On Fri, May 20, 2016 at 8:00 PM, Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx> wrote:
> On Fri, 2016-05-20 at 17:29 +0100, Al Viro wrote:
>> 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.
>
> My fault for even suggesting it.
Mimi, it is not your fault. I should have never missed something so
ridiculously trivial as this. I was simply being an idiot and rolled a
patch with my brain completely shut off and without stopping for
a single second to think about what I was doing. I agre with Al that
in that state of mind one should not be anywhere near a keyboard,
let alone trying to write kernel code. Got what I deserved.
>> I would need to see more context to be able to comment on the problem in
>> question, but this patch is almost certainly broken.
>
> We deferred __fput() back in 2012 in order for IMA to safely take the
> i_mutex and write security.ima. Writing the security.ima xattr now
> triggers overlayfs to write the xattr, but overlayfs doesn't
> differentiate between callers - as a result of userspace or as described
> here in __fput(). All calls to ovl_setxattr() should call vfs_sexattr,
> except the one triggered by __fput(). Refer to the original lockdep
> report -
> http://thread.gmane.org/gmane.linux.file-systems.union/640
How about the other (maybe kludgy) option of adding a dedicated inode
flag for this, setting it in the call to __vfs_setxattr_noperm in ima_appraise
and forcibly clearing it in vfs_setxattr. ovl_setxattr could then test it and
branch accordingly...
>
> Al, any help in resolving this lockdep would be much appreciated.
>
> Mimi
>
Cheers,
Krisztian the Idiot, hiding in a quiet corner, bowing his head in shame
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