Re: [PATCH v2 22/35] vfs: don't open real

[Vivek Goyal]

On Fri, May 11, 2018 at 03:42:48PM -0400, Vivek Goyal wrote:
> On Fri, May 11, 2018 at 02:54:30PM -0400, Vivek Goyal wrote:
> > On Mon, May 07, 2018 at 10:37:54AM +0200, Miklos Szeredi wrote:
> > > Let overlayfs do its thing when opening a file.
> > > 
> > > This enables stacking and fixes the corner case when a file is opened for
> > > read, modified through a writable open, and data is read from the read-only
> > > file.  After this patch the read-only open will not return stale data even
> > > in this case.
> > 
> > [CC Dan, Steven, Paul, linux-security-module list]
> > 
> > Hi Miklos,
> > 
> > I was running selinux-testsuite and one of the tests seems to fail. I
> > think this is side effect of installing overlay inode in file->f_inode
> > instead of real underlying inode.
> > 
> > Following test is failing.
> > 
> > sub test_90_1 {
> >     print "Attempting to enter domain with bad entrypoint, should fail.\n";
> >     $result = system(
> > "runcon -t test_overlay_client_t -l s0:c10,c20 $basedir/container1/merged/badentrypoint >/dev/null 2>&1"
> >     );
> >     ok($result);
> >     return;
> > }
> 
> I am wondering, shouldn't do_open_execat() have failed. It should have called
> into inode_permission(MAY_EXEC). And then ovl_inode_permission()
> will in turn call inode_permission(realinode, MAY_EXEC) with mounter's
> creds. Shouldn't selinux_inode_permission() have returned that mounter
> does not have MAY_EXEC permission on inode.

Ok, I noticed that current policy in tests gives exec permission to 
mounter for ro_t file and that's why inode_permission(MAY_EXEC) does
not fail.

can_exec(test_overlay_mounter_t, test_overlay_files_ro_t)

Talked to Dan and he mentioned that he was trying to test entrypoint
failure (and not exec failure) and that's whey he might have allowed exec
to mounter.

I think that current entrypoint test's expectations are wrong.
User process sees overlay inode lablel which is rwx_t and that means
overlay layer will allow entrypoint into that executable. This will be the
behavior on a normal file system where underlying file's label will be
completely overridden by context=.

So in my opinion, we should modify testsuite and not run this test with
context= mounts.

Only little thing to argue is that should we check if mounter has the
permission for this entrypoint. And currently SELinux checks are not
two level checks. So this can be implemented once SELinux is made
aware of multiple levels (if we ever do that).

Thanks
Vivek

> 
> Dan, I am wondering if this is a selinux policy issue? In my testing
> on upstream kernel, do_open_execat() succeeds and it fails much later.
> I am wondering why that's the case. Is it expected.
> 
> Thanks
> Vivek
> 
> 
> > 
> > Basically, this test has an executable named "badentrypoint" with selinux
> > label "unconfined_u:object_r:test_overlay_files_ro_t:s0". And we mount
> > overlay with context=unconfined_u:object_r:test_overlay_files_rwx_t:s0:c10,c20
> > 
> > So effectively overlay inode of "badentrypoint" now gets the label
> > specified by "context=".
> > 
> > I think intent of test is that this file's real label is "...ro_t". That
> > means this file is not supposed to be executed and any attempt to execute
> > it should be denied.
> > 
> > Currently test works and execution fails with following avc.
> > 
> > AVC avc:  denied  { entrypoint } for  pid=1425 comm="runcon" path="/root/git/selinux-testsuite/tests/overlay/container1/merged/badentrypoint" dev="dm-0" ino=34515261 scontext=unconfined_u:unconfined_r:test_overlay_client_t:s0:c10,c20 tcontext=unconfined_u:object_r:test_overlay_files_ro_t:s0 tclass=file permissive=0
> > 
> > But with new patches, this test starts passing. 
> > 
> > I think currently selinux_bprm_set_creds() returns error. It does
> > checks on inode returned by file_inode() and as of now that inode is
> > real inode and that inode has real lable of "...ro_t" and permission
> > to execute that file is denied.
> > 
> > But after the patches file_inode() returns overlay inode. Which has
> > the label specified by context= mount option "...rwx_t". And that
> > label allows executing file, so file execution is not blocked by
> > selinux.
> > 
> > I feel that even now code is working accidently. Ideally our theme was
> > that task's credential as checked against overlay inode and mounter's
> > creds are checked against underlying inode to determine if certain
> > permission is allowed. So ideally mounter should not have been allwed
> > to execute a file of type "...ro_t". But we don't have that workflow
> > and VFS calls into selinux and selinux checks the underlying file's
> > label against task.
> > 
> > It worked so far but the moment we install overlay inode in file, selinux
> > checks it against overlay inode label and allows permission to execute and
> > mounter is never checked against real inode.
> > 
> > I am not sure what's the right solution. So far selinux is not aware of
> > two levels of checks and if two levels of checks are to be performed, it
> > somehow needs to be enforced by overlay and call same hook on two levels.
> > 
> > Thought of atleast starting a conversation on this.
> > 
> > Thanks
> > Vivek
> > 
> > 
> > > 
> > > Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
> > > ---
> > >  fs/open.c | 7 +------
> > >  1 file changed, 1 insertion(+), 6 deletions(-)
> > > 
> > > diff --git a/fs/open.c b/fs/open.c
> > > index 6e52fd6fea7c..244cd2ecfefd 100644
> > > --- a/fs/open.c
> > > +++ b/fs/open.c
> > > @@ -897,13 +897,8 @@ EXPORT_SYMBOL(file_path);
> > >  int vfs_open(const struct path *path, struct file *file,
> > >  	     const struct cred *cred)
> > >  {
> > > -	struct dentry *dentry = d_real(path->dentry, NULL, file->f_flags, 0);
> > > -
> > > -	if (IS_ERR(dentry))
> > > -		return PTR_ERR(dentry);
> > > -
> > >  	file->f_path = *path;
> > > -	return do_dentry_open(file, d_backing_inode(dentry), NULL, cred);
> > > +	return do_dentry_open(file, d_backing_inode(path->dentry), NULL, cred);
> > >  }
> > >  
> > >  /**
> > > -- 
> > > 2.14.3
> > > 
> > > --
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