On Thu, Aug 9, 2018 at 4:24 PM, David Howells <dhowells@xxxxxxxxxx> wrote: > Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote: > >> First let me thank you for adding both FSCONFIG_CMD_CREATE and >> FSCONFIG_CMD_RECONFIGURE. Unfortunately the implementation is currently >> broken. So this patch gets my: >> >> This is broken in two specific ways. >> ... >> 2) FSCONFIG_CMD_CREATE will succeed even if the superblock already >> exists and it can not use all of the superblock parameters. >> >> This happens because vfs_get_super will only call fill_super >> if the super block is created. Which is reasonable on the face >> of it. But it in practice this introduces security problems. >> >> a) Either through reconfiguring a shared super block you did not >> realize was shared (as we saw with devpts). >> >> b) Mounting a super block and not honoring it's mount options >> because something has already mounted it. As we see today >> with proc. Leaving userspace to think the filesystem will behave >> one way when in fact it behaves another. >> >> I have already explained this several times, and apparently I have been >> ignored. This fundamental usability issue that leads to security >> problems. > > I've also explained why you're wrong or at least only partially right. I *do* > *not* want to implement sget() in userspace with the ability for userspace to > lock out other mount requests - which is what it appears that you've been > asking for. > > However, as I have said, I *am* willing to add one of more flags to help with > this, but I can't make any "legacy" fs honour them as this requires the > fs_context to be passed down to sget_fc() and the filesystem - which is why I > was considering leaving it for later. You can determine at fsopen() time whether the filesystem is able to support the O_EXCL behavior? If so, then it's trivial to enable this conditionally. I think that's what Eric is asking for, it's obviously not fair to ask for a change in behavior of the legacy interface. Thanks, Miklos -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
