On Fri, May 20, 2016 at 10:21:27AM -0400, Mimi Zohar wrote: > > + if (mutex_is_locked(&upper->d_inode->i_mutex)) > > + err = __vfs_setxattr_noperm(upper, name, value, size, flags); > > As far as I'm aware, the only time that i_mutex is taken, is during > __fput() when IMA writes security.ima. Previous versions of this patch > checked whether the xattr being written was security.ima. It would > probably be a good idea not to make that assumption here. The question > is what should happen if the i_mutex is locked, but the xattr isn't > security.ima. At minimum it should be audited. Al, any comments? ITYM "printable", and that's somewhat harder. OK, let me try: Anybody using ..._is_lock() kind of primitives that way ought to be (re)educated before they are allowed near any kind of multithreaded code _anywhere_. mutex could've been held by a different thread of execution and dropped just as mutex_is_locked() returns. Or at any subsequent point. This is 100% bogus; one should *never* write that kind of code. As in "here's your well-earned F-, better luck next semester". I haven't seen the full patch (you've quoted only a part of that gem), but about the only way for it to be correct is to have it continue with + else + err = <identical call> Practically all calls of mutex_is_locked() are of form WARN_ON(!mutex_is_locked(...)) or equivalent thereof. And the rest contains similar... wonders - for example, take a look at drivers/media/rc/imon.c; imon_ir_change_protocol() has this if (!mutex_is_locked(&ictx->lock)) { unlock = true; mutex_lock(&ictx->lock); } retval = send_packet(ictx); if (retval) goto out; ictx->rc_type = *rc_type; ictx->pad_mouse = false; out: if (unlock) mutex_unlock(&ictx->lock); Finding why it's exploitably racy is left as a trivial exercise for readers... Folks, if you see something of that sort in the code, it's a huge red flag. There are legitimate uses of mutex_is_locked other than asserts, but those are extremely rare. I would need to see more context to be able to comment on the problem in question, but this patch is almost certainly broken. -- To unsubscribe from this list: send the line "unsubscribe linux-unionfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html