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![Mattias Nissler](/assets/img/avatar_default.png)
For mounts that have the new "nosymfollow" option, don't follow symlinks when resolving paths. The new option is similar in spirit to the existing "nodev", "noexec", and "nosuid" options, as well as to the LOOKUP_NO_SYMLINKS resolve flag in the openat2(2) syscall. Various BSD variants have been supporting the "nosymfollow" mount option for a long time with equivalent implementations. Note that symlinks may still be created on file systems mounted with the "nosymfollow" option present. readlink() remains functional, so user space code that is aware of symlinks can still choose to follow them explicitly. Setting the "nosymfollow" mount option helps prevent privileged writers from modifying files unintentionally in case there is an unexpected link along the accessed path. The "nosymfollow" option is thus useful as a defensive measure for systems that need to deal with untrusted file systems in privileged contexts. More information on the history and motivation for this patch can be found here: https://sites.google.com/a/chromium.org/dev/chromium-os/chromiumos-design-docs/hardening-against-malicious-stateful-data#TOC-Restricting-symlink-traversal Signed-off-by: Mattias Nissler <mnissler@chromium.org> Signed-off-by: Ross Zwisler <zwisler@google.com> Reviewed-by: Aleksa Sarai <cyphar@cyphar.com> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
342 lines
7.9 KiB
C
342 lines
7.9 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/*
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* fs/proc_namespace.c - handling of /proc/<pid>/{mounts,mountinfo,mountstats}
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*
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* In fact, that's a piece of procfs; it's *almost* isolated from
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* the rest of fs/proc, but has rather close relationships with
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* fs/namespace.c, thus here instead of fs/proc
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*
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*/
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#include <linux/mnt_namespace.h>
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#include <linux/nsproxy.h>
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#include <linux/security.h>
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#include <linux/fs_struct.h>
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#include <linux/sched/task.h>
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#include "proc/internal.h" /* only for get_proc_task() in ->open() */
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#include "pnode.h"
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#include "internal.h"
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static __poll_t mounts_poll(struct file *file, poll_table *wait)
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{
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struct seq_file *m = file->private_data;
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struct proc_mounts *p = m->private;
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struct mnt_namespace *ns = p->ns;
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__poll_t res = EPOLLIN | EPOLLRDNORM;
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int event;
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poll_wait(file, &p->ns->poll, wait);
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event = READ_ONCE(ns->event);
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if (m->poll_event != event) {
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m->poll_event = event;
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res |= EPOLLERR | EPOLLPRI;
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}
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return res;
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}
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struct proc_fs_opts {
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int flag;
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const char *str;
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};
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static int show_sb_opts(struct seq_file *m, struct super_block *sb)
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{
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static const struct proc_fs_opts fs_opts[] = {
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{ SB_SYNCHRONOUS, ",sync" },
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{ SB_DIRSYNC, ",dirsync" },
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{ SB_MANDLOCK, ",mand" },
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{ SB_LAZYTIME, ",lazytime" },
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{ 0, NULL }
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};
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const struct proc_fs_opts *fs_infop;
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for (fs_infop = fs_opts; fs_infop->flag; fs_infop++) {
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if (sb->s_flags & fs_infop->flag)
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seq_puts(m, fs_infop->str);
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}
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return security_sb_show_options(m, sb);
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}
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static void show_mnt_opts(struct seq_file *m, struct vfsmount *mnt)
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{
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static const struct proc_fs_opts mnt_opts[] = {
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{ MNT_NOSUID, ",nosuid" },
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{ MNT_NODEV, ",nodev" },
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{ MNT_NOEXEC, ",noexec" },
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{ MNT_NOATIME, ",noatime" },
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{ MNT_NODIRATIME, ",nodiratime" },
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{ MNT_RELATIME, ",relatime" },
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{ MNT_NOSYMFOLLOW, ",nosymfollow" },
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{ 0, NULL }
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};
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const struct proc_fs_opts *fs_infop;
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for (fs_infop = mnt_opts; fs_infop->flag; fs_infop++) {
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if (mnt->mnt_flags & fs_infop->flag)
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seq_puts(m, fs_infop->str);
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}
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}
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static inline void mangle(struct seq_file *m, const char *s)
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{
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seq_escape(m, s, " \t\n\\");
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}
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static void show_type(struct seq_file *m, struct super_block *sb)
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{
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mangle(m, sb->s_type->name);
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if (sb->s_subtype) {
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seq_putc(m, '.');
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mangle(m, sb->s_subtype);
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}
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}
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static int show_vfsmnt(struct seq_file *m, struct vfsmount *mnt)
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{
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struct proc_mounts *p = m->private;
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struct mount *r = real_mount(mnt);
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struct path mnt_path = { .dentry = mnt->mnt_root, .mnt = mnt };
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struct super_block *sb = mnt_path.dentry->d_sb;
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int err;
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if (sb->s_op->show_devname) {
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err = sb->s_op->show_devname(m, mnt_path.dentry);
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if (err)
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goto out;
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} else {
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mangle(m, r->mnt_devname ? r->mnt_devname : "none");
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}
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seq_putc(m, ' ');
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/* mountpoints outside of chroot jail will give SEQ_SKIP on this */
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err = seq_path_root(m, &mnt_path, &p->root, " \t\n\\");
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if (err)
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goto out;
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seq_putc(m, ' ');
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show_type(m, sb);
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seq_puts(m, __mnt_is_readonly(mnt) ? " ro" : " rw");
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err = show_sb_opts(m, sb);
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if (err)
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goto out;
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show_mnt_opts(m, mnt);
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if (sb->s_op->show_options)
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err = sb->s_op->show_options(m, mnt_path.dentry);
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seq_puts(m, " 0 0\n");
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out:
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return err;
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}
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static int show_mountinfo(struct seq_file *m, struct vfsmount *mnt)
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{
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struct proc_mounts *p = m->private;
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struct mount *r = real_mount(mnt);
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struct super_block *sb = mnt->mnt_sb;
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struct path mnt_path = { .dentry = mnt->mnt_root, .mnt = mnt };
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int err;
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seq_printf(m, "%i %i %u:%u ", r->mnt_id, r->mnt_parent->mnt_id,
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MAJOR(sb->s_dev), MINOR(sb->s_dev));
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if (sb->s_op->show_path) {
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err = sb->s_op->show_path(m, mnt->mnt_root);
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if (err)
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goto out;
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} else {
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seq_dentry(m, mnt->mnt_root, " \t\n\\");
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}
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seq_putc(m, ' ');
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/* mountpoints outside of chroot jail will give SEQ_SKIP on this */
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err = seq_path_root(m, &mnt_path, &p->root, " \t\n\\");
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if (err)
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goto out;
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seq_puts(m, mnt->mnt_flags & MNT_READONLY ? " ro" : " rw");
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show_mnt_opts(m, mnt);
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/* Tagged fields ("foo:X" or "bar") */
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if (IS_MNT_SHARED(r))
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seq_printf(m, " shared:%i", r->mnt_group_id);
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if (IS_MNT_SLAVE(r)) {
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int master = r->mnt_master->mnt_group_id;
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int dom = get_dominating_id(r, &p->root);
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seq_printf(m, " master:%i", master);
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if (dom && dom != master)
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seq_printf(m, " propagate_from:%i", dom);
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}
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if (IS_MNT_UNBINDABLE(r))
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seq_puts(m, " unbindable");
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/* Filesystem specific data */
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seq_puts(m, " - ");
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show_type(m, sb);
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seq_putc(m, ' ');
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if (sb->s_op->show_devname) {
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err = sb->s_op->show_devname(m, mnt->mnt_root);
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if (err)
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goto out;
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} else {
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mangle(m, r->mnt_devname ? r->mnt_devname : "none");
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}
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seq_puts(m, sb_rdonly(sb) ? " ro" : " rw");
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err = show_sb_opts(m, sb);
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if (err)
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goto out;
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if (sb->s_op->show_options)
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err = sb->s_op->show_options(m, mnt->mnt_root);
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seq_putc(m, '\n');
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out:
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return err;
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}
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static int show_vfsstat(struct seq_file *m, struct vfsmount *mnt)
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{
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struct proc_mounts *p = m->private;
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struct mount *r = real_mount(mnt);
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struct path mnt_path = { .dentry = mnt->mnt_root, .mnt = mnt };
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struct super_block *sb = mnt_path.dentry->d_sb;
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int err;
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/* device */
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if (sb->s_op->show_devname) {
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seq_puts(m, "device ");
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err = sb->s_op->show_devname(m, mnt_path.dentry);
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if (err)
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goto out;
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} else {
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if (r->mnt_devname) {
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seq_puts(m, "device ");
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mangle(m, r->mnt_devname);
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} else
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seq_puts(m, "no device");
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}
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/* mount point */
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seq_puts(m, " mounted on ");
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/* mountpoints outside of chroot jail will give SEQ_SKIP on this */
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err = seq_path_root(m, &mnt_path, &p->root, " \t\n\\");
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if (err)
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goto out;
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seq_putc(m, ' ');
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/* file system type */
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seq_puts(m, "with fstype ");
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show_type(m, sb);
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/* optional statistics */
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if (sb->s_op->show_stats) {
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seq_putc(m, ' ');
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err = sb->s_op->show_stats(m, mnt_path.dentry);
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}
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seq_putc(m, '\n');
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out:
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return err;
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}
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static int mounts_open_common(struct inode *inode, struct file *file,
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int (*show)(struct seq_file *, struct vfsmount *))
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{
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struct task_struct *task = get_proc_task(inode);
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struct nsproxy *nsp;
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struct mnt_namespace *ns = NULL;
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struct path root;
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struct proc_mounts *p;
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struct seq_file *m;
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int ret = -EINVAL;
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if (!task)
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goto err;
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task_lock(task);
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nsp = task->nsproxy;
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if (!nsp || !nsp->mnt_ns) {
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task_unlock(task);
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put_task_struct(task);
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goto err;
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}
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ns = nsp->mnt_ns;
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get_mnt_ns(ns);
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if (!task->fs) {
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task_unlock(task);
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put_task_struct(task);
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ret = -ENOENT;
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goto err_put_ns;
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}
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get_fs_root(task->fs, &root);
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task_unlock(task);
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put_task_struct(task);
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ret = seq_open_private(file, &mounts_op, sizeof(struct proc_mounts));
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if (ret)
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goto err_put_path;
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m = file->private_data;
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m->poll_event = ns->event;
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p = m->private;
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p->ns = ns;
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p->root = root;
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p->show = show;
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INIT_LIST_HEAD(&p->cursor.mnt_list);
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p->cursor.mnt.mnt_flags = MNT_CURSOR;
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return 0;
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err_put_path:
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path_put(&root);
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err_put_ns:
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put_mnt_ns(ns);
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err:
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return ret;
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}
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static int mounts_release(struct inode *inode, struct file *file)
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{
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struct seq_file *m = file->private_data;
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struct proc_mounts *p = m->private;
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path_put(&p->root);
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mnt_cursor_del(p->ns, &p->cursor);
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put_mnt_ns(p->ns);
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return seq_release_private(inode, file);
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}
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static int mounts_open(struct inode *inode, struct file *file)
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{
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return mounts_open_common(inode, file, show_vfsmnt);
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}
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static int mountinfo_open(struct inode *inode, struct file *file)
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{
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return mounts_open_common(inode, file, show_mountinfo);
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}
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static int mountstats_open(struct inode *inode, struct file *file)
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{
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return mounts_open_common(inode, file, show_vfsstat);
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}
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const struct file_operations proc_mounts_operations = {
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.open = mounts_open,
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.read = seq_read,
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.llseek = seq_lseek,
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.release = mounts_release,
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.poll = mounts_poll,
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};
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const struct file_operations proc_mountinfo_operations = {
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.open = mountinfo_open,
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.read = seq_read,
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.llseek = seq_lseek,
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.release = mounts_release,
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.poll = mounts_poll,
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};
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const struct file_operations proc_mountstats_operations = {
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.open = mountstats_open,
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.read = seq_read,
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.llseek = seq_lseek,
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.release = mounts_release,
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};
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