diff mbox series

[f2fs-dev,v2,4/6] fs: Establish locking order for unrelated directories

Message ID 20230601105830.13168-4-jack@suse.cz (mailing list archive)
State New
Headers show
Series fs: Fix directory corruption when moving directories | expand

Commit Message

Jan Kara June 1, 2023, 10:58 a.m. UTC
Currently the locking order of inode locks for directories that are not
in ancestor relationship is not defined because all operations that
needed to lock two directories like this were serialized by
sb->s_vfs_rename_mutex. However some filesystems need to lock two
subdirectories for RENAME_EXCHANGE operations and for this we need the
locking order established even for two tree-unrelated directories.
Provide a helper function lock_two_inodes() that establishes lock
ordering for any two inodes and use it in lock_two_directories().

CC: stable@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/inode.c    | 42 ++++++++++++++++++++++++++++++++++++++++++
 fs/internal.h |  2 ++
 fs/namei.c    |  4 ++--
 3 files changed, 46 insertions(+), 2 deletions(-)

Comments

Christian Brauner June 1, 2023, 1:58 p.m. UTC | #1
On Thu, Jun 01, 2023 at 12:58:24PM +0200, Jan Kara wrote:
> Currently the locking order of inode locks for directories that are not
> in ancestor relationship is not defined because all operations that
> needed to lock two directories like this were serialized by
> sb->s_vfs_rename_mutex. However some filesystems need to lock two
> subdirectories for RENAME_EXCHANGE operations and for this we need the
> locking order established even for two tree-unrelated directories.
> Provide a helper function lock_two_inodes() that establishes lock
> ordering for any two inodes and use it in lock_two_directories().
> 
> CC: stable@vger.kernel.org
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/inode.c    | 42 ++++++++++++++++++++++++++++++++++++++++++
>  fs/internal.h |  2 ++
>  fs/namei.c    |  4 ++--
>  3 files changed, 46 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/inode.c b/fs/inode.c
> index 577799b7855f..4000ab08bbc0 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -1103,6 +1103,48 @@ void discard_new_inode(struct inode *inode)
>  }
>  EXPORT_SYMBOL(discard_new_inode);
>  
> +/**
> + * lock_two_inodes - lock two inodes (may be regular files but also dirs)
> + *
> + * Lock any non-NULL argument. The caller must make sure that if he is passing
> + * in two directories, one is not ancestor of the other.  Zero, one or two
> + * objects may be locked by this function.
> + *
> + * @inode1: first inode to lock
> + * @inode2: second inode to lock
> + * @subclass1: inode lock subclass for the first lock obtained
> + * @subclass2: inode lock subclass for the second lock obtained
> + */
> +void lock_two_inodes(struct inode *inode1, struct inode *inode2,
> +		     unsigned subclass1, unsigned subclass2)
> +{
> +	if (!inode1 || !inode2)

I think you forgot the opening bracket...
I can just fix this up for you though.

> +		/*
> +		 * Make sure @subclass1 will be used for the acquired lock.
> +		 * This is not strictly necessary (no current caller cares) but
> +		 * let's keep things consistent.
> +		 */
> +		if (!inode1)
> +			swap(inode1, inode2);
> +		goto lock;
> +	}
Jan Kara June 1, 2023, 3:24 p.m. UTC | #2
On Thu 01-06-23 15:58:58, Christian Brauner wrote:
> On Thu, Jun 01, 2023 at 12:58:24PM +0200, Jan Kara wrote:
> > Currently the locking order of inode locks for directories that are not
> > in ancestor relationship is not defined because all operations that
> > needed to lock two directories like this were serialized by
> > sb->s_vfs_rename_mutex. However some filesystems need to lock two
> > subdirectories for RENAME_EXCHANGE operations and for this we need the
> > locking order established even for two tree-unrelated directories.
> > Provide a helper function lock_two_inodes() that establishes lock
> > ordering for any two inodes and use it in lock_two_directories().
> > 
> > CC: stable@vger.kernel.org
> > Signed-off-by: Jan Kara <jack@suse.cz>
> > ---
> >  fs/inode.c    | 42 ++++++++++++++++++++++++++++++++++++++++++
> >  fs/internal.h |  2 ++
> >  fs/namei.c    |  4 ++--
> >  3 files changed, 46 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/inode.c b/fs/inode.c
> > index 577799b7855f..4000ab08bbc0 100644
> > --- a/fs/inode.c
> > +++ b/fs/inode.c
> > @@ -1103,6 +1103,48 @@ void discard_new_inode(struct inode *inode)
> >  }
> >  EXPORT_SYMBOL(discard_new_inode);
> >  
> > +/**
> > + * lock_two_inodes - lock two inodes (may be regular files but also dirs)
> > + *
> > + * Lock any non-NULL argument. The caller must make sure that if he is passing
> > + * in two directories, one is not ancestor of the other.  Zero, one or two
> > + * objects may be locked by this function.
> > + *
> > + * @inode1: first inode to lock
> > + * @inode2: second inode to lock
> > + * @subclass1: inode lock subclass for the first lock obtained
> > + * @subclass2: inode lock subclass for the second lock obtained
> > + */
> > +void lock_two_inodes(struct inode *inode1, struct inode *inode2,
> > +		     unsigned subclass1, unsigned subclass2)
> > +{
> > +	if (!inode1 || !inode2)
> 
> I think you forgot the opening bracket...
> I can just fix this up for you though.

Oh, yes. Apparently I forgot to rerun git-format-patch after fixing up this
bit. I'm sorry for that. The patch series has survived full ext4 fstests
run on my end.

								Honza
David Laight June 1, 2023, 3:37 p.m. UTC | #3
...
> > > + * Lock any non-NULL argument. The caller must make sure that if he is passing
> > > + * in two directories, one is not ancestor of the other

Not directly relevant to this change but is the 'not an ancestor'
check actually robust?

I found a condition in which the kernel 'pwd' code (which follows
the inode chain) failed to stop at the base of a chroot.

I suspect that the ancestor check would fail the same way.

IIRC the problematic code used unshare() to 'escape' from
a network natespace.
If it was inside a chroot (that wasn't on a mount point) there
ware two copies of the 'chroot /' inode and the match failed.

I might be able to find the test case.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
Christian Brauner June 1, 2023, 3:59 p.m. UTC | #4
On Thu, Jun 01, 2023 at 05:24:49PM +0200, Jan Kara wrote:
> On Thu 01-06-23 15:58:58, Christian Brauner wrote:
> > On Thu, Jun 01, 2023 at 12:58:24PM +0200, Jan Kara wrote:
> > > Currently the locking order of inode locks for directories that are not
> > > in ancestor relationship is not defined because all operations that
> > > needed to lock two directories like this were serialized by
> > > sb->s_vfs_rename_mutex. However some filesystems need to lock two
> > > subdirectories for RENAME_EXCHANGE operations and for this we need the
> > > locking order established even for two tree-unrelated directories.
> > > Provide a helper function lock_two_inodes() that establishes lock
> > > ordering for any two inodes and use it in lock_two_directories().
> > > 
> > > CC: stable@vger.kernel.org
> > > Signed-off-by: Jan Kara <jack@suse.cz>
> > > ---
> > >  fs/inode.c    | 42 ++++++++++++++++++++++++++++++++++++++++++
> > >  fs/internal.h |  2 ++
> > >  fs/namei.c    |  4 ++--
> > >  3 files changed, 46 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/fs/inode.c b/fs/inode.c
> > > index 577799b7855f..4000ab08bbc0 100644
> > > --- a/fs/inode.c
> > > +++ b/fs/inode.c
> > > @@ -1103,6 +1103,48 @@ void discard_new_inode(struct inode *inode)
> > >  }
> > >  EXPORT_SYMBOL(discard_new_inode);
> > >  
> > > +/**
> > > + * lock_two_inodes - lock two inodes (may be regular files but also dirs)
> > > + *
> > > + * Lock any non-NULL argument. The caller must make sure that if he is passing
> > > + * in two directories, one is not ancestor of the other.  Zero, one or two
> > > + * objects may be locked by this function.
> > > + *
> > > + * @inode1: first inode to lock
> > > + * @inode2: second inode to lock
> > > + * @subclass1: inode lock subclass for the first lock obtained
> > > + * @subclass2: inode lock subclass for the second lock obtained
> > > + */
> > > +void lock_two_inodes(struct inode *inode1, struct inode *inode2,
> > > +		     unsigned subclass1, unsigned subclass2)
> > > +{
> > > +	if (!inode1 || !inode2)
> > 
> > I think you forgot the opening bracket...
> > I can just fix this up for you though.
> 
> Oh, yes. Apparently I forgot to rerun git-format-patch after fixing up this
> bit. I'm sorry for that. The patch series has survived full ext4 fstests

No problem at all!
Jan Kara June 1, 2023, 4:13 p.m. UTC | #5
On Thu 01-06-23 15:37:32, David Laight wrote:
> ...
> > > > + * Lock any non-NULL argument. The caller must make sure that if he is passing
> > > > + * in two directories, one is not ancestor of the other
> 
> Not directly relevant to this change but is the 'not an ancestor'
> check actually robust?
> 
> I found a condition in which the kernel 'pwd' code (which follows
> the inode chain) failed to stop at the base of a chroot.
> 
> I suspect that the ancestor check would fail the same way.

Honestly, I'm not sure how this could be the case but I'm not a dcache
expert. d_ancestor() works on dentries and the whole dcache code pretty
much relies on the fact that there always is at most one dentry for any
directory. Also in case we call d_ancestor() from this code, we have the
whole filesystem locked from any other directory moves so the ancestor
relationship of two dirs cannot change (which is different from pwd code
AFAIK). So IMHO no failure is possible in our case.

								Honza

> 
> IIRC the problematic code used unshare() to 'escape' from
> a network natespace.
> If it was inside a chroot (that wasn't on a mount point) there
> ware two copies of the 'chroot /' inode and the match failed.
> 
> I might be able to find the test case.
> 
> 	David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
>
Christian Brauner June 1, 2023, 4:21 p.m. UTC | #6
On Thu, Jun 01, 2023 at 06:13:53PM +0200, Jan Kara wrote:
> On Thu 01-06-23 15:37:32, David Laight wrote:
> > ...
> > > > > + * Lock any non-NULL argument. The caller must make sure that if he is passing
> > > > > + * in two directories, one is not ancestor of the other
> > 
> > Not directly relevant to this change but is the 'not an ancestor'
> > check actually robust?
> > 
> > I found a condition in which the kernel 'pwd' code (which follows
> > the inode chain) failed to stop at the base of a chroot.
> > 
> > I suspect that the ancestor check would fail the same way.
> 
> Honestly, I'm not sure how this could be the case but I'm not a dcache
> expert. d_ancestor() works on dentries and the whole dcache code pretty
> much relies on the fact that there always is at most one dentry for any
> directory. Also in case we call d_ancestor() from this code, we have the
> whole filesystem locked from any other directory moves so the ancestor
> relationship of two dirs cannot change (which is different from pwd code
> AFAIK). So IMHO no failure is possible in our case.

Yes, this is a red herring. What matters is that the tree topology can't
change which is up to the caller to guarantee. And where it's called
we're under s_vfs_rename_mutex. It's also literally mentioned in the
directory locking documentation.
David Laight June 1, 2023, 4:33 p.m. UTC | #7
From: Jan Kara <jack@suse.cz>
> Sent: 01 June 2023 17:14
> 
> On Thu 01-06-23 15:37:32, David Laight wrote:
> > ...
> > > > > + * Lock any non-NULL argument. The caller must make sure that if he is passing
> > > > > + * in two directories, one is not ancestor of the other
> >
> > Not directly relevant to this change but is the 'not an ancestor'
> > check actually robust?
> >
> > I found a condition in which the kernel 'pwd' code (which follows
> > the inode chain) failed to stop at the base of a chroot.
> >
> > I suspect that the ancestor check would fail the same way.
> 
> Honestly, I'm not sure how this could be the case but I'm not a dcache
> expert. d_ancestor() works on dentries and the whole dcache code pretty
> much relies on the fact that there always is at most one dentry for any
> directory. Also in case we call d_ancestor() from this code, we have the
> whole filesystem locked from any other directory moves so the ancestor
> relationship of two dirs cannot change (which is different from pwd code
> AFAIK). So IMHO no failure is possible in our case.

I've found the test program.
This uses readlinkat() to get the full path /proc/self/fd/0.
It should be inside the chroot, but the comparison done
to detect the 'root' fails.

Now maybe any rename that would hit this is invalid
for other reasons.
But something is awry somewhere.

	David

The program below reproduces this when run with stdin
redirected to a file in the current directory.

This sequence is used by 'ip netns exec' so isn't actually
that unusual.

	David

#define _GNU_SOURCE
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <sched.h>

static void print_link(const char *where, int fd)
{
        char buf[256];

        printf("%s: %.*s\n", where, (int)readlinkat(fd, "", buf, sizeof buf), buf);
}

int main(int argc, char **argv)
{
        int link_fd = open("/proc/self/fd/0", O_PATH | O_NOFOLLOW);

        print_link("initial", link_fd);
        if (chroot("."))
                return 1;
        print_link("after chroot", link_fd);
        if (unshare(CLONE_NEWNS))
                return 2;
        print_link("after unshare", link_fd);
        return 0;
}

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
kernel test robot June 2, 2023, 1:36 a.m. UTC | #8
Hi Jan,

kernel test robot noticed the following build errors:

[auto build test ERROR on tytso-ext4/dev]
[also build test ERROR on jaegeuk-f2fs/dev-test jaegeuk-f2fs/dev linus/master v6.4-rc4 next-20230601]
[cannot apply to vfs-idmapping/for-next]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Jan-Kara/ext4-Remove-ext4-locking-of-moved-directory/20230601-225100
base:   https://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4.git dev
patch link:    https://lore.kernel.org/r/20230601105830.13168-4-jack%40suse.cz
patch subject: [PATCH v2 4/6] fs: Establish locking order for unrelated directories
config: um-i386_defconfig (https://download.01.org/0day-ci/archive/20230602/202306020948.TBmCxtVw-lkp@intel.com/config)
compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
reproduce (this is a W=1 build):
        # https://github.com/intel-lab-lkp/linux/commit/234d970a1de0d79e372cc04d6a8112d2aec56c44
        git remote add linux-review https://github.com/intel-lab-lkp/linux
        git fetch --no-tags linux-review Jan-Kara/ext4-Remove-ext4-locking-of-moved-directory/20230601-225100
        git checkout 234d970a1de0d79e372cc04d6a8112d2aec56c44
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=um SUBARCH=i386 olddefconfig
        make W=1 O=build_dir ARCH=um SUBARCH=i386 SHELL=/bin/bash

If you fix the issue, kindly add following tag where applicable
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202306020948.TBmCxtVw-lkp@intel.com/

All error/warnings (new ones prefixed by >>):

   fs/inode.c: In function 'lock_two_inodes':
>> fs/inode.c:1121:9: warning: this 'if' clause does not guard... [-Wmisleading-indentation]
    1121 |         if (!inode1 || !inode2)
         |         ^~
   fs/inode.c:1129:17: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'if'
    1129 |                 goto lock;
         |                 ^~~~
>> fs/inode.c:1129:17: error: label 'lock' used but not defined
   fs/inode.c: At top level:
>> fs/inode.c:1136:9: error: expected identifier or '(' before 'if'
    1136 |         if (S_ISDIR(inode2->i_mode) == S_ISDIR(inode1->i_mode)) {
         |         ^~
>> fs/inode.c:1139:11: error: expected identifier or '(' before 'else'
    1139 |         } else if (!S_ISDIR(inode1->i_mode))
         |           ^~~~
   In file included from include/linux/kernel.h:27,
                    from include/linux/cpumask.h:10,
                    from include/linux/smp.h:13,
                    from include/linux/lockdep.h:14,
                    from include/linux/spinlock.h:63,
                    from include/linux/wait.h:9,
                    from include/linux/wait_bit.h:8,
                    from include/linux/fs.h:6,
                    from fs/inode.c:7:
>> include/linux/minmax.h:167:63: error: expected identifier or '(' before 'while'
     167 |         do { typeof(a) __tmp = (a); (a) = (b); (b) = __tmp; } while (0)
         |                                                               ^~~~~
   fs/inode.c:1140:17: note: in expansion of macro 'swap'
    1140 |                 swap(inode1, inode2);
         |                 ^~~~
>> fs/inode.c:1141:5: error: expected '=', ',', ';', 'asm' or '__attribute__' before ':' token
    1141 | lock:
         |     ^
   fs/inode.c:1144:9: error: expected identifier or '(' before 'if'
    1144 |         if (inode2 && inode2 != inode1)
         |         ^~
>> fs/inode.c:1146:1: error: expected identifier or '(' before '}' token
    1146 | }
         | ^


vim +/lock +1129 fs/inode.c

  1105	
  1106	/**
  1107	 * lock_two_inodes - lock two inodes (may be regular files but also dirs)
  1108	 *
  1109	 * Lock any non-NULL argument. The caller must make sure that if he is passing
  1110	 * in two directories, one is not ancestor of the other.  Zero, one or two
  1111	 * objects may be locked by this function.
  1112	 *
  1113	 * @inode1: first inode to lock
  1114	 * @inode2: second inode to lock
  1115	 * @subclass1: inode lock subclass for the first lock obtained
  1116	 * @subclass2: inode lock subclass for the second lock obtained
  1117	 */
  1118	void lock_two_inodes(struct inode *inode1, struct inode *inode2,
  1119			     unsigned subclass1, unsigned subclass2)
  1120	{
> 1121		if (!inode1 || !inode2)
  1122			/*
  1123			 * Make sure @subclass1 will be used for the acquired lock.
  1124			 * This is not strictly necessary (no current caller cares) but
  1125			 * let's keep things consistent.
  1126			 */
  1127			if (!inode1)
  1128				swap(inode1, inode2);
> 1129			goto lock;
  1130		}
  1131	
  1132		/*
  1133		 * If one object is directory and the other is not, we must make sure
  1134		 * to lock directory first as the other object may be its child.
  1135		 */
> 1136		if (S_ISDIR(inode2->i_mode) == S_ISDIR(inode1->i_mode)) {
  1137			if (inode1 > inode2)
  1138				swap(inode1, inode2);
> 1139		} else if (!S_ISDIR(inode1->i_mode))
  1140			swap(inode1, inode2);
> 1141	lock:
  1142		if (inode1)
  1143			inode_lock_nested(inode1, subclass1);
  1144		if (inode2 && inode2 != inode1)
  1145			inode_lock_nested(inode2, subclass2);
> 1146	}
  1147
Christian Brauner June 2, 2023, 12:34 p.m. UTC | #9
On Thu, Jun 01, 2023 at 04:33:58PM +0000, David Laight wrote:
> From: Jan Kara <jack@suse.cz>
> > Sent: 01 June 2023 17:14
> > 
> > On Thu 01-06-23 15:37:32, David Laight wrote:
> > > ...
> > > > > > + * Lock any non-NULL argument. The caller must make sure that if he is passing
> > > > > > + * in two directories, one is not ancestor of the other
> > >
> > > Not directly relevant to this change but is the 'not an ancestor'
> > > check actually robust?
> > >
> > > I found a condition in which the kernel 'pwd' code (which follows
> > > the inode chain) failed to stop at the base of a chroot.
> > >
> > > I suspect that the ancestor check would fail the same way.
> > 
> > Honestly, I'm not sure how this could be the case but I'm not a dcache
> > expert. d_ancestor() works on dentries and the whole dcache code pretty
> > much relies on the fact that there always is at most one dentry for any
> > directory. Also in case we call d_ancestor() from this code, we have the
> > whole filesystem locked from any other directory moves so the ancestor
> > relationship of two dirs cannot change (which is different from pwd code
> > AFAIK). So IMHO no failure is possible in our case.
> 
> I've found the test program.
> This uses readlinkat() to get the full path /proc/self/fd/0.
> It should be inside the chroot, but the comparison done
> to detect the 'root' fails.

That's intentional and relied-upon behavior. In glibc alone for tty
validation it wants the full link path returned. So any change in this
is an immediate widespread userspace regression.

> 
> Now maybe any rename that would hit this is invalid
> for other reasons.
> But something is awry somewhere.

It really isn't.

> 
> 	David
> 
> The program below reproduces this when run with stdin
> redirected to a file in the current directory.
> 
> This sequence is used by 'ip netns exec' so isn't actually

Fwiw, it doesn't use chroot() at all. 

> that unusual.
> 
> 	David
> 
> #define _GNU_SOURCE
> #include <unistd.h>
> #include <stdio.h>
> #include <fcntl.h>
> #include <sched.h>
> 
> static void print_link(const char *where, int fd)
> {
>         char buf[256];
> 
>         printf("%s: %.*s\n", where, (int)readlinkat(fd, "", buf, sizeof buf), buf);
> }
> 
> int main(int argc, char **argv)
> {
>         int link_fd = open("/proc/self/fd/0", O_PATH | O_NOFOLLOW);
> 
>         print_link("initial", link_fd);
>         if (chroot("."))

chroot(2):

"This call changes an ingredient in the pathname resolution process and
does nothing else. In particular, it is not intended [...] to restrict
filesystem system calls.

[...]

This call does not close open file descriptors, and such file
descriptors may allow access to files outside the chroot tree."

>                 return 1;
>         print_link("after chroot", link_fd);
>         if (unshare(CLONE_NEWNS))
>                 return 2;
>         print_link("after unshare", link_fd);
>         return 0;
> }

But anyway, the code sample you provided is using O_PATH | O_NOFOLLOW to
open magic link, i.e., /proc/<pid>/fd/<nr>. That means whatever the
magic link refers to isn't really reopened. You can create these magic
link references trivially for every path:

        int fd = open("/tmp", 0);

        // create fd referencing magic link
        sprintf(buf, "/proc/self/fd/%d", fd);
        int link_fd = open(buf, O_PATH | O_NOFOLLOW);

In fact, you don't even need magic links for that. You can get the same
behavior with any symlink:

        ln -sf /usr /BLUB
        linkt_fd = open("/BLUB", O_PATH | O_NOFOLLOW);

If you pass such a fd to readlinkat() then d_path() will give you the
full path whether it's accessible in your namespace/chroot/pivot_root()
or not.

Look at __prepend_path() current->fs->root is only used to terminate the
walk for fds that are scopable _beneath_ your chroot:

        mkdir -p /A/B/C
        touch /A/B/C/D

        chroot("/A/B/C");
        int fd = open("/A/B/C/D", 0);
        sprintf(buf, "/proc/self/fd/%d", fd);
        int link_fd = open(buf, O_PATH | O_NOFOLLOW);

In this case, you'll see that after chroot("/A/B/C") it'll print:

        /D

And this actually makes a lot of sense. The fd for /A/B/C/D is scoped
beneath your chroot(). But an fd pointing outside of your chroot is not
scoped by the chroot because you can also very well do:

        fchdir(fd-outside-chroot)

And btw, orderings such as:

        chroot()
        unshare(CLONE_NEWNS)

aren't intuitive. It seems that you're under the impression that the
unshare(CLONE_NEWNS) doesn't have any effect on the chroot() but it
does. Going back to the previous example:

        mkdir -p /A/B/C
        touch /A/B/C/D

        chroot("/A/B/C");
        int fd = open("/A/B/C/D", 0);
        sprintf(buf, "/proc/self/fd/%d", fd);
        int link_fd = open(buf, O_PATH | O_NOFOLLOW);

Compare what gets printed after the chroot() and after
unshare(CLONE_NEWNS). You'll see /D after the chroot() but again the
full path /A/B/C/D after the unshare(). Why?

The reason is that if the mount that you're currently chroot()ed into is
copied as part of the unshare(CLONE_NEWNS) then current->fs->root will
be updated to refer to the copy.

But since this is a copy it means that __prepend_path() doesn't
terminate the walk at /D. That's seemingly counterintuitive but makes
sense if you consider that you were moved into a new mount namespace.
The mount the fd refers to is now inaccessible from your mount namespace
and so the full path is returned again.

Yes, that's not straightforward but heavily relied upon so even if we
could change it to be less surprising it would break the hell out of
everyone.

And most of this doesn't have anything to do with ancestor relationships
per se since this code is able to detect concurrent tree modifications
through rename_lock seqlock iirc. That's a related but different
problem. The effects you're seeing are caused by mount semantics more
than anything else.

And btw about /proc/self/fd/0 specifically... Not verifying an fd
pointing to a pty device in any type of sandbox in the age of containers
is ripe for confusion. Quoting from work I did on glibc years ago:

"It's a common practice among container managers to allocate
 a PTY master/slave pair in the host's mount namespace (the slave having
 a path like "/dev/pty/$X"), bind mount the slave to "/dev/console" in
 the container's mount namespace, and send the slave FD to a process in
 the container. Inside of the container, the slave-end isn't available
 at its original path ("/dev/pts/$X"), since the container mount
 namespace has a separate devpts instance from the host (that path may
 or may not exist in the container; if it does exist, it's not the same
 PTY slave device)."
diff mbox series

Patch

diff --git a/fs/inode.c b/fs/inode.c
index 577799b7855f..4000ab08bbc0 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -1103,6 +1103,48 @@  void discard_new_inode(struct inode *inode)
 }
 EXPORT_SYMBOL(discard_new_inode);
 
+/**
+ * lock_two_inodes - lock two inodes (may be regular files but also dirs)
+ *
+ * Lock any non-NULL argument. The caller must make sure that if he is passing
+ * in two directories, one is not ancestor of the other.  Zero, one or two
+ * objects may be locked by this function.
+ *
+ * @inode1: first inode to lock
+ * @inode2: second inode to lock
+ * @subclass1: inode lock subclass for the first lock obtained
+ * @subclass2: inode lock subclass for the second lock obtained
+ */
+void lock_two_inodes(struct inode *inode1, struct inode *inode2,
+		     unsigned subclass1, unsigned subclass2)
+{
+	if (!inode1 || !inode2)
+		/*
+		 * Make sure @subclass1 will be used for the acquired lock.
+		 * This is not strictly necessary (no current caller cares) but
+		 * let's keep things consistent.
+		 */
+		if (!inode1)
+			swap(inode1, inode2);
+		goto lock;
+	}
+
+	/*
+	 * If one object is directory and the other is not, we must make sure
+	 * to lock directory first as the other object may be its child.
+	 */
+	if (S_ISDIR(inode2->i_mode) == S_ISDIR(inode1->i_mode)) {
+		if (inode1 > inode2)
+			swap(inode1, inode2);
+	} else if (!S_ISDIR(inode1->i_mode))
+		swap(inode1, inode2);
+lock:
+	if (inode1)
+		inode_lock_nested(inode1, subclass1);
+	if (inode2 && inode2 != inode1)
+		inode_lock_nested(inode2, subclass2);
+}
+
 /**
  * lock_two_nondirectories - take two i_mutexes on non-directory objects
  *
diff --git a/fs/internal.h b/fs/internal.h
index bd3b2810a36b..377030a50aca 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -152,6 +152,8 @@  extern long prune_icache_sb(struct super_block *sb, struct shrink_control *sc);
 int dentry_needs_remove_privs(struct mnt_idmap *, struct dentry *dentry);
 bool in_group_or_capable(struct mnt_idmap *idmap,
 			 const struct inode *inode, vfsgid_t vfsgid);
+void lock_two_inodes(struct inode *inode1, struct inode *inode2,
+		     unsigned subclass1, unsigned subclass2);
 
 /*
  * fs-writeback.c
diff --git a/fs/namei.c b/fs/namei.c
index e4fe0879ae55..148570aabe74 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3028,8 +3028,8 @@  static struct dentry *lock_two_directories(struct dentry *p1, struct dentry *p2)
 		return p;
 	}
 
-	inode_lock_nested(p1->d_inode, I_MUTEX_PARENT);
-	inode_lock_nested(p2->d_inode, I_MUTEX_PARENT2);
+	lock_two_inodes(p1->d_inode, p2->d_inode,
+			I_MUTEX_PARENT, I_MUTEX_PARENT2);
 	return NULL;
 }