Re: vfs: INFO: possible circular locking dependency detected

Al Viro
2012-05-09T16:12:07Z

On Wed, May 09, 2012 at 05:25:14PM +0200, Sasha Levin wrote:
> Hi all,
>
> I've started seeing the following warning while fuzzing inside a KVM guest with the latest -next:

[->read() may grab ->cred_guard_mutex, but it may itself be called by
prepare_binprm() after having ->cred_guard_mutex grabbed]

Nasty, that... What's more, it's not just prepare_binprm() itself -
->load_binary() might end up calling read(); it doesn't have to
limit itself to mmap(), so essentially anything that can be grabbed
by ->read() of a regular file might nest under ->cred_guard_mutex.

AFAICS, /proc/*/stack, /proc/*/syscall, /proc/*/personality,
/proc/*/io_accounting, /proc/*/auxv, /proc/*/environ, /proc/*/*maps
and /proc/*/pagemap have ->cred_guard_mutex grabbed on read. seq_file
is a red herring here - io_accounting has the same issue and it does
things directly, without seq_read().

It's not a realistic attack, fortunately, since you need root
to get past open_exec() on any of those... Wait. How _did_ you get
past open_exec(), anyway? MAY_EXEC is not supposed to be granted on
anything that has no exec bits at all and AFAICS none of those files
have them.