[v4,0/7] vfs: make immutable files actually immutable
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Message ID 156116141046.1664939.11424021489724835645.stgit@magnolia
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  • vfs: make immutable files actually immutable
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Darrick J. Wong June 21, 2019, 11:56 p.m. UTC
Hi all,

The chattr(1) manpage has this to say about the immutable bit that
system administrators can set on files:

"A file with the 'i' attribute cannot be modified: it cannot be deleted
or renamed, no link can be created to this file, most of the file's
metadata can not be modified, and the file can not be opened in write
mode."

Given the clause about how the file 'cannot be modified', it is
surprising that programs holding writable file descriptors can continue
to write to and truncate files after the immutable flag has been set,
but they cannot call other things such as utimes, fallocate, unlink,
link, setxattr, or reflink.

Since the immutable flag is only settable by administrators, resolve
this inconsistent behavior in favor of the documented behavior -- once
the flag is set, the file cannot be modified, period.  We presume that
administrators must be trusted to know what they're doing, and that
cutting off programs with writable fds will probably break them.

Therefore, add immutability checks to the relevant VFS functions, then
refactor the SETFLAGS and FSSETXATTR implementations to use common
argument checking functions so that we can then force pagefaults on all
the file data when setting immutability.

Note that various distro manpages points out the inconsistent behavior
of the various Linux filesystems w.r.t. immutable.  This fixes all that.

I also discovered that userspace programs can write and create writable
memory mappings to active swap files.  This is extremely bad because
this allows anyone with write privileges to corrupt system memory.  The
final patch in this series closes off that hole, at least for swap
files.

If you're going to start using this mess, you probably ought to just
pull from my git trees, which are linked below.

This has been lightly tested with fstests.  Enjoy!
Comments and questions are, as always, welcome.

--D

kernel git tree:
https://git.kernel.org/cgit/linux/kernel/git/djwong/xfs-linux.git/log/?h=immutable-files

fstests git tree:
https://git.kernel.org/cgit/linux/kernel/git/djwong/xfstests-dev.git/log/?h=immutable-files

Comments

Christoph Hellwig June 25, 2019, 10:36 a.m. UTC | #1
On Fri, Jun 21, 2019 at 04:56:50PM -0700, Darrick J. Wong wrote:
> Hi all,
> 
> The chattr(1) manpage has this to say about the immutable bit that
> system administrators can set on files:
> 
> "A file with the 'i' attribute cannot be modified: it cannot be deleted
> or renamed, no link can be created to this file, most of the file's
> metadata can not be modified, and the file can not be opened in write
> mode."
> 
> Given the clause about how the file 'cannot be modified', it is
> surprising that programs holding writable file descriptors can continue
> to write to and truncate files after the immutable flag has been set,
> but they cannot call other things such as utimes, fallocate, unlink,
> link, setxattr, or reflink.

I still think living code beats documentation.  And as far as I can
tell the immutable bit never behaved as documented or implemented
in this series on Linux, and it originated on Linux.

If you want  hard cut off style immutable flag it should really be a
new API, but I don't really see the point.  It isn't like the usual
workload is to set the flag on a file actively in use.
Darrick J. Wong June 25, 2019, 6:03 p.m. UTC | #2
On Tue, Jun 25, 2019 at 03:36:31AM -0700, Christoph Hellwig wrote:
> On Fri, Jun 21, 2019 at 04:56:50PM -0700, Darrick J. Wong wrote:
> > Hi all,
> > 
> > The chattr(1) manpage has this to say about the immutable bit that
> > system administrators can set on files:
> > 
> > "A file with the 'i' attribute cannot be modified: it cannot be deleted
> > or renamed, no link can be created to this file, most of the file's
> > metadata can not be modified, and the file can not be opened in write
> > mode."
> > 
> > Given the clause about how the file 'cannot be modified', it is
> > surprising that programs holding writable file descriptors can continue
> > to write to and truncate files after the immutable flag has been set,
> > but they cannot call other things such as utimes, fallocate, unlink,
> > link, setxattr, or reflink.
> 
> I still think living code beats documentation.  And as far as I can
> tell the immutable bit never behaved as documented or implemented
> in this series on Linux, and it originated on Linux.

The behavior has never been consistent -- since the beginning you can
keep write()ing to a fd after the file becomes immutable, but you can't
ftruncate() it.  I would really like to make the behavior consistent.
Since the authors of nearly every new system call and ioctl since the
late 1990s have interpreted S_IMMUTABLE to mean "immutable takes effect
everywhere immediately" I resolved the inconsistency in favor of that
interpretation.

I asked Ted what he thought that that userspace having the ability to
continue writing to an immutable file, and he thought it was an
implementation bug that had been there for 25 years.  Even he thought
that immutable should take effect immediately everywhere.

> If you want  hard cut off style immutable flag it should really be a
> new API, but I don't really see the point.  It isn't like the usual
> workload is to set the flag on a file actively in use.

FWIW Ted also thought that since it's rare for admins to set +i on a
file actively in use we could just change it without forcing everyone
onto a new api.

--D
Andreas Dilger June 25, 2019, 8:37 p.m. UTC | #3
On Jun 25, 2019, at 12:03 PM, Darrick J. Wong <darrick.wong@oracle.com> wrote:
> 
> On Tue, Jun 25, 2019 at 03:36:31AM -0700, Christoph Hellwig wrote:
>> On Fri, Jun 21, 2019 at 04:56:50PM -0700, Darrick J. Wong wrote:
>>> Hi all,
>>> 
>>> The chattr(1) manpage has this to say about the immutable bit that
>>> system administrators can set on files:
>>> 
>>> "A file with the 'i' attribute cannot be modified: it cannot be deleted
>>> or renamed, no link can be created to this file, most of the file's
>>> metadata can not be modified, and the file can not be opened in write
>>> mode."
>>> 
>>> Given the clause about how the file 'cannot be modified', it is
>>> surprising that programs holding writable file descriptors can continue
>>> to write to and truncate files after the immutable flag has been set,
>>> but they cannot call other things such as utimes, fallocate, unlink,
>>> link, setxattr, or reflink.
>> 
>> I still think living code beats documentation.  And as far as I can
>> tell the immutable bit never behaved as documented or implemented
>> in this series on Linux, and it originated on Linux.
> 
> The behavior has never been consistent -- since the beginning you can
> keep write()ing to a fd after the file becomes immutable, but you can't
> ftruncate() it.  I would really like to make the behavior consistent.
> Since the authors of nearly every new system call and ioctl since the
> late 1990s have interpreted S_IMMUTABLE to mean "immutable takes effect
> everywhere immediately" I resolved the inconsistency in favor of that
> interpretation.
> 
> I asked Ted what he thought that that userspace having the ability to
> continue writing to an immutable file, and he thought it was an
> implementation bug that had been there for 25 years.  Even he thought
> that immutable should take effect immediately everywhere.
> 
>> If you want  hard cut off style immutable flag it should really be a
>> new API, but I don't really see the point.  It isn't like the usual
>> workload is to set the flag on a file actively in use.
> 
> FWIW Ted also thought that since it's rare for admins to set +i on a
> file actively in use we could just change it without forcing everyone
> onto a new api.

On the flip side, it is possible to continue to write to an open fd
after removing the write permission, and this is a problem we've hit
in the real world with NFS export, so real applications do this.

It may be the same case with immutable files, where an application sets
the immutable flag immediately after creation, but continues to write
until it closes the file, so that the file can't be modified by other
processes, and there isn't a risk that the file is missing the immutable
flag if the writing process dies before setting it at the end.

Cheers, Andreas