| Message ID | 20200916161737.38028-5-vgoyal@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | fuse: Implement FUSE_HANDLE_KILLPRIV_V2 and enable SB_NOSEC | expand |
On Wed, Sep 16, 2020 at 6:18 PM Vivek Goyal <vgoyal@redhat.com> wrote: > But if this is non-truncate setattr then server will not kill suid/sgid. > So continue to send ATTR_MODE to kill suid/sgid for non-truncate setattr, > even if ->handle_killpriv_v2 is enabled. Sending ATTR_MODE doesn't make sense, since that is racy. The refresh-recalculate makes the race window narrower, but it doesn't eliminate it. I think I suggested sending write synchronously if suid/sgid/caps are set. Do you see a problem with this? Does this affect anything other than cached writes? Thanks, Miklos
On Tue, Sep 22, 2020 at 03:56:47PM +0200, Miklos Szeredi wrote: > On Wed, Sep 16, 2020 at 6:18 PM Vivek Goyal <vgoyal@redhat.com> wrote: > > > But if this is non-truncate setattr then server will not kill suid/sgid. > > So continue to send ATTR_MODE to kill suid/sgid for non-truncate setattr, > > even if ->handle_killpriv_v2 is enabled. > > Sending ATTR_MODE doesn't make sense, since that is racy. The > refresh-recalculate makes the race window narrower, but it doesn't > eliminate it. Hi Miklos, Agreed that it does not eliminate that race. > > I think I suggested sending write synchronously if suid/sgid/caps are > set. Do you see a problem with this? Sorry, I might have missed it. So you are saying that for the case of ->writeback_cache, force a synchronous WRITE if suid/sgid is set. But this will only work if client sees the suid/sgid bits. If client B set the suid/sgid which client A does not see then all the WRITEs will be cached in client A and not clear suid/sgid bits. Also another problem is that if client sees suid/sgid and we make WRITE synchronous, client's suid/sgid attrs are still cached till next refresh (both for ->writeback_cache and non writeback_cache case). So server is clearing suid/sgid bits but client still keeps them cached. I hope none of the code paths end up using this stale value and refresh attrs before using suid/sgid. Shall we refresh attrs after WRITE if suid/sgid is set and client expects it to clear after WRITE finishes to solve this problem. Or this is something which is actually not a real problem and I am overdesigning. Thanks Vivek > > Does this affect anything other than cached writes? > > Thanks, > Miklos >
On Tue, Sep 22, 2020 at 10:08 PM Vivek Goyal <vgoyal@redhat.com> wrote: > > On Tue, Sep 22, 2020 at 03:56:47PM +0200, Miklos Szeredi wrote: > > On Wed, Sep 16, 2020 at 6:18 PM Vivek Goyal <vgoyal@redhat.com> wrote: > > > > > But if this is non-truncate setattr then server will not kill suid/sgid. > > > So continue to send ATTR_MODE to kill suid/sgid for non-truncate setattr, > > > even if ->handle_killpriv_v2 is enabled. > > > > Sending ATTR_MODE doesn't make sense, since that is racy. The > > refresh-recalculate makes the race window narrower, but it doesn't > > eliminate it. > > Hi Miklos, > > Agreed that it does not eliminate that race. > > > > > I think I suggested sending write synchronously if suid/sgid/caps are > > set. Do you see a problem with this? > > Sorry, I might have missed it. So you are saying that for the case of > ->writeback_cache, force a synchronous WRITE if suid/sgid is set. But > this will only work if client sees the suid/sgid bits. If client B > set the suid/sgid which client A does not see then all the WRITEs > will be cached in client A and not clear suid/sgid bits. Unless the attributes are invalidated (either by timeout or explicitly) there's no way that in that situation the suid/sgid bits can be cleared. That's true of your patch as well. > > Also another problem is that if client sees suid/sgid and we make > WRITE synchronous, client's suid/sgid attrs are still cached till > next refresh (both for ->writeback_cache and non writeback_cache > case). So server is clearing suid/sgid bits but client still > keeps them cached. I hope none of the code paths end up using > this stale value and refresh attrs before using suid/sgid. > > Shall we refresh attrs after WRITE if suid/sgid is set and client > expects it to clear after WRITE finishes to solve this problem. Or > this is something which is actually not a real problem and I am > overdesigning. The fuse_perform_write() path already has the attribute invalidation, which will trigger GETATTR from fuse_update_attributes() in the next write. So I think all that should work fine. Thanks, Miklos
On Tue, Sep 22, 2020 at 11:25:30PM +0200, Miklos Szeredi wrote: > On Tue, Sep 22, 2020 at 10:08 PM Vivek Goyal <vgoyal@redhat.com> wrote: > > > > On Tue, Sep 22, 2020 at 03:56:47PM +0200, Miklos Szeredi wrote: > > > On Wed, Sep 16, 2020 at 6:18 PM Vivek Goyal <vgoyal@redhat.com> wrote: > > > > > > > But if this is non-truncate setattr then server will not kill suid/sgid. > > > > So continue to send ATTR_MODE to kill suid/sgid for non-truncate setattr, > > > > even if ->handle_killpriv_v2 is enabled. > > > > > > Sending ATTR_MODE doesn't make sense, since that is racy. The > > > refresh-recalculate makes the race window narrower, but it doesn't > > > eliminate it. > > > > Hi Miklos, > > > > Agreed that it does not eliminate that race. > > > > > > > > I think I suggested sending write synchronously if suid/sgid/caps are > > > set. Do you see a problem with this? > > > > Sorry, I might have missed it. So you are saying that for the case of > > ->writeback_cache, force a synchronous WRITE if suid/sgid is set. But > > this will only work if client sees the suid/sgid bits. If client B > > set the suid/sgid which client A does not see then all the WRITEs > > will be cached in client A and not clear suid/sgid bits. > > Unless the attributes are invalidated (either by timeout or > explicitly) there's no way that in that situation the suid/sgid bits > can be cleared. That's true of your patch as well. Right. And that's why I mentioned that handle_killpriv_v2 is not fully compatible with ->writeback_cache. > > > > > Also another problem is that if client sees suid/sgid and we make > > WRITE synchronous, client's suid/sgid attrs are still cached till > > next refresh (both for ->writeback_cache and non writeback_cache > > case). So server is clearing suid/sgid bits but client still > > keeps them cached. I hope none of the code paths end up using > > this stale value and refresh attrs before using suid/sgid. > > > > Shall we refresh attrs after WRITE if suid/sgid is set and client > > expects it to clear after WRITE finishes to solve this problem. Or > > this is something which is actually not a real problem and I am > > overdesigning. > > The fuse_perform_write() path already has the attribute invalidation, > which will trigger GETATTR from fuse_update_attributes() in the next > write. Ok. So if there is any path which potentially can make use of cached suid/sgid, we just need to make sure fuse_update_attributes() has been called in that path. > > So I think all that should work fine. Sounds good. I will give it a try and see if I notice any other issues. Thanks Vivek
diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c index ecdb7895c156..4b0fe0828e36 100644 --- a/fs/fuse/dir.c +++ b/fs/fuse/dir.c @@ -1655,6 +1655,21 @@ static int fuse_setattr(struct dentry *entry, struct iattr *attr) return -EACCES; if (attr->ia_valid & (ATTR_KILL_SUID | ATTR_KILL_SGID)) { + bool kill_sugid = true; + bool is_truncate = !!(attr->ia_valid & ATTR_SIZE); + + if (fc->handle_killpriv || + (fc->handle_killpriv_v2 && is_truncate)) { + /* + * If this is truncate and ->handle_killpriv_v2 is + * enabled, we don't have to send ATTR_MODE to + * kill suid/sgid as server will do it anyway as + * part of truncate. But if this is not truncate + * then kill suid/sgid by sending ATTR_MODE. + */ + kill_sugid = false; + } + attr->ia_valid &= ~(ATTR_KILL_SUID | ATTR_KILL_SGID | ATTR_MODE); @@ -1664,7 +1679,7 @@ static int fuse_setattr(struct dentry *entry, struct iattr *attr) * * This should be done on write(), truncate() and chown(). */ - if (!fc->handle_killpriv) { + if (kill_sugid) { /* * ia_mode calculation may have used stale i_mode. * Refresh and recalculate.
If a truncate is happening with ->handle_killpriv_v2 is enabled, then we don't have to send ATTR_MODE to kill suid/sgid as server will kill it as part of the protocol. But if this is non-truncate setattr then server will not kill suid/sgid. So continue to send ATTR_MODE to kill suid/sgid for non-truncate setattr, even if ->handle_killpriv_v2 is enabled. This path is taken when client does a write on a file which has suid/ sgid is set. VFS will first kill suid/sgid and then proceed with WRITE. One can argue that why not simply ignore ATTR_MODE because a WRITE will follow and ->handle_killpriv_v2 will kill suid/sgid that time. I feel this is a safer approach for following reasons. - With ->writeback_cache enabled, WRITE will not go to server. I feel that for this reason ->writeback_cache mode is not fully compatible with ->handle_killpriv_v2. But if we kill suid/sgid now, this will solve this particular issue for ->writeback_cache mode too. Again, I will not solve all the issues around ->writeback_cache but makes things better. - If we rely on WRITE killing suid/sgid, then after cache becomes out of sync w.r.t host. Client will still have suid/sgid set but subsequent WRITE will clear suid/sgid. Well WRITE will also invalidate client cache so further access to inode->i_mode should result in a ->getattr. Hmm..., for the case of ->writeback_cache, I am kind of inclined to send ATTR_MODE. - We are sending setattr(ATTR_FORCE) anyway (even if we clear ATTR_MODE). So if we are not saving on setattr(), why not kill suid/sgid now. Signed-off-by: Vivek Goyal <vgoyal@redhat.com> --- fs/fuse/dir.c | 17 ++++++++++++++++- 1 file changed, 16 insertions(+), 1 deletion(-)