Re: [PATCH 2/2] ovl: remove privs in ovl_fallocate()

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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()
	          // 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()



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