[BUG] fbdev: sm712fb: Page fault in smtcfb_read

I found a minor in the smtcfb_read() function of the driver sm712fb.

This read function can not handle the case that the size of the
buffer is 3 and does not check for it, which may cause a page fault.

Here is a simple PoC:

#include <endian.h>
#include <fcntl.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>

int main(void)
{
    char buf[10];

    int fd = open("/dev/fb0", O_RDWR);
    read(fd, buf, 3);

    return 0;
}

The following log reveals it:

[ 2432.614490] BUG: unable to handle page fault for address: ffffc90001ffffff
[ 2432.618474] RIP: 0010:smtcfb_read+0x230/0x3e0
[ 2432.626551] Call Trace:
[ 2432.626770]  <TASK>
[ 2432.626950]  vfs_read+0x198/0xa00
[ 2432.627225]  ? do_sys_openat2+0x27d/0x350
[ 2432.627552]  ? __fget_light+0x54/0x340
[ 2432.627871]  ksys_read+0xce/0x190
[ 2432.628143]  do_syscall_64+0x43/0x90

Regards,
Zheyu Ma

* Zheyu Ma [off-list ref]:

I found a minor in the smtcfb_read() function of the driver sm712fb.

This read function can not handle the case that the size of the
buffer is 3 and does not check for it, which may cause a page fault.

The following log reveals it:

[ 2432.614490] BUG: unable to handle page fault for address: ffffc90001ffffff
[ 2432.618474] RIP: 0010:smtcfb_read+0x230/0x3e0
[ 2432.626551] Call Trace:
[ 2432.626770]  <TASK>
[ 2432.626950]  vfs_read+0x198/0xa00
[ 2432.627225]  ? do_sys_openat2+0x27d/0x350
[ 2432.627552]  ? __fget_light+0x54/0x340
[ 2432.627871]  ksys_read+0xce/0x190
[ 2432.628143]  do_syscall_64+0x43/0x90

Could you try the attached patch ?

diff --git a/drivers/video/fbdev/sm712fb.c b/drivers/video/fbdev/sm712fb.c
index 0dbc6bf8268a..ab45212ccf66 100644
--- 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;

@@ -1059,25 +1059,11 @@ 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);
+		for (i = (c + 3) >> 2; i--;) {
+			u32 val = fb_readl(src++);
+			*dst = big_swap(val);
 			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;

On Sat, Feb 26, 2022 at 11:03 PM Helge Deller [off-list ref] wrote:

* Zheyu Ma [off-list ref]:

quoted

I found a minor in the smtcfb_read() function of the driver sm712fb.

This read function can not handle the case that the size of the
buffer is 3 and does not check for it, which may cause a page fault.

The following log reveals it:

[ 2432.614490] BUG: unable to handle page fault for address: ffffc90001ffffff
[ 2432.618474] RIP: 0010:smtcfb_read+0x230/0x3e0
[ 2432.626551] Call Trace:
[ 2432.626770]  <TASK>
[ 2432.626950]  vfs_read+0x198/0xa00
[ 2432.627225]  ? do_sys_openat2+0x27d/0x350
[ 2432.627552]  ? __fget_light+0x54/0x340
[ 2432.627871]  ksys_read+0xce/0x190
[ 2432.628143]  do_syscall_64+0x43/0x90

Could you try the attached patch ?

Good, the patch works for me, thanks.

Tested-by: Zheyu Ma <redacted>

Regards,
Zheyu Ma