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From 3291d66b98445bd7f7d02eac7f2bca2ac2c56942 Mon Sep 17 00:00:00 2001
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From: Aleksa Sarai <asarai@suse.de>
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Date: Sat, 21 Dec 2019 23:40:17 +1100
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Subject: [PATCH] rootfs: do not permit /proc mounts to non-directories
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mount(2) will blindly follow symlinks, which is a problem because it
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allows a malicious container to trick runc into mounting /proc to an
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entirely different location (and thus within the attacker's control for
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a rename-exchange attack).
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This is just a hotfix (to "stop the bleeding"), and the more complete
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fix would be finish libpathrs and port runc to it (to avoid these types
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of attacks entirely, and defend against a variety of other /proc-related
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attacks). It can be bypased by someone having "/" be a volume controlled
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by another container.
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Fixes: CVE-2019-19921
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Signed-off-by: Aleksa Sarai <asarai@suse.de>
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---
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libcontainer/rootfs_linux.go | 12 ++++++++++++
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1 file changed, 12 insertions(+)
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diff --git a/libcontainer/rootfs_linux.go b/libcontainer/rootfs_linux.go
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index 291021440..106c4c2b9 100644
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--- a/libcontainer/rootfs_linux.go
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+++ b/libcontainer/rootfs_linux.go
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@@ -299,6 +299,18 @@ func mountToRootfs(m *configs.Mount, rootfs, mountLabel string, enableCgroupns b
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switch m.Device {
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case "proc", "sysfs":
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+ // If the destination already exists and is not a directory, we bail
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+ // out This is to avoid mounting through a symlink or similar -- which
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+ // has been a "fun" attack scenario in the past.
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+ // TODO: This won't be necessary once we switch to libpathrs and we can
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+ // stop all of these symlink-exchange attacks.
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+ if fi, err := os.Lstat(dest); err != nil {
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+ if !os.IsNotExist(err) {
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+ return err
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+ }
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+ } else if fi.Mode()&os.ModeDir == 0 {
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+ return fmt.Errorf("filesystem %q must be mounted on ordinary directory", m.Device)
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+ }
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if err := os.MkdirAll(dest, 0755); err != nil {
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return err
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}
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