Commit 378c6520 authored by Jann Horn's avatar Jann Horn Committed by Linus Torvalds
Browse files

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_p...
parent 1333ab03
......@@ -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));
......
......@@ -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;
......
......@@ -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);
......
......@@ -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);
......
......@@ -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);
......
......@@ -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;
......
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