On Tue, Oct 04, 2022 at 12:59:37PM +0200, Christian Brauner wrote:
> On Mon, Oct 03, 2022 at 03:30:40PM +0300, Amir Goldstein wrote:
> > Underlying fs doesn't remove privs because fallocate is called with
> > privileged mounter credentials.
> >
> > This fixes some failure in fstests generic/683..687.
> >
> > Fixes: aab8848cee5e ("ovl: add ovl_fallocate()")
> > Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
> > ---
> > fs/overlayfs/file.c | 12 +++++++++++-
> > 1 file changed, 11 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> > index c8308da8909a..e90ac5376456 100644
> > --- a/fs/overlayfs/file.c
> > +++ b/fs/overlayfs/file.c
> > @@ -517,9 +517,16 @@ static long ovl_fallocate(struct file *file, int mode, loff_t offset, loff_t len
> > const struct cred *old_cred;
> > int ret;
> >
> > + inode_lock(inode);
> > + /* Update mode */
> > + ovl_copyattr(inode);
> > + ret = file_remove_privs(file);
>
> First, thank you for picking this up!
>
> Let me analyze generic/683 failure of Test1 to see why you still see
> failures in this test:
>
> echo "Test 1 - qa_user, non-exec file $verb"
> setup_testfile
> chmod a+rws $junk_file
> commit_and_check "$qa_user" "$verb" 64k 64k
>
> So this creates a file with 6666 permissions. While the file has the
> S_ISUID and S_ISGID bits set it does not have the S_IXGRP set. This is
> important in a little bit.
>
> On a regular filesystem like xfs what will happen is:
>
> sys_fallocate()
> -> vfs_fallocate()
> -> xfs_file_fallocate()
> -> file_modified()
> -> __file_remove_privs()
> -> dentry_needs_remove_privs()
> -> should_remove_suid()
> -> __remove_privs()
> newattrs.ia_valid = ATTR_FORCE | kill;
> -> notify_change()
>
> In should_remove_suid() we can see that ATTR_KILL_SUID is raised
> unconditionally because the file in the test has S_ISUID set.
>
> But we also see that ATTR_KILL_SGID won't be set because while the file
> is S_ISGID it is not S_IXGRP (see above) which is a condition for
> ATTR_KILL_SGID being raised.
>
> So by the time we call notify_change() we have attr->ia_valid set to
> ATTR_KILL_SUID | ATTR_FORCE. Now notify_change() sees that
> ATTR_KILL_SUID is set and does:
>
> ia_valid = attr->ia_valid |= ATTR_MODE
> attr->ia_mode = (inode->i_mode & ~S_ISUID);
>
> which means that when we call setattr_copy() later we will definitely
> update inode->i_mode. Note that attr->ia_mode still contain S_ISGID.
>
> Now we call into the filesystem's ->setattr() inode operation which will end up
> calling setattr_copy(). Since ATTR_MODE is set we will hit:
>
> if (ia_valid & ATTR_MODE) {
> umode_t mode = attr->ia_mode;
> vfsgid_t vfsgid = i_gid_into_vfsgid(mnt_userns, inode);
> if (!vfsgid_in_group_p(vfsgid) &&
> !capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
> mode &= ~S_ISGID;
> inode->i_mode = mode;
> }
>
> and since the caller in the test is neither capable nor in the group of the
> inode the S_ISGID bit is stripped.
>
> But now contrast this with overlayfs even after your changes. When
> ovl_setattr() is hit from ovl_fallocate()'s call to file_remove_privs()
> and calls ovl_do_notify_change() then we are doing this under the
> mounter's creds and so the S_ISGID bit is retained:
>
> sys_fallocate()
> -> vfs_fallocate()
> -> ovl_fallocate()
> -> file_remove_privs()
> -> dentry_needs_remove_privs()
> -> should_remove_suid()
> -> __remove_privs()
> newattrs.ia_valid = attr_force | kill;
> -> notify_change()
> -> ovl_setattr()
Note also that the setattr_prepare() call skips setgid checks because
ATTR_FORCE is passed from file_remove_privs().
> // TAKE ON MOUNTER'S CREDS
> -> ovl_do_notify_change()
> // GIVE UP MOUNTER'S CREDS
> // TAKE ON MOUNTER'S CREDS
> -> vfs_fallocate()
> -> xfs_file_fallocate()
> -> file_modified()
> -> __file_remove_privs()
> -> dentry_needs_remove_privs()
> -> should_remove_suid()
> -> __remove_privs()
> newattrs.ia_valid = attr_force | kill;
> -> notify_change()
