tty,vcs: lseek/VC-release race fix

there's a race between vcs's lseek handler and VC release.

The lseek handler does not hold console_lock and touches
VC's size info. If during this the VC got released, there's
an access violation.

Following program triggers the issue for me:

[SNIP]
#define _BSD_SOURCE
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <linux/vt.h>
#include <unistd.h>
#include <errno.h>

static int run_seek(void)
{
while(1) {
int fd;
fd = open("./vcs30", O_RDWR);
while(lseek(fd, 0, 0) != -1);
close(fd);
}
}

static int open_ioctl_tty(void)
{
return open("/dev/tty1", O_RDWR);
}

static int do_ioctl(int fd, int req, int i)
{
return ioctl(fd, req, i);
}

#define INIT(i) do_ioctl(ioctl_fd, VT_ACTIVATE, i)
#define SHUT(i) do_ioctl(ioctl_fd, VT_DISALLOCATE, i)

int main(int argc, char **argv)
{
int ioctl_fd = open_ioctl_tty();

if (ioctl < 0) {
perror("open tty1 failed\n");
return -1;
}

if ((-1 == mknod("vcs30", S_IFCHR|0666, makedev(7, 30))) &&
(errno != EEXIST)) {
printf("errno %d\n", errno);
perror("failed to create vcs30");
return -1;
}

do_ioctl(ioctl_fd, VT_LOCKSWITCH, 0);

if (!fork())
run_seek();

while(1) {
INIT(30);
SHUT(30);
}

return 0;
}
[SNIP]

Signed-off-by: Jiri Olsa <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/vt/vc_screen.c

···
 	int size;
 
 	mutex_lock(&con_buf_mtx);
+	console_lock();
 	size = vcs_size(file->f_path.dentry->d_inode);
+	console_unlock();
+	if (size < 0) {
+		mutex_unlock(&con_buf_mtx);
+		return size;
+	}
 	switch (orig) {
 		default:
 			mutex_unlock(&con_buf_mtx);
···
 		 * could sleep.
 		 */
 		size = vcs_size(inode);
+		if (size < 0) {
+			if (read)
+				break;
+			ret = size;
+			goto unlock_out;
+		}
 		if (pos >= size)
 			break;
 		if (count > size - pos)
···
 		 * Return data written up to now on failure.
 		 */
 		size = vcs_size(inode);
+		if (size < 0) {
+			if (written)
+				break;
+			ret = size;
+			goto unlock_out;
+		}
 		if (pos >= size)
 			break;
 		if (this_round > size - pos)