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Subject[PATCH 3.2 052/115] fs/coredump: prevent fsuid=0 dumps into user-controlled directories
3.2.80-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jann Horn <jann@thejh.net>

commit 378c6520e7d29280f400ef2ceaf155c86f05a71a upstream.

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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2: adjust filename, context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
arch/um/drivers/mconsole_kern.c | 2 +-
fs/exec.c | 30 ++++++++++++++++++++++++++----
fs/fhandle.c | 2 +-
fs/open.c | 6 ++----
include/linux/fs.h | 2 +-
kernel/sysctl_binary.c | 2 +-
6 files changed, 32 insertions(+), 12 deletions(-)

--- a/arch/um/drivers/mconsole_kern.c
+++ b/arch/um/drivers/mconsole_kern.c
@@ -133,7 +133,7 @@ void mconsole_proc(struct mc_request *re
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);
goto out;
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -55,6 +55,9 @@
#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 <asm/uaccess.h>
#include <asm/mmu_context.h>
@@ -2246,6 +2249,8 @@ void do_coredump(long signr, int exit_co
}
} 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;
@@ -2284,10 +2289,27 @@ void do_coredump(long signr, int exit_co
* 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;

--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -229,7 +229,7 @@ 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);
--- a/fs/open.c
+++ b/fs/open.c
@@ -958,12 +958,10 @@ struct file *filp_open(const char *filen
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 lookup = build_open_flags(flags, 0, &op);
- if (flags & O_CREAT)
- return ERR_PTR(-EINVAL);
+ int lookup = build_open_flags(flags, mode, &op);
if (!filename && (flags & O_DIRECTORY))
if (!dentry->d_inode->i_op->lookup)
return ERR_PTR(-ENOTDIR);
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2073,7 +2073,7 @@ extern long do_sys_open(int dfd, const c
int mode);
extern struct file *filp_open(const char *, int, int);
extern struct file *file_open_root(struct dentry *, struct vfsmount *,
- const char *, int);
+ const char *, int, umode_t);
extern struct file * dentry_open(struct dentry *, struct vfsmount *, int,
const struct cred *);
extern int filp_close(struct file *, fl_owner_t id);
--- a/kernel/sysctl_binary.c
+++ b/kernel/sysctl_binary.c
@@ -1346,7 +1346,7 @@ static ssize_t binary_sysctl(const int *
}

mnt = current->nsproxy->pid_ns->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;
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