Changes in v2:
* Reordered addition to ioctls.h to follow correct hex ordering.
Aleksa Sarai (1):
tty: add TIOCGPTPEER ioctl
arch/alpha/include/uapi/asm/ioctls.h | 1 +
arch/mips/include/uapi/asm/ioctls.h | 1 +
arch/parisc/include/uapi/asm/ioctls.h | 1 +
arch/powerpc/include/uapi/asm/ioctls.h | 1 +
arch/sh/include/uapi/asm/ioctls.h | 1 +
arch/sparc/include/uapi/asm/ioctls.h | 3 +-
arch/xtensa/include/uapi/asm/ioctls.h | 1 +
drivers/tty/pty.c | 71 ++++++++++++++++++++++++++++++++--
include/uapi/asm-generic/ioctls.h | 1 +
9 files changed, 76 insertions(+), 5 deletions(-)
--
2.13.0
When opening the slave end of a PTY, it is not possible for userspace to
safely ensure that /dev/pts/$num is actually a slave (in cases where the
mount namespace in which devpts was mounted is controlled by an
untrusted process). In addition, there are several unresolvable
race conditions if userspace were to attempt to detect attacks through
stat(2) and other similar methods [in addition it is not clear how
userspace could detect attacks involving FUSE].
Resolve this by providing an interface for userpace to safely open the
"peer" end of a PTY file descriptor by using the dentry cached by
devpts. Since it is not possible to have an open master PTY without
having its slave exposed in /dev/pts this interface is safe. This
interface currently does not provide a way to get the master pty (since
it is not clear whether such an interface is safe or even useful).
Cc: Christian Brauner <[email protected]>
Cc: Valentin Rothberg <[email protected]>
Signed-off-by: Aleksa Sarai <[email protected]>
---
arch/alpha/include/uapi/asm/ioctls.h | 1 +
arch/mips/include/uapi/asm/ioctls.h | 1 +
arch/parisc/include/uapi/asm/ioctls.h | 1 +
arch/powerpc/include/uapi/asm/ioctls.h | 1 +
arch/sh/include/uapi/asm/ioctls.h | 1 +
arch/sparc/include/uapi/asm/ioctls.h | 3 +-
arch/xtensa/include/uapi/asm/ioctls.h | 1 +
drivers/tty/pty.c | 71 ++++++++++++++++++++++++++++++++--
include/uapi/asm-generic/ioctls.h | 1 +
9 files changed, 76 insertions(+), 5 deletions(-)
diff --git a/arch/alpha/include/uapi/asm/ioctls.h b/arch/alpha/include/uapi/asm/ioctls.h
index f30c94ae1bdb..ff67b8373bf7 100644
--- a/arch/alpha/include/uapi/asm/ioctls.h
+++ b/arch/alpha/include/uapi/asm/ioctls.h
@@ -100,6 +100,7 @@
#define TIOCGPKT _IOR('T', 0x38, int) /* Get packet mode state */
#define TIOCGPTLCK _IOR('T', 0x39, int) /* Get Pty lock state */
#define TIOCGEXCL _IOR('T', 0x40, int) /* Get exclusive mode state */
+#define TIOCGPTPEER _IOR('T', 0x41, int) /* Safely open the slave */
#define TIOCSERCONFIG 0x5453
#define TIOCSERGWILD 0x5454
diff --git a/arch/mips/include/uapi/asm/ioctls.h b/arch/mips/include/uapi/asm/ioctls.h
index 740219c2c894..68e19b689a00 100644
--- a/arch/mips/include/uapi/asm/ioctls.h
+++ b/arch/mips/include/uapi/asm/ioctls.h
@@ -91,6 +91,7 @@
#define TIOCGPKT _IOR('T', 0x38, int) /* Get packet mode state */
#define TIOCGPTLCK _IOR('T', 0x39, int) /* Get Pty lock state */
#define TIOCGEXCL _IOR('T', 0x40, int) /* Get exclusive mode state */
+#define TIOCGPTPEER _IOR('T', 0x41, int) /* Safely open the slave */
/* I hope the range from 0x5480 on is free ... */
#define TIOCSCTTY 0x5480 /* become controlling tty */
diff --git a/arch/parisc/include/uapi/asm/ioctls.h b/arch/parisc/include/uapi/asm/ioctls.h
index b6572f051b67..674c68a5bbd0 100644
--- a/arch/parisc/include/uapi/asm/ioctls.h
+++ b/arch/parisc/include/uapi/asm/ioctls.h
@@ -60,6 +60,7 @@
#define TIOCGPKT _IOR('T', 0x38, int) /* Get packet mode state */
#define TIOCGPTLCK _IOR('T', 0x39, int) /* Get Pty lock state */
#define TIOCGEXCL _IOR('T', 0x40, int) /* Get exclusive mode state */
+#define TIOCGPTPEER _IOR('T', 0x41, int) /* Safely open the slave */
#define FIONCLEX 0x5450 /* these numbers need to be adjusted. */
#define FIOCLEX 0x5451
diff --git a/arch/powerpc/include/uapi/asm/ioctls.h b/arch/powerpc/include/uapi/asm/ioctls.h
index 49a25796a61a..bfd609a3e928 100644
--- a/arch/powerpc/include/uapi/asm/ioctls.h
+++ b/arch/powerpc/include/uapi/asm/ioctls.h
@@ -100,6 +100,7 @@
#define TIOCGPKT _IOR('T', 0x38, int) /* Get packet mode state */
#define TIOCGPTLCK _IOR('T', 0x39, int) /* Get Pty lock state */
#define TIOCGEXCL _IOR('T', 0x40, int) /* Get exclusive mode state */
+#define TIOCGPTPEER _IOR('T', 0x41, int) /* Safely open the slave */
#define TIOCSERCONFIG 0x5453
#define TIOCSERGWILD 0x5454
diff --git a/arch/sh/include/uapi/asm/ioctls.h b/arch/sh/include/uapi/asm/ioctls.h
index c9903e56ccf4..eec7901e9e65 100644
--- a/arch/sh/include/uapi/asm/ioctls.h
+++ b/arch/sh/include/uapi/asm/ioctls.h
@@ -93,6 +93,7 @@
#define TIOCGPKT _IOR('T', 0x38, int) /* Get packet mode state */
#define TIOCGPTLCK _IOR('T', 0x39, int) /* Get Pty lock state */
#define TIOCGEXCL _IOR('T', 0x40, int) /* Get exclusive mode state */
+#define TIOCGPTPEER _IOR('T', 0x41, int) /* Safely open the slave */
#define TIOCSERCONFIG _IO('T', 83) /* 0x5453 */
#define TIOCSERGWILD _IOR('T', 84, int) /* 0x5454 */
diff --git a/arch/sparc/include/uapi/asm/ioctls.h b/arch/sparc/include/uapi/asm/ioctls.h
index 06b3f6c3bb9a..6d27398632ea 100644
--- a/arch/sparc/include/uapi/asm/ioctls.h
+++ b/arch/sparc/include/uapi/asm/ioctls.h
@@ -27,7 +27,7 @@
#define TIOCGRS485 _IOR('T', 0x41, struct serial_rs485)
#define TIOCSRS485 _IOWR('T', 0x42, struct serial_rs485)
-/* Note that all the ioctls that are not available in Linux have a
+/* Note that all the ioctls that are not available in Linux have a
* double underscore on the front to: a) avoid some programs to
* think we support some ioctls under Linux (autoconfiguration stuff)
*/
@@ -88,6 +88,7 @@
#define TIOCGPTN _IOR('t', 134, unsigned int) /* Get Pty Number */
#define TIOCSPTLCK _IOW('t', 135, int) /* Lock/unlock PTY */
#define TIOCSIG _IOW('t', 136, int) /* Generate signal on Pty slave */
+#define TIOCGPTPEER _IOR('t', 137, int) /* Safely open the slave */
/* Little f */
#define FIOCLEX _IO('f', 1)
diff --git a/arch/xtensa/include/uapi/asm/ioctls.h b/arch/xtensa/include/uapi/asm/ioctls.h
index 518954e74e6d..98b004e24e85 100644
--- a/arch/xtensa/include/uapi/asm/ioctls.h
+++ b/arch/xtensa/include/uapi/asm/ioctls.h
@@ -105,6 +105,7 @@
#define TIOCGPKT _IOR('T', 0x38, int) /* Get packet mode state */
#define TIOCGPTLCK _IOR('T', 0x39, int) /* Get Pty lock state */
#define TIOCGEXCL _IOR('T', 0x40, int) /* Get exclusive mode state */
+#define TIOCGPTPEER _IOR('T', 0x41, int) /* Safely open the slave */
#define TIOCSERCONFIG _IO('T', 83)
#define TIOCSERGWILD _IOR('T', 84, int)
diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index 65799575c666..19c134ca47bb 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -24,6 +24,9 @@
#include <linux/slab.h>
#include <linux/mutex.h>
#include <linux/poll.h>
+#include <linux/mount.h>
+#include <linux/file.h>
+#include <linux/ioctl.h>
#undef TTY_DEBUG_HANGUP
#ifdef TTY_DEBUG_HANGUP
@@ -66,8 +69,13 @@ static void pty_close(struct tty_struct *tty, struct file *filp)
#ifdef CONFIG_UNIX98_PTYS
if (tty->driver == ptm_driver) {
mutex_lock(&devpts_mutex);
- if (tty->link->driver_data)
- devpts_pty_kill(tty->link->driver_data);
+ if (tty->link->driver_data) {
+ struct path *path = tty->link->driver_data;
+
+ devpts_pty_kill(path->dentry);
+ path_put(path);
+ kfree(path);
+ }
mutex_unlock(&devpts_mutex);
}
#endif
@@ -440,6 +448,48 @@ static int pty_common_install(struct tty_driver *driver, struct tty_struct *tty,
return retval;
}
+/**
+ * pty_open_peer - open the peer of a pty
+ * @tty: the peer of the pty being opened
+ *
+ * Open the cached dentry in tty->link, providing a safe way for userspace
+ * to get the slave end of a pty (where they have the master fd and cannot
+ * access or trust the mount namespace /dev/pts was mounted inside).
+ */
+static struct file *pty_open_peer(struct tty_struct *tty, int flags)
+{
+ if (tty->driver->subtype != PTY_TYPE_MASTER)
+ return ERR_PTR(-EIO);
+ return dentry_open(tty->link->driver_data, flags, current_cred());
+}
+
+static int pty_get_peer(struct tty_struct *tty, int flags)
+{
+ int fd = -1;
+ struct file *filp = NULL;
+ int retval = -EINVAL;
+
+ fd = get_unused_fd_flags(0);
+ if (fd < 0) {
+ retval = fd;
+ goto err;
+ }
+
+ filp = pty_open_peer(tty, flags);
+ if (IS_ERR(filp)) {
+ retval = PTR_ERR(filp);
+ goto err_put;
+ }
+
+ fd_install(fd, filp);
+ return fd;
+
+err_put:
+ put_unused_fd(fd);
+err:
+ return retval;
+}
+
static void pty_cleanup(struct tty_struct *tty)
{
tty_port_put(tty->port);
@@ -602,6 +652,8 @@ static int pty_unix98_ioctl(struct tty_struct *tty,
return pty_get_pktmode(tty, (int __user *)arg);
case TIOCGPTN: /* Get PT Number */
return put_user(tty->index, (unsigned int __user *)arg);
+ case TIOCGPTPEER: /* Open the other end */
+ return pty_get_peer(tty, (int) arg);
case TIOCSIG: /* Send signal to other side of pty */
return pty_signal(tty, (int) arg);
}
@@ -718,6 +770,7 @@ static int ptmx_open(struct inode *inode, struct file *filp)
{
struct pts_fs_info *fsi;
struct tty_struct *tty;
+ struct path *pts_path;
struct dentry *dentry;
int retval;
int index;
@@ -771,16 +824,26 @@ static int ptmx_open(struct inode *inode, struct file *filp)
retval = PTR_ERR(dentry);
goto err_release;
}
- tty->link->driver_data = dentry;
+ /* We need to cache a fake path for TIOCGPTPEER. */
+ pts_path = kmalloc(sizeof(struct path), GFP_KERNEL);
+ if (!pts_path)
+ goto err_release;
+ pts_path->mnt = filp->f_path.mnt;
+ pts_path->dentry = dentry;
+ path_get(pts_path);
+ tty->link->driver_data = pts_path;
retval = ptm_driver->ops->open(tty, filp);
if (retval)
- goto err_release;
+ goto err_path_put;
tty_debug_hangup(tty, "opening (count=%d)\n", tty->count);
tty_unlock(tty);
return 0;
+err_path_put:
+ path_put(pts_path);
+ kfree(pts_path);
err_release:
tty_unlock(tty);
// This will also put-ref the fsi
diff --git a/include/uapi/asm-generic/ioctls.h b/include/uapi/asm-generic/ioctls.h
index 143dacbb7d9a..06d5f7ddf84e 100644
--- a/include/uapi/asm-generic/ioctls.h
+++ b/include/uapi/asm-generic/ioctls.h
@@ -77,6 +77,7 @@
#define TIOCGPKT _IOR('T', 0x38, int) /* Get packet mode state */
#define TIOCGPTLCK _IOR('T', 0x39, int) /* Get Pty lock state */
#define TIOCGEXCL _IOR('T', 0x40, int) /* Get exclusive mode state */
+#define TIOCGPTPEER _IOR('T', 0x41, int) /* Safely open the slave */
#define FIONCLEX 0x5450
#define FIOCLEX 0x5451
--
2.13.0
On Fri, Jun 2, 2017 at 10:48 AM, Aleksa Sarai <[email protected]> wrote:
> When opening the slave end of a PTY, it is not possible for userspace to
> safely ensure that /dev/pts/$num is actually a slave (in cases where the
> mount namespace in which devpts was mounted is controlled by an
> untrusted process). In addition, there are several unresolvable
> race conditions if userspace were to attempt to detect attacks through
> stat(2) and other similar methods [in addition it is not clear how
> userspace could detect attacks involving FUSE].
>
> Resolve this by providing an interface for userpace to safely open the
> "peer" end of a PTY file descriptor by using the dentry cached by
> devpts. Since it is not possible to have an open master PTY without
> having its slave exposed in /dev/pts this interface is safe. This
> interface currently does not provide a way to get the master pty (since
> it is not clear whether such an interface is safe or even useful).
This should be added to the compat ioctls as well. There are two ways
of doing this:
a) like the other ioctls handled by pty_unix98_ioctl(), add it to the
list in fs/compat_ioctl.c, and check that the list is still up-to-date with
the current driver (someone else may have missed one)
b) remove all compat handling for ioctls that are specific to
pty_unix98_ioctl() and pty_bsd_ioctl() from fs/compat_ioctl.c
and add compat_ioctl callback pointers to master_pty_ops_bsd
and ptm_unix98_ops, pointing to the respective ioctl handlers.
I would recommend b) as it avoids the problem in the future,
but it is a little more upfront work, and should be done as a
separate preparation patch.
All the ioctls here are compatible between 32-bit and 64-bit
user space, so no special translation is needed.
Arnd