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

Greg Kroah-Hartman
2022-04-05T07:50:45Z

From: Helge Deller <deller@gmx.de>

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>
---
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;