[PATCH 1/1] selinux,smack: don't bypass permissions check in inode_setsecctx hook

[Casey Schaufler]

On 8/28/2024 2:05 PM, Paul Moore wrote:
> On Wed, Aug 28, 2024 at 3:51 PM Scott Mayhew <smayhew at redhat.com> wrote:
>> Marek Gresko reports that the root user on an NFS client is able to
>> change the security labels on files on an NFS filesystem that is
>> exported with root squashing enabled.
>>
>> The end of the kerneldoc comment for __vfs_setxattr_noperm() states:
>>
>>  *  This function requires the caller to lock the inode's i_mutex before it
>>  *  is executed. It also assumes that the caller will make the appropriate
>>  *  permission checks.
>>
>> nfsd_setattr() does do permissions checking via fh_verify() and
>> nfsd_permission(), but those don't do all the same permissions checks
>> that are done by security_inode_setxattr() and its related LSM hooks do.
>>
>> Since nfsd_setattr() is the only consumer of security_inode_setsecctx(),
>> simplest solution appears to be to replace the call to
>> __vfs_setxattr_noperm() with a call to __vfs_setxattr_locked().  This
>> fixes the above issue and has the added benefit of causing nfsd to
>> recall conflicting delegations on a file when a client tries to change
>> its security label.
>>
>> Reported-by: Marek Gresko <marek.gresko at protonmail.com>
>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=218809
>> Signed-off-by: Scott Mayhew <smayhew at redhat.com>
>> ---
>>  security/selinux/hooks.c   | 4 ++--
>>  security/smack/smack_lsm.c | 4 ++--
>>  2 files changed, 4 insertions(+), 4 deletions(-)
> Thanks Scott, this looks good to me, but since it touches Smack too
> I'd also like to get Casey's ACK on this patch;

Testing labeled NFS has always been a challenge for the somewhat
limited resources available to the Smack project. I'll Ack the patch,
but won't claim to have tested it.

>  if for some reason we
> don't hear from Casey after a bit I'll go ahead and merge it.
> Speaking of merging, since this touches both SELinux and Smack I'll
> likely pull this in via the LSM tree, with a marking for the stable
> kernels, if anyone has any objections to that please let me know.
>