Do not copy up when volume is not empty

When Docker performs a copy up, it first verifies that the volume
being copied into is empty; thus, for volumes that have been
modified elsewhere (e.g. manually copying into then), the copy up
will not be performed at all. Duplicate this behavior in Podman
by checking if the volume is empty before copying.

Furthermore, move setting copyup to false further up. This will
prevent a potential race where copy up could happen more than
once if Podman was killed after some files had been copied but
before the DB was updated.

This resolves CVE-2020-1726.

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

1 files changed, 22 insertions, 6 deletions

diff --git a/libpod/container_internal.go b/libpod/container_internal.go
index 78ec09f29..216bbe669 100644
--- a/libpod/container_internal.go
+++ b/libpod/container_internal.go
@@ -1383,18 +1383,34 @@ func (c *Container) mountNamedVolume(v *ContainerNamedVolume, mountpoint string)
 	}
 	if vol.state.NeedsCopyUp {
 		logrus.Debugf("Copying up contents from container %s to volume %s", c.ID(), vol.Name())
+
+		// Set NeedsCopyUp to false immediately, so we don't try this
+		// again when there are already files copied.
+		vol.state.NeedsCopyUp = false
+		if err := vol.save(); err != nil {
+			return nil, err
+		}
+
+		// If the volume is not empty, we should not copy up.
+		volMount := vol.MountPoint()
+		contents, err := ioutil.ReadDir(volMount)
+		if err != nil {
+			return nil, errors.Wrapf(err, "error listing contents of volume %s mountpoint when copying up from container %s", vol.Name(), c.ID())
+		}
+		if len(contents) > 0 {
+			// The volume is not empty. It was likely modified
+			// outside of Podman. For safety, let's not copy up into
+			// it. Fixes CVE-2020-1726.
+			return vol, nil
+		}
+
 		srcDir, err := securejoin.SecureJoin(mountpoint, v.Dest)
 		if err != nil {
 			return nil, errors.Wrapf(err, "error calculating destination path to copy up container %s volume %s", c.ID(), vol.Name())
 		}
-		if err := c.copyWithTarFromImage(srcDir, vol.MountPoint()); err != nil && !os.IsNotExist(err) {
+		if err := c.copyWithTarFromImage(srcDir, volMount); err != nil && !os.IsNotExist(err) {
 			return nil, errors.Wrapf(err, "error copying content from container %s into volume %s", c.ID(), vol.Name())
 		}
-
-		vol.state.NeedsCopyUp = false
-		if err := vol.save(); err != nil {
-			return nil, err
-		}
 	}
 	return vol, nil
 }