fuse: don't send ATTR_MODE to kill suid/sgid for handle_killpriv_v2
If client does a write() on a suid/sgid file, VFS will first call fuse_setattr() with ATTR_KILL_S[UG]ID set. This requires sending setattr to file server with ATTR_MODE set to kill suid/sgid. But to do that client needs to know latest mode otherwise it is racy. To reduce the race window, current code first call fuse_do_getattr() to get latest ->i_mode and then resets suid/sgid bits and sends rest to server with setattr(ATTR_MODE). This does not reduce the race completely but narrows race window significantly. With fc->handle_killpriv_v2 enabled, it should be possible to remove this race completely. Do not kill suid/sgid with ATTR_MODE at all. It will be killed by server when WRITE request is sent to server soon. This is similar to fc->handle_killpriv logic. V2 is just more refined version of protocol. Hence this patch does not send ATTR_MODE to kill suid/sgid if fc->handle_killpriv_v2 is enabled. This creates an issue if fc->writeback_cache is enabled. In that case WRITE can be cached in guest and server might not see WRITE request and hence will not kill suid/sgid. Miklos suggested that in such cases, we should fallback to a writethrough WRITE instead and that will generate WRITE request and kill suid/sgid. This patch implements that too. But this relies on client seeing the suid/sgid set. If another client sets suid/sgid and this client does not see it immideately, then we will not fallback to writethrough WRITE. So this is one limitation with both fc->handle_killpriv_v2 and fc->writeback_cache enabled. Both the options are not fully compatible. But might be good enough for many use cases. Note: This patch is not checking whether security.capability is set or not when falling back to writethrough path. If suid/sgid is not set and only security.capability is set, that will be taken care of by file_remove_privs() call in ->writeback_cache path. Signed-off-by:Vivek Goyal <vgoyal@redhat.com> Signed-off-by:
Miklos Szeredi <mszeredi@redhat.com>
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31792161
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