Message-Id: <20210305145535.f5e41f54290f91968687f474@virtuozzo.com>
Date:   Fri, 5 Mar 2021 14:55:35 +0300
From:   Alexander Mikhalitsyn <alexander.mikhalitsyn@...tuozzo.com>
To:     Ian Kent <raven@...maw.net>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>, autofs@...r.kernel.org,
        linux-kernel@...r.kernel.org, Miklos Szeredi <mszeredi@...hat.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Ross Zwisler <zwisler@...gle.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Eric Biggers <ebiggers@...gle.com>,
        Mattias Nissler <mnissler@...omium.org>,
        linux-fsdevel@...r.kernel.org, alexander@...alicyn.com
Subject: Re: [RFC PATCH] autofs: find_autofs_mount overmounted parent
 support

On Fri, 05 Mar 2021 18:10:02 +0800
Ian Kent <raven@...maw.net> wrote:

> On Thu, 2021-03-04 at 13:11 +0300, Alexander Mikhalitsyn wrote:
> > On Thu, 04 Mar 2021 14:54:11 +0800
> > Ian Kent <raven@...maw.net> wrote:
> > 
> > > On Wed, 2021-03-03 at 18:28 +0300, Alexander Mikhalitsyn wrote:
> > > > It was discovered that find_autofs_mount() function
> > > > in autofs not support cases when autofs mount
> > > > parent is overmounted. In this case this function will
> > > > always return -ENOENT.
> > > 
> > > Ok, I get this shouldn't happen.
> > > 
> > > > Real-life reproducer is fairly simple.
> > > > Consider the following mounts on root mntns:
> > > > --
> > > > 35 24 0:36 / /proc/sys/fs/binfmt_misc ... shared:16 - autofs
> > > > systemd-
> > > > 1 ...
> > > > 654 35 0:57 / /proc/sys/fs/binfmt_misc ... shared:322 -
> > > > binfmt_misc
> > > > ...
> > > > --
> > > > and some process which calls ioctl(AUTOFS_DEV_IOCTL_OPENMOUNT)
> > > > $ unshare -m -p --fork --mount-proc ./process-bin
> > > > 
> > > > Due to "mount-proc" /proc will be overmounted and
> > > > ioctl() will fail with -ENOENT
> > > 
> > > I think I need a better explanation ...
> > 
> > Thank you for the quick reply, Ian.
> > I'm sorry If my patch description was not sufficiently clear and
> > detailed.
> > 
> > That problem connected with CRIU (Checkpoint-Restore in Userspace)
> > project.
> > In CRIU we have support of autofs mounts C/R. To acheive that we need
> > to use
> > ioctl's from /dev/autofs to get data about mounts, restore mount as
> > catatonic
> > (if needed), change pipe fd and so on. But the problem is that during
> > CRIU
> > dump we may meet situation when VFS subtree where autofs mount
> > present was
> > overmounted as whole.
> > 
> > Simpliest example is /proc/sys/fs/binfmt_misc. This mount present on
> > most
> > GNU/Linux distributions by default. For instance on my Fedora 33:
> 
> Yes, I don't know why systemd uses this direct mount, there must
> have been a reason for it.
> 
> > 
> > trigger automount of binfmt_misc
> > $ ls /proc/sys/fs/binfmt_misc
> > 
> > $ cat /proc/1/mountinfo | grep binfmt
> > 35 24 0:36 / /proc/sys/fs/binfmt_misc rw,relatime shared:16 - autofs
> > systemd-1 rw,...,direct,pipe_ino=223
> > 632 35 0:56 / /proc/sys/fs/binfmt_misc rw,...,relatime shared:315 -
> > binfmt_misc binfmt_misc rw
> 
> Yes, I think this looks normal.
> 
> > 
> > $ sudo unshare -m -p --fork --mount-proc sh
> > # cat /proc/self/mountinfo | grep "/proc"
> > 828 809 0:23 / /proc rw,nosuid,nodev,noexec,relatime - proc proc rw
> > 829 828 0:36 / /proc/sys/fs/binfmt_misc rw,relatime - autofs systemd-
> > 1 rw,...,direct,pipe_ino=223
> > 943 829 0:56 / /proc/sys/fs/binfmt_misc rw,...,relatime - binfmt_misc
> > binfmt_misc rw
> > 949 828 0:57 / /proc rw...,relatime - proc proc rw
> 
> Isn't this screwed up, /proc is on top of the binfmt_misc mount ...
> 
> Is this what's seen from the root namespace?

No-no, after issuing
$ sudo unshare -m -p --fork --mount-proc sh

we enter to the pid+mount namespace and:

# cat /proc/self/mountinfo | grep "/proc"

So, it's picture from inside namespaces.

> 
> > 
> > As we can see now autofs mount /proc/sys/fs/binfmt_misc is
> > inaccessible.
> > If we do something like:
> > 
> > struct autofs_dev_ioctl *param;
> > param = malloc(...);
> > devfd = open("/dev/autofs", O_RDONLY);
> > init_autofs_dev_ioctl(param);
> > param->size = size;
> > strcpy(param->path, "/proc/sys/fs/binfmt_misc");
> > param->openmount.devid = 36;
> > err = ioctl(devfd, AUTOFS_DEV_IOCTL_OPENMOUNT, param)
> > 
> > now we get err = -ENOENT.
> 
> Maybe that should be EINVAL, not sure about cases though.

in current version -ENOENT is returned in this particular case

> 
> > 
> > > What's being said here?
> > > 
> > > For a start your talking about direct mounts, I'm pretty sure this
> > > use case can't occur with indirect mounts in the sense that the
> > > indirect mount base should/must never be over mounted and IIRC that
> > > base can't be /proc (but maybe that's just mounts inside proc ...),
> > > can't remember now but from a common sense POV an indirect mount
> > > won't/can't be on /proc.
> > > 
> > > And why is this ioctl be called?
> > 
> > We call this ioctl during criu dump stage to open fd from autofs
> > mount dentry. This fd is used later to call
> > ioctl(AUTOFS_IOC_CATATONIC)
> > (we do that on criu dump if we see that control process of autofs
> > mount
> > is dead or pipe is dead).
> 
> Right so your usage "is" the way it's intended, ;)

That's good! ;)

> 
> > 
> > > If the mount is over mounted should that prevent expiration of the
> > > over mounted /proc anyway, so maybe the return is correct ... or
> > > not ...
> > 
> > I agree that case with overmounted subtree with autofs mount is weird
> > case.
> > But it may be easily created by user and we in CRIU try to handle
> > that.
> 
> I'm not yet ready to make a call on how I think this this should
> be done.
> 
> Since you seem to be clear on what this should be used for I'll
> need to look more closely at the patch.
> 
> But, at first glance, it looked like it would break the existing
> function of the ioctl.
> 
> Could you explain how the patch works, in particular why it doesn't
> break the existing functionality.

Sure. With pleasure. Idea of patch is naive:
1. find_autofs_mount() function called only from syscall context, so,
we always can determine current mount namespace of caller.
So, I've introduced

> > > > + int lookup_mount_path(struct mnt_namespace *ns,
> > > > +		      struct path *res,
> > > > +		      int test(const struct path *mnt, void *data),
> > > > +		      void *data)

lookup_mount_path() helper function, which allows to traverse mounts list of
mount namespace and find proper autofs mount by user-provided helper test().

2. Helper function is fairly simple:
a) it checks that mount is autofs mount (by magic number on superblock)
b) it calculates full path to mount point of each mount in mount namespace
and compare it with path which user was provided to the ioctl(AUTOFS_DEV_IOCTL_OPENMOUNT)
parameters.

Problem here is case when user provided relative path in ioctl parameters
(struct autofs_dev_ioctl). In this case we may fail to resolve user provided path to
struct path. For instance:

# cat /proc/self/mountinfo | grep "/proc"
828 809 0:23 / /proc rw,nosuid,nodev,noexec,relatime - proc proc rw
829 828 0:36 / /proc/sys/fs/binfmt_misc rw,relatime - autofs systemd-
1 rw,...,direct,pipe_ino=223
943 829 0:56 / /proc/sys/fs/binfmt_misc rw,...,relatime - binfmt_misc
binfmt_misc rw
949 828 0:57 / /proc rw...,relatime - proc proc rw

in this case 
kern_path("/proc/sys/fs/binfmt_misc", LOOKUP_MOUNTPOINT, &path) == -ENOENT

To overcome this issue, if kern_path() failed with -ENOENT
AND user-provided mount path looks like fullpath (starts from /)
we just try to find autofs mount in mounts list just by searching
autofs mounts in mounts list with mount point path equal to user-provided
path. This covers our problem case.

This patch is fully compatible with old behaviour - if parent mounts of
autofs mount is not overmounted - then
kern_path("/proc/sys/fs/binfmt_misc", LOOKUP_MOUNTPOINT, &path)
will not fail, and we also easily find needed autofs mount in mounts list
of caller mount namespace.

> 
> Long ago I'm pretty sure I continued to follow up but IIRC that
> went away and was replaced by a single follow_up(), but since
> the changes didn't break the existing function of autofs I
> didn't pay that much attention to them, I'll need to look at
> that too. Not only that, the namespace code has moved a long
> way too however there's still little attention given to
> sanitizing the mounts in the new namespace by anything that I'm
> aware of that uses the feature. TBH I'm not sure why I don't
> see a lot more problems of that nature.
> 
> I have to wonder if what's needed is attention to the follow up
> but that /proc covering the earlier mounts is a bit of a concern.
> 
> > 
> > > I get that the mount namespaces should be independent and
> > > intuitively
> > > this is a bug but what is the actual use and expected result.
> > > 
> > > But anyway, aren't you saying that the VFS path walk isn't handling
> > > mount namespaces properly or are you saying that a process outside
> > > this new mount namespace becomes broken because of it?
> > 
> > No-no, it's only about opening autofs mount by device id + path.
> 
> That's right, specifically getting a file handle to a covered autofs
> mount for things like bringing it back to life etc. But that silently
> implies the same mount namespace.
> 
> Let me look at the patch and think about it a bit.
> I'll probably need to run some tests too.
> I am a little busy right now so it may take a bit of time.
> 
> Ian 

Thank you very much for your attention to the patch and comments.

Regards,
Alex

> > 
> > > Either way the solution looks more complicated than I'd expect so
> > > some explanation along these lines would be good.
> > > 
> > > Ian
> > 
> > Regards,
> > Alex
> > 
> > > > Cc: Matthew Wilcox <willy@...radead.org>
> > > > Cc: Al Viro <viro@...iv.linux.org.uk>
> > > > Cc: Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
> > > > Cc: Kirill Tkhai <ktkhai@...tuozzo.com>
> > > > Cc: autofs@...r.kernel.org
> > > > Cc: linux-kernel@...r.kernel.org
> > > > Signed-off-by: Alexander Mikhalitsyn <
> > > > alexander.mikhalitsyn@...tuozzo.com>
> > > > ---
> > > >  fs/autofs/dev-ioctl.c | 127
> > > > +++++++++++++++++++++++++++++++++++++---
> > > > --
> > > >  fs/namespace.c        |  44 +++++++++++++++
> > > >  include/linux/mount.h |   5 ++
> > > >  3 files changed, 162 insertions(+), 14 deletions(-)
> > > > 
> > > > diff --git a/fs/autofs/dev-ioctl.c b/fs/autofs/dev-ioctl.c
> > > > index 5bf781ea6d67..55edd3eba8ce 100644
> > > > --- a/fs/autofs/dev-ioctl.c
> > > > +++ b/fs/autofs/dev-ioctl.c
> > > > @@ -10,6 +10,7 @@
> > > >  #include <linux/fdtable.h>
> > > >  #include <linux/magic.h>
> > > >  #include <linux/nospec.h>
> > > > +#include <linux/nsproxy.h>
> > > >  
> > > >  #include "autofs_i.h"
> > > >  
> > > > @@ -179,32 +180,130 @@ static int
> > > > autofs_dev_ioctl_protosubver(struct
> > > > file *fp,
> > > >  	return 0;
> > > >  }
> > > >  
> > > > +struct filter_autofs_data {
> > > > +	char *pathbuf;
> > > > +	const char *fpathname;
> > > > +	int (*test)(const struct path *path, void *data);
> > > > +	void *data;
> > > > +};
> > > > +
> > > > +static int filter_autofs(const struct path *path, void *p)
> > > > +{
> > > > +	struct filter_autofs_data *data = p;
> > > > +	char *name;
> > > > +	int err;
> > > > +
> > > > +	if (path->mnt->mnt_sb->s_magic != AUTOFS_SUPER_MAGIC)
> > > > +		return 0;
> > > > +
> > > > +	name = d_path(path, data->pathbuf, PATH_MAX);
> > > > +	if (IS_ERR(name)) {
> > > > +		err = PTR_ERR(name);
> > > > +		pr_err("d_path failed, errno %d\n", err);
> > > > +		return 0;
> > > > +	}
> > > > +
> > > > +	if (strncmp(data->fpathname, name, PATH_MAX))
> > > > +		return 0;
> > > > +
> > > > +	if (!data->test(path, data->data))
> > > > +		return 0;
> > > > +
> > > > +	return 1;
> > > > +}
> > > > +
> > > >  /* Find the topmost mount satisfying test() */
> > > >  static int find_autofs_mount(const char *pathname,
> > > >  			     struct path *res,
> > > >  			     int test(const struct path *path, void
> > > > *data),
> > > >  			     void *data)
> > > >  {
> > > > -	struct path path;
> > > > +	struct filter_autofs_data mdata = {
> > > > +		.pathbuf = NULL,
> > > > +		.test = test,
> > > > +		.data = data,
> > > > +	};
> > > > +	struct mnt_namespace *mnt_ns = current->nsproxy->mnt_ns;
> > > > +	struct path path = {};
> > > > +	char *fpathbuf = NULL;
> > > >  	int err;
> > > >  
> > > > +	/*
> > > > +	 * In most cases user will provide full path to autofs mount
> > > > point
> > > > +	 * as it is in /proc/X/mountinfo. But if not, then we need to
> > > > +	 * open provided relative path and calculate full path.
> > > > +	 * It will not work in case when parent mount of autofs mount
> > > > +	 * is overmounted:
> > > > +	 * cd /root
> > > > +	 * ./autofs_mount /root/autofs_yard/mnt
> > > > +	 * mount -t tmpfs tmpfs /root/autofs_yard/mnt
> > > > +	 * mount -t tmpfs tmpfs /root/autofs_yard
> > > > +	 * ./call_ioctl /root/autofs_yard/mnt <- all fine here because
> > > > we
> > > > +	 * 					 have full path and
> > > > don't
> > > > +	 * 					 need to call
> > > > kern_path()
> > > > +	 * 					 and d_path()
> > > > +	 * ./call_ioctl autofs_yard/mnt <- will fail because
> > > > kern_path()
> > > > +	 * 				   can't lookup
> > > > /root/autofs_yard/mnt
> > > > +	 * 				   (/root/autofs_yard
> > > > directory is
> > > > +	 * 				    empty)
> > > > +	 *
> > > > +	 * TO DISCUSS: we can write special algorithm for relative path
> > > > case
> > > > +	 * by getting cwd path combining it with relative path from
> > > > user. But
> > > > +	 * is it worth it? User also may use paths with symlinks in
> > > > components
> > > > +	 * of path.
> > > > +	 *
> > > > +	 */
> > > >  	err = kern_path(pathname, LOOKUP_MOUNTPOINT, &path);
> > > > -	if (err)
> > > > -		return err;
> > > > -	err = -ENOENT;
> > > > -	while (path.dentry == path.mnt->mnt_root) {
> > > > -		if (path.dentry->d_sb->s_magic == AUTOFS_SUPER_MAGIC) {
> > > > -			if (test(&path, data)) {
> > > > -				path_get(&path);
> > > > -				*res = path;
> > > > -				err = 0;
> > > > -				break;
> > > > -			}
> > > > +	if (err) {
> > > > +		if (pathname[0] == '/') {
> > > > +			/*
> > > > +			 * pathname looks like full path let's try to
> > > > use it
> > > > +			 * as it is when searching autofs mount
> > > > +			 */
> > > > +			mdata.fpathname = pathname;
> > > > +			err = 0;
> > > > +			pr_debug("kern_path failed on %s, errno %d.
> > > > Will use path as it is to search mount\n",
> > > > +				 pathname, err);
> > > > +		} else {
> > > > +			pr_err("kern_path failed on %s, errno %d\n",
> > > > +			       pathname, err);
> > > > +			return err;
> > > > +		}
> > > > +	} else {
> > > > +		pr_debug("find_autofs_mount: let's resolve full path
> > > > %s\n",
> > > > +			 pathname);
> > > > +
> > > > +		fpathbuf = kmalloc(PATH_MAX, GFP_KERNEL);
> > > > +		if (!fpathbuf) {
> > > > +			err = -ENOMEM;
> > > > +			goto err;
> > > > +		}
> > > > +
> > > > +		/*
> > > > +		 * We have pathname from user but it may be relative,
> > > > we need to
> > > > +		 * have full path because we want to compare it with
> > > > mountpoints
> > > > +		 * paths later.
> > > > +		 */
> > > > +		mdata.fpathname = d_path(&path, fpathbuf, PATH_MAX);
> > > > +		if (IS_ERR(mdata.fpathname)) {
> > > > +			err = PTR_ERR(mdata.fpathname);
> > > > +			pr_err("d_path failed, errno %d\n", err);
> > > > +			goto err;
> > > >  		}
> > > > -		if (!follow_up(&path))
> > > > -			break;
> > > >  	}
> > > > +
> > > > +	mdata.pathbuf = kmalloc(PATH_MAX, GFP_KERNEL);
> > > > +	if (!mdata.pathbuf) {
> > > > +		err = -ENOMEM;
> > > > +		goto err;
> > > > +	}
> > > > +
> > > > +	err = lookup_mount_path(mnt_ns, res, filter_autofs, &mdata);
> > > > +
> > > > +err:
> > > >  	path_put(&path);
> > > > +	kfree(fpathbuf);
> > > > +	kfree(mdata.pathbuf);
> > > >  	return err;
> > > >  }
> > > >  
> > > > diff --git a/fs/namespace.c b/fs/namespace.c
> > > > index 56bb5a5fdc0d..e1d006dbdfe2 100644
> > > > --- a/fs/namespace.c
> > > > +++ b/fs/namespace.c
> > > > @@ -1367,6 +1367,50 @@ void mnt_cursor_del(struct mnt_namespace
> > > > *ns,
> > > > struct mount *cursor)
> > > >  }
> > > >  #endif  /* CONFIG_PROC_FS */
> > > >  
> > > > +/**
> > > > + * lookup_mount_path - traverse all mounts in mount namespace
> > > > + *                     and filter using test() probe callback
> > > > + * As a result struct path will be provided.
> > > > + * @ns: root of mount tree
> > > > + * @res: struct path pointer where resulting path will be
> > > > written
> > > > + * @test: filter callback
> > > > + * @data: will be provided as argument to test() callback
> > > > + *
> > > > + */
> > > > +int lookup_mount_path(struct mnt_namespace *ns,
> > > > +		      struct path *res,
> > > > +		      int test(const struct path *mnt, void *data),
> > > > +		      void *data)
> > > > +{
> > > > +	struct mount *mnt;
> > > > +	int err = -ENOENT;
> > > > +
> > > > +	down_read(&namespace_sem);
> > > > +	lock_ns_list(ns);
> > > > +	list_for_each_entry(mnt, &ns->list, mnt_list) {
> > > > +		struct path tmppath;
> > > > +
> > > > +		if (mnt_is_cursor(mnt))
> > > > +			continue;
> > > > +
> > > > +		tmppath.dentry = mnt->mnt.mnt_root;
> > > > +		tmppath.mnt = &mnt->mnt;
> > > > +
> > > > +		if (test(&tmppath, data)) {
> > > > +			path_get(&tmppath);
> > > > +			*res = tmppath;
> > > > +			err = 0;
> > > > +			break;
> > > > +		}
> > > > +	}
> > > > +	unlock_ns_list(ns);
> > > > +	up_read(&namespace_sem);
> > > > +
> > > > +	return err;
> > > > +}
> > > > +
> > > > +EXPORT_SYMBOL(lookup_mount_path);
> > > > +
> > > >  /**
> > > >   * may_umount_tree - check if a mount tree is busy
> > > >   * @mnt: root of mount tree
> > > > diff --git a/include/linux/mount.h b/include/linux/mount.h
> > > > index 5d92a7e1a742..a79e6392e38e 100644
> > > > --- a/include/linux/mount.h
> > > > +++ b/include/linux/mount.h
> > > > @@ -118,6 +118,11 @@ extern unsigned int sysctl_mount_max;
> > > >  
> > > >  extern bool path_is_mountpoint(const struct path *path);
> > > >  
> > > > +extern int lookup_mount_path(struct mnt_namespace *ns,
> > > > +			     struct path *res,
> > > > +			     int test(const struct path *mnt, void
> > > > *data),
> > > > +			     void *data);
> > > > +
> > > >  extern void kern_unmount_array(struct vfsmount *mnt[], unsigned
> > > > int
> > > > num);
> > > >  
> > > >  #endif /* _LINUX_MOUNT_H */
> 

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