fs/coredump: prevent fsuid=0 dumps into user-controlled directories

This commit fixes the following security hole affecting systems where
all of the following conditions are fulfilled:

 - The fs.suid_dumpable sysctl is set to 2.
 - The kernel.core_pattern sysctl's value starts with "/". (Systems
   where kernel.core_pattern starts with "|/" are not affected.)
 - Unprivileged user namespace creation is permitted. (This is
   true on Linux >=3.8, but some distributions disallow it by
   default using a distro patch.)

Under these conditions, if a program executes under secure exec rules,
causing it to run with the SUID_DUMP_ROOT flag, then unshares its user
namespace, changes its root directory and crashes, the coredump will be
written using fsuid=0 and a path derived from kernel.core_pattern - but
this path is interpreted relative to the root directory of the process,
allowing the attacker to control where a coredump will be written with
root privileges.

To fix the security issue, always interpret core_pattern for dumps that
are written under SUID_DUMP_ROOT relative to the root directory of init.

Signed-off-by: Jann Horn <jann@thejh.net>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

6 files changed, 32 insertions, 12 deletions

diff --git a/arch/um/drivers/mconsole_kern.c b/arch/um/drivers/mconsole_kern.c
index b821b13d343a..8a6b57108ac2 100644
--- a/arch/um/drivers/mconsole_kern.c
+++ b/arch/um/drivers/mconsole_kern.c
@@ -133,7 +133,7 @@ void mconsole_proc(struct mc_request *req)
         ptr += strlen("proc");
         ptr = skip_spaces(ptr);
 
-        file = file_open_root(mnt->mnt_root, mnt, ptr, O_RDONLY);
+        file = file_open_root(mnt->mnt_root, mnt, ptr, O_RDONLY, 0);
         if (IS_ERR(file)) {
                 mconsole_reply(req, "Failed to open file", 1, 0);
                 printk(KERN_ERR "open /proc/%s: %ld\n", ptr, PTR_ERR(file));
diff --git a/fs/coredump.c b/fs/coredump.c
index 9ea87e9fdccf..47c32c3bfa1d 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -32,6 +32,9 @@
 #include <linux/pipe_fs_i.h>
 #include <linux/oom.h>
 #include <linux/compat.h>
+#include <linux/sched.h>
+#include <linux/fs.h>
+#include <linux/path.h>
 #include <linux/timekeeping.h>
 
 #include <asm/uaccess.h>
@@ -649,6 +652,8 @@ void do_coredump(const siginfo_t *siginfo)
                 }
         } else {
                 struct inode *inode;
+                int open_flags = O_CREAT | O_RDWR | O_NOFOLLOW |
+                                 O_LARGEFILE | O_EXCL;
 
                 if (cprm.limit < binfmt->min_coredump)
                         goto fail_unlock;
@@ -687,10 +692,27 @@ void do_coredump(const siginfo_t *siginfo)
                  * what matters is that at least one of the two processes
                  * writes its coredump successfully, not which one.
                  */
-                cprm.file = filp_open(cn.corename,
-                                 O_CREAT | 2 | O_NOFOLLOW |
-                                 O_LARGEFILE | O_EXCL,
-                                 0600);
+                if (need_suid_safe) {
+                        /*
+                         * Using user namespaces, normal user tasks can change
+                         * their current->fs->root to point to arbitrary
+                         * directories. Since the intention of the "only dump
+                         * with a fully qualified path" rule is to control where
+                         * coredumps may be placed using root privileges,
+                         * current->fs->root must not be used. Instead, use the
+                         * root directory of init_task.
+                         */
+                        struct path root;
+
+                        task_lock(&init_task);
+                        get_fs_root(init_task.fs, &root);
+                        task_unlock(&init_task);
+                        cprm.file = file_open_root(root.dentry, root.mnt,
+                                cn.corename, open_flags, 0600);
+                        path_put(&root);
+                } else {
+                        cprm.file = filp_open(cn.corename, open_flags, 0600);
+                }
                 if (IS_ERR(cprm.file))
                         goto fail_unlock;
 
diff --git a/fs/fhandle.c b/fs/fhandle.c
index d59712dfa3e7..ca3c3dd01789 100644
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -228,7 +228,7 @@ long do_handle_open(int mountdirfd,
                 path_put(&path);
                 return fd;
         }
-        file = file_open_root(path.dentry, path.mnt, "", open_flag);
+        file = file_open_root(path.dentry, path.mnt, "", open_flag, 0);
         if (IS_ERR(file)) {
                 put_unused_fd(fd);
                 retval =  PTR_ERR(file);
diff --git a/fs/open.c b/fs/open.c
index 55bdc75e2172..17cb6b1dab75 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -992,14 +992,12 @@ struct file *filp_open(const char *filename, int flags, umode_t mode)
 EXPORT_SYMBOL(filp_open);
 
 struct file *file_open_root(struct dentry *dentry, struct vfsmount *mnt,
-                            const char *filename, int flags)
+                            const char *filename, int flags, umode_t mode)
 {
         struct open_flags op;
-        int err = build_open_flags(flags, 0, &op);
+        int err = build_open_flags(flags, mode, &op);
         if (err)
                 return ERR_PTR(err);
-        if (flags & O_CREAT)
-                return ERR_PTR(-EINVAL);
         return do_file_open_root(dentry, mnt, filename, &op);
 }
 EXPORT_SYMBOL(file_open_root);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 35d99266ca9a..14a97194b34b 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2263,7 +2263,7 @@ extern long do_sys_open(int dfd, const char __user *filename, int flags,
 extern struct file *file_open_name(struct filename *, int, umode_t);
 extern struct file *filp_open(const char *, int, umode_t);
 extern struct file *file_open_root(struct dentry *, struct vfsmount *,
-                                   const char *, int);
+                                   const char *, int, umode_t);
 extern struct file * dentry_open(const struct path *, int, const struct cred *);
 extern int filp_close(struct file *, fl_owner_t id);
 
diff --git a/kernel/sysctl_binary.c b/kernel/sysctl_binary.c
index 7e7746a42a62..10a1d7dc9313 100644
--- a/kernel/sysctl_binary.c
+++ b/kernel/sysctl_binary.c
@@ -1321,7 +1321,7 @@ static ssize_t binary_sysctl(const int *name, int nlen,
         }
 
         mnt = task_active_pid_ns(current)->proc_mnt;
-        file = file_open_root(mnt->mnt_root, mnt, pathname, flags);
+        file = file_open_root(mnt->mnt_root, mnt, pathname, flags, 0);
         result = PTR_ERR(file);
         if (IS_ERR(file))
                 goto out_putname;