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Subject[PATCH v3 4/4] Allow unprivileged chroot when safe
Date
Chroot can easily be used to subvert setuid programs.  If no_new_privs,
then setuid programs don't gain any privilege, so allow chroot.

Because chroot is an easy way to break out of chroot jail, CAP_SYS_ADMIN
is still required if the caller is already chrooted.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
fs/open.c | 46 ++++++++++++++++++++++++++++++++++++++++++++--
1 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index 77becc0..2e2887a 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -418,10 +418,39 @@ out:
return error;
}

+static bool is_chrooted(struct fs_struct *fs)
+{
+ bool ret;
+
+ /*
+ * This is equivalent to checking whether "/.." is the same
+ * directory as "/", where the ".." part ignores the current
+ * root. This logic is the same as follow_dotdot except that we
+ * ignore fs->root and we don't need to follow the final
+ * mountpoint we end up on.
+ */
+ struct path path = fs->root;
+ path_get(&path);
+ while (true) {
+ if (path.dentry != path.mnt->mnt_root) {
+ ret = true; /* .. moves up within a vfsmount. */
+ break;
+ }
+
+ if (!follow_up(&path)) {
+ ret = false; /* We've hit the real root. */
+ break;
+ }
+ }
+ path_put(&path);
+ return ret;
+}
+
SYSCALL_DEFINE1(chroot, const char __user *, filename)
{
struct path path;
int error;
+ struct fs_struct *fs = current->fs;

error = user_path_dir(filename, &path);
if (error)
@@ -432,13 +461,26 @@ SYSCALL_DEFINE1(chroot, const char __user *, filename)
goto dput_and_out;

error = -EPERM;
- if (!capable(CAP_SYS_CHROOT))
+ /*
+ * Chroot is dangerous unless no_new_privs is set, and we also
+ * don't want to allow unprivileged users to break out of chroot
+ * jail with another chroot call.
+ *
+ * We therefore allow chroot under one of two circumstances:
+ * a) no_new_privs (so setuid and similar programs can't be
+ * exploited), fs not shared (to avoid bypassing no_new_privs),
+ * and not already chrooted (so there's no chroot jail to break
+ * out of)
+ * b) CAP_SYS_CHROOT
+ */
+ if (!(current->no_new_privs && fs->users == 1 && !is_chrooted(fs)) &&
+ !capable(CAP_SYS_CHROOT))
goto dput_and_out;
error = security_path_chroot(&path);
if (error)
goto dput_and_out;

- set_fs_root(current->fs, &path);
+ set_fs_root(fs, &path);
error = 0;
dput_and_out:
path_put(&path);
--
1.7.7.6


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