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 <jolsa@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/tty/vt/vc_screen.c

@@ -159,7 +159,13 @@ static loff_t vcs_lseek(struct file *file, loff_t offset, int orig)
 	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);
@@ -237,6 +243,12 @@ vcs_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
 		 * 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)
@@ -436,6 +448,12 @@ vcs_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos)
 		 * 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)