da1452ffef0707c3da3d809978 100644 (file)
--- a/drivers/video/fbdev/sm712fb.c
+++ b/drivers/video/fbdev/sm712fb.c
@@ -1047,7 +1047,7 @@ static ssize_t smtcfb_read(struct fb_info *info, char __user *buf,
        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;
 
@@ -1059,24 +1059,13 @@ static ssize_t smtcfb_read(struct fb_info *info, char __user *buf,
        while (count) {
                c = (count > PAGE_SIZE) ? PAGE_SIZE : count;
                dst = buffer;
-               for (i = c >> 2; i--;) {
-                       *dst = fb_readl(src++);
-                       *dst = big_swap(*dst);
-                       dst++;
-               }
-               if (c & 3) {
-                       u8 *dst8 = (u8 *)dst;
-                       u8 __iomem *src8 = (u8 __iomem *)src;
+               for (i = (c + 3) >> 2; i--;) {
+                       u32 val;
 
-                       for (i = c & 3; i--;) {
-                               if (i & 1) {
-                                       *dst8++ = fb_readb(++src8);
-                               } else {
-                                       *dst8++ = fb_readb(--src8);
-                                       src8 += 2;
-                               }
-                       }
-                       src = (u32 __iomem *)src8;
+                       val = fb_readl(src);
+                       *dst = big_swap(val);
+                       src++;
+                       dst++;
                }
 
                if (copy_to_user(buf, buffer, c)) {