This is a note to let you know that I've just added the patch titled fs: Better permission checking for submounts to the 4.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: fs-better-permission-checking-for-submounts.patch and it can be found in the queue-4.9 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 93faccbbfa958a9668d3ab4e30f38dd205cee8d8 Mon Sep 17 00:00:00 2001 From: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx> Date: Wed, 1 Feb 2017 06:06:16 +1300 Subject: fs: Better permission checking for submounts From: Eric W. Biederman <ebiederm@xxxxxxxxxxxx> commit 93faccbbfa958a9668d3ab4e30f38dd205cee8d8 upstream. To support unprivileged users mounting filesystems two permission checks have to be performed: a test to see if the user allowed to create a mount in the mount namespace, and a test to see if the user is allowed to access the specified filesystem. The automount case is special in that mounting the original filesystem grants permission to mount the sub-filesystems, to any user who happens to stumble across the their mountpoint and satisfies the ordinary filesystem permission checks. Attempting to handle the automount case by using override_creds almost works. It preserves the idea that permission to mount the original filesystem is permission to mount the sub-filesystem. Unfortunately using override_creds messes up the filesystems ordinary permission checks. Solve this by being explicit that a mount is a submount by introducing vfs_submount, and using it where appropriate. vfs_submount uses a new mount internal mount flags MS_SUBMOUNT, to let sget and friends know that a mount is a submount so they can take appropriate action. sget and sget_userns are modified to not perform any permission checks on submounts. follow_automount is modified to stop using override_creds as that has proven problemantic. do_mount is modified to always remove the new MS_SUBMOUNT flag so that we know userspace will never by able to specify it. autofs4 is modified to stop using current_real_cred that was put in there to handle the previous version of submount permission checking. cifs is modified to pass the mountpoint all of the way down to vfs_submount. debugfs is modified to pass the mountpoint all of the way down to trace_automount by adding a new parameter. To make this change easier a new typedef debugfs_automount_t is introduced to capture the type of the debugfs automount function. Fixes: 069d5ac9ae0d ("autofs: Fix automounts by using current_real_cred()->uid") Fixes: aeaa4a79ff6a ("fs: Call d_automount with the filesystems creds") Reviewed-by: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx> Reviewed-by: Seth Forshee <seth.forshee@xxxxxxxxxxxxx> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- fs/afs/mntpt.c | 2 +- fs/autofs4/waitq.c | 4 ++-- fs/cifs/cifs_dfs_ref.c | 7 ++++--- fs/debugfs/inode.c | 8 ++++---- fs/namei.c | 3 --- fs/namespace.c | 17 ++++++++++++++++- fs/nfs/namespace.c | 2 +- fs/nfs/nfs4namespace.c | 2 +- fs/super.c | 13 ++++++++++--- include/linux/debugfs.h | 3 ++- include/linux/mount.h | 3 +++ include/uapi/linux/fs.h | 1 + kernel/trace/trace.c | 4 ++-- 13 files changed, 47 insertions(+), 22 deletions(-) --- a/fs/afs/mntpt.c +++ b/fs/afs/mntpt.c @@ -202,7 +202,7 @@ static struct vfsmount *afs_mntpt_do_aut /* try and do the mount */ _debug("--- attempting mount %s -o %s ---", devname, options); - mnt = vfs_kern_mount(&afs_fs_type, 0, devname, options); + mnt = vfs_submount(mntpt, &afs_fs_type, devname, options); _debug("--- mount result %p ---", mnt); free_page((unsigned long) devname); --- a/fs/autofs4/waitq.c +++ b/fs/autofs4/waitq.c @@ -431,8 +431,8 @@ int autofs4_wait(struct autofs_sb_info * memcpy(&wq->name, &qstr, sizeof(struct qstr)); wq->dev = autofs4_get_dev(sbi); wq->ino = autofs4_get_ino(sbi); - wq->uid = current_real_cred()->uid; - wq->gid = current_real_cred()->gid; + wq->uid = current_cred()->uid; + wq->gid = current_cred()->gid; wq->pid = pid; wq->tgid = tgid; wq->status = -EINTR; /* Status return if interrupted */ --- a/fs/cifs/cifs_dfs_ref.c +++ b/fs/cifs/cifs_dfs_ref.c @@ -245,7 +245,8 @@ compose_mount_options_err: * @fullpath: full path in UNC format * @ref: server's referral */ -static struct vfsmount *cifs_dfs_do_refmount(struct cifs_sb_info *cifs_sb, +static struct vfsmount *cifs_dfs_do_refmount(struct dentry *mntpt, + struct cifs_sb_info *cifs_sb, const char *fullpath, const struct dfs_info3_param *ref) { struct vfsmount *mnt; @@ -259,7 +260,7 @@ static struct vfsmount *cifs_dfs_do_refm if (IS_ERR(mountdata)) return (struct vfsmount *)mountdata; - mnt = vfs_kern_mount(&cifs_fs_type, 0, devname, mountdata); + mnt = vfs_submount(mntpt, &cifs_fs_type, devname, mountdata); kfree(mountdata); kfree(devname); return mnt; @@ -334,7 +335,7 @@ static struct vfsmount *cifs_dfs_do_auto mnt = ERR_PTR(-EINVAL); break; } - mnt = cifs_dfs_do_refmount(cifs_sb, + mnt = cifs_dfs_do_refmount(mntpt, cifs_sb, full_path, referrals + i); cifs_dbg(FYI, "%s: cifs_dfs_do_refmount:%s , mnt:%p\n", __func__, referrals[i].node_name, mnt); --- a/fs/debugfs/inode.c +++ b/fs/debugfs/inode.c @@ -187,9 +187,9 @@ static const struct super_operations deb static struct vfsmount *debugfs_automount(struct path *path) { - struct vfsmount *(*f)(void *); - f = (struct vfsmount *(*)(void *))path->dentry->d_fsdata; - return f(d_inode(path->dentry)->i_private); + debugfs_automount_t f; + f = (debugfs_automount_t)path->dentry->d_fsdata; + return f(path->dentry, d_inode(path->dentry)->i_private); } static const struct dentry_operations debugfs_dops = { @@ -504,7 +504,7 @@ EXPORT_SYMBOL_GPL(debugfs_create_dir); */ struct dentry *debugfs_create_automount(const char *name, struct dentry *parent, - struct vfsmount *(*f)(void *), + debugfs_automount_t f, void *data) { struct dentry *dentry = start_creating(name, parent); --- a/fs/namei.c +++ b/fs/namei.c @@ -1100,7 +1100,6 @@ static int follow_automount(struct path bool *need_mntput) { struct vfsmount *mnt; - const struct cred *old_cred; int err; if (!path->dentry->d_op || !path->dentry->d_op->d_automount) @@ -1129,9 +1128,7 @@ static int follow_automount(struct path if (nd->total_link_count >= 40) return -ELOOP; - old_cred = override_creds(&init_cred); mnt = path->dentry->d_op->d_automount(path); - revert_creds(old_cred); if (IS_ERR(mnt)) { /* * The filesystem is allowed to return -EISDIR here to indicate --- a/fs/namespace.c +++ b/fs/namespace.c @@ -995,6 +995,21 @@ vfs_kern_mount(struct file_system_type * } EXPORT_SYMBOL_GPL(vfs_kern_mount); +struct vfsmount * +vfs_submount(const struct dentry *mountpoint, struct file_system_type *type, + const char *name, void *data) +{ + /* Until it is worked out how to pass the user namespace + * through from the parent mount to the submount don't support + * unprivileged mounts with submounts. + */ + if (mountpoint->d_sb->s_user_ns != &init_user_ns) + return ERR_PTR(-EPERM); + + return vfs_kern_mount(type, MS_SUBMOUNT, name, data); +} +EXPORT_SYMBOL_GPL(vfs_submount); + static struct mount *clone_mnt(struct mount *old, struct dentry *root, int flag) { @@ -2779,7 +2794,7 @@ long do_mount(const char *dev_name, cons flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE | MS_BORN | MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT | - MS_STRICTATIME | MS_NOREMOTELOCK); + MS_STRICTATIME | MS_NOREMOTELOCK | MS_SUBMOUNT); if (flags & MS_REMOUNT) retval = do_remount(&path, flags & ~MS_REMOUNT, mnt_flags, --- a/fs/nfs/namespace.c +++ b/fs/nfs/namespace.c @@ -226,7 +226,7 @@ static struct vfsmount *nfs_do_clone_mou const char *devname, struct nfs_clone_mount *mountdata) { - return vfs_kern_mount(&nfs_xdev_fs_type, 0, devname, mountdata); + return vfs_submount(mountdata->dentry, &nfs_xdev_fs_type, devname, mountdata); } /** --- a/fs/nfs/nfs4namespace.c +++ b/fs/nfs/nfs4namespace.c @@ -279,7 +279,7 @@ static struct vfsmount *try_location(str mountdata->hostname, mountdata->mnt_path); - mnt = vfs_kern_mount(&nfs4_referral_fs_type, 0, page, mountdata); + mnt = vfs_submount(mountdata->dentry, &nfs4_referral_fs_type, page, mountdata); if (!IS_ERR(mnt)) break; } --- a/fs/super.c +++ b/fs/super.c @@ -470,7 +470,7 @@ struct super_block *sget_userns(struct f struct super_block *old; int err; - if (!(flags & MS_KERNMOUNT) && + if (!(flags & (MS_KERNMOUNT|MS_SUBMOUNT)) && !(type->fs_flags & FS_USERNS_MOUNT) && !capable(CAP_SYS_ADMIN)) return ERR_PTR(-EPERM); @@ -500,7 +500,7 @@ retry: } if (!s) { spin_unlock(&sb_lock); - s = alloc_super(type, flags, user_ns); + s = alloc_super(type, (flags & ~MS_SUBMOUNT), user_ns); if (!s) return ERR_PTR(-ENOMEM); goto retry; @@ -541,8 +541,15 @@ struct super_block *sget(struct file_sys { struct user_namespace *user_ns = current_user_ns(); + /* We don't yet pass the user namespace of the parent + * mount through to here so always use &init_user_ns + * until that changes. + */ + if (flags & MS_SUBMOUNT) + user_ns = &init_user_ns; + /* Ensure the requestor has permissions over the target filesystem */ - if (!(flags & MS_KERNMOUNT) && !ns_capable(user_ns, CAP_SYS_ADMIN)) + if (!(flags & (MS_KERNMOUNT|MS_SUBMOUNT)) && !ns_capable(user_ns, CAP_SYS_ADMIN)) return ERR_PTR(-EPERM); return sget_userns(type, test, set, flags, user_ns, data); --- a/include/linux/debugfs.h +++ b/include/linux/debugfs.h @@ -96,9 +96,10 @@ struct dentry *debugfs_create_dir(const struct dentry *debugfs_create_symlink(const char *name, struct dentry *parent, const char *dest); +typedef struct vfsmount *(*debugfs_automount_t)(struct dentry *, void *); struct dentry *debugfs_create_automount(const char *name, struct dentry *parent, - struct vfsmount *(*f)(void *), + debugfs_automount_t f, void *data); void debugfs_remove(struct dentry *dentry); --- a/include/linux/mount.h +++ b/include/linux/mount.h @@ -90,6 +90,9 @@ struct file_system_type; extern struct vfsmount *vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void *data); +extern struct vfsmount *vfs_submount(const struct dentry *mountpoint, + struct file_system_type *type, + const char *name, void *data); extern void mnt_set_expiry(struct vfsmount *mnt, struct list_head *expiry_list); extern void mark_mounts_for_expiry(struct list_head *mounts); --- a/include/uapi/linux/fs.h +++ b/include/uapi/linux/fs.h @@ -132,6 +132,7 @@ struct inodes_stat_t { #define MS_LAZYTIME (1<<25) /* Update the on-disk [acm]times lazily */ /* These sb flags are internal to the kernel */ +#define MS_SUBMOUNT (1<<26) #define MS_NOREMOTELOCK (1<<27) #define MS_NOSEC (1<<28) #define MS_BORN (1<<29) --- a/kernel/trace/trace.c +++ b/kernel/trace/trace.c @@ -7241,7 +7241,7 @@ init_tracer_tracefs(struct trace_array * ftrace_init_tracefs(tr, d_tracer); } -static struct vfsmount *trace_automount(void *ingore) +static struct vfsmount *trace_automount(struct dentry *mntpt, void *ingore) { struct vfsmount *mnt; struct file_system_type *type; @@ -7254,7 +7254,7 @@ static struct vfsmount *trace_automount( type = get_fs_type("tracefs"); if (!type) return NULL; - mnt = vfs_kern_mount(type, 0, "tracefs", NULL); + mnt = vfs_submount(mntpt, type, "tracefs", NULL); put_filesystem(type); if (IS_ERR(mnt)) return NULL; Patches currently in stable-queue which might be from ebiederm@xxxxxxxxxxxx are queue-4.9/mnt-tuck-mounts-under-others-instead-of-creating-shadow-side-mounts.patch queue-4.9/fs-better-permission-checking-for-submounts.patch