[RFC PATCH 0/1] xattr: Allow user.* xattr on symlink/special files if caller has CAP_SYS_RESOURCE

Vivek Goyal vgoyal at redhat.com
Thu Jul 1 13:10:30 UTC 2021

On Thu, Jul 01, 2021 at 09:48:33AM +0100, Dr. David Alan Gilbert wrote:
> * Theodore Ts'o (tytso at mit.edu) wrote:
> > On Wed, Jun 30, 2021 at 04:01:42PM +0100, Dr. David Alan Gilbert wrote:
> > > 
> > > Even if you fix symlinks, I don't think it fixes device nodes or
> > > anything else where the permissions bitmap isn't purely used as the
> > > permissions on the inode.
> > 
> > I think we're making a mountain out of a molehill.  Again, very few
> > people are using quota these days.  And if you give someone write
> > access to a 8TB disk, do you really care if they can "steal" 32k worth
> > of space (which is the maximum size of an xattr, enforced by the VFS).
> > 
> > OK, but what about character mode devices?  First of all, most users
> > don't have access to huge number of devices, but let's assume
> > something absurd.  Let's say that a user has write access to *1024*
> > devices.  (My /dev has 233 character mode devices, and I have write
> > access to well under a dozen.)
> > 
> > An 8TB disk costs about $200.  So how much of the "stolen" quota space
> > are we talking about, assuming the user has access to 1024 devices,
> > and the file system actually supports a 32k xattr.
> > 
> >     32k * 1024 * $200 / 8TB / (1024*1024*1024) = $0.000763 = 0.0763 cents
> > 
> > A 2TB SSD is less around $180, so even if we calculate the prices
> > based on SSD space, we're still talking about a quarter of a penny.
> > 
> > Why are we worrying about this?
> I'm not worrying about storage cost, but we would need to define what
> the rules are on who can write and change a user.* xattr on a device
> node.  It doesn't feel sane to make it anyone who can write to the
> device; then everyone can start leaving droppings on /dev/null.
> The other evilness I can imagine, is if there's a 32k limit on xattrs on
> a node, an evil user could write almost 32k of junk to the node
> and then break the next login that tries to add an acl or breaks the
> next relabel.

I guess 64k is per xattr VFS size limit.

#define XATTR_SIZE_MAX 65536

I just wrote a simple program to write "user.<N>" xattrs of size 1K
each and could easily write 1M xattrs. So that 1G worth data right
there. I did not try to push it further.

So a user can write lot of data in the form of user.* xattrs on
symlinks and device nodes if were to open it unconditionally. Hence
permission semantics will probably will have to defined properly.

I am wondering will it be alright if owner of the file (or CAP_FOWNER),
is allowed to write user.* xattrs on symlinks and special files.


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