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Date: Thu, 10 Feb 2011 17:27:55 -0800 (PST)
From: Davide Libenzi <davidel@...ilserver.org>
To: Nelson Elhage <nelhage@...lice.com>
cc: linux-fsdevel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
security@...nel.org
Subject: Re: [PATCH] epoll: Prevent deadlock through unsafe ->f_op->poll()
calls.
On Mon, 7 Feb 2011, Nelson Elhage wrote:
> The point of grabbing epmutex is to make sure that the loop-check and the insert
> happen atomically with respect to any other such pair of operations, so you need
> to hold epmutex across both the check and the insert. Something like the
> following. It's not particularly pretty, but it seems to work.
Right we need to hold epmutex, and this is getting uglier than I though :|
> I also had to change the "cookie" value in ep_loop_check_proc from 'ep' to
> 'epi->ffd.file->private_data'; I think this is correct -- you want to avoid
> visiting the same _containing_ 'struct eventpoll' more than once. Without that
> fix, it doesn't detect the loop in my test case.
Also, an 'unlikely' on that 'if (did_lock_epmutex)' is due.
It probably deserves a few lines of comment at the top comment, where we
describe the locking.
Do you mind posting a revised patch, while I go buying a brownbag to help
me getting over those hacks?
having said NO to the 'epoll inside epoll' thing would have saved us a lot
of headaches.
> This is minimally tested.
>
> ---
> fs/eventpoll.c | 88 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 files changed, 88 insertions(+), 0 deletions(-)
>
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index cc8a9b7..8a05f33 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -224,6 +224,9 @@ static long max_user_watches __read_mostly;
> */
> static DEFINE_MUTEX(epmutex);
>
> +/* Used to check for epoll file descriptor inclusion loops */
> +static struct nested_calls poll_loop_ncalls;
> +
> /* Used for safe wake up implementation */
> static struct nested_calls poll_safewake_ncalls;
>
> @@ -1188,6 +1191,62 @@ retry:
> return res;
> }
>
> +/**
> + * ep_loop_check_proc - Callback function to be passed to the @ep_call_nested()
> + * API, to verify that adding an epoll file inside another
> + * epoll structure, does not violate the constraints, in
> + * terms of closed loops, or too deep chains (which can
> + * result in excessive stack usage).
> + *
> + * @priv: Pointer to the epoll file to be currently checked.
> + * @cookie: Original cookie for this call. This is the top-of-the-chain epoll
> + * data structure pointer.
> + * @call_nests: Current dept of the @ep_call_nested() call stack.
> + *
> + * Returns: Returns zero if adding the epoll @file inside current epoll
> + * structure @ep does not violate the constraints, or -1 otherwise.
> + */
> +static int ep_loop_check_proc(void *priv, void *cookie, int call_nests)
> +{
> + int error = 0;
> + struct file *file = priv;
> + struct eventpoll *ep = file->private_data;
> + struct rb_node *rbp;
> + struct epitem *epi;
> +
> + mutex_lock(&ep->mtx);
> + for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {
> + epi = rb_entry(rbp, struct epitem, rbn);
> + if (unlikely(is_file_epoll(epi->ffd.file))) {
> + error = ep_call_nested(&poll_loop_ncalls, EP_MAX_NESTS,
> + ep_loop_check_proc, epi->ffd.file,
> + epi->ffd.file->private_data, current);
> + if (error != 0)
> + break;
> + }
> + }
> + mutex_unlock(&ep->mtx);
> +
> + return error;
> +}
> +
> +/**
> + * ep_loop_check - Performs a check to verify that adding an epoll file (@file)
> + * another epoll file (represented by @ep) does not create
> + * closed loops or too deep chains.
> + *
> + * @ep: Pointer to the epoll private data structure.
> + * @file: Pointer to the epoll file to be checked.
> + *
> + * Returns: Returns zero if adding the epoll @file inside current epoll
> + * structure @ep does not violate the constraints, or -1 otherwise.
> + */
> +static int ep_loop_check(struct eventpoll *ep, struct file *file)
> +{
> + return ep_call_nested(&poll_loop_ncalls, EP_MAX_NESTS,
> + ep_loop_check_proc, file, ep, current);
> +}
> +
> /*
> * Open an eventpoll file descriptor.
> */
> @@ -1236,6 +1295,7 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
> struct epoll_event __user *, event)
> {
> int error;
> + int did_lock_epmutex = 0;
> struct file *file, *tfile;
> struct eventpoll *ep;
> struct epitem *epi;
> @@ -1277,6 +1337,25 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
> */
> ep = file->private_data;
>
> + /*
> + * When we insert an epoll file descriptor, inside another epoll file
> + * descriptor, there is the change of creating closed loops, which are
> + * better be handled here, than in more critical paths.
> + *
> + * We hold epmutex across the loop check and the insert in this case, in
> + * order to prevent two separate inserts from racing and each doing the
> + * insert "at the same time" such that ep_loop_check passes on both
> + * before either one does the insert, thereby creating a cycle.
> + */
> + if (unlikely(is_file_epoll(tfile) && op == EPOLL_CTL_ADD)) {
> + mutex_lock(&epmutex);
> + did_lock_epmutex = 1;
> + error = -ELOOP;
> + if (ep_loop_check(ep, tfile) != 0)
> + goto error_tgt_fput;
> + }
> +
> +
> mutex_lock(&ep->mtx);
>
> /*
> @@ -1312,6 +1391,9 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
> mutex_unlock(&ep->mtx);
>
> error_tgt_fput:
> + if (did_lock_epmutex)
> + mutex_unlock(&epmutex);
> +
> fput(tfile);
> error_fput:
> fput(file);
> @@ -1431,6 +1513,12 @@ static int __init eventpoll_init(void)
> EP_ITEM_COST;
> BUG_ON(max_user_watches < 0);
>
> + /*
> + * Initialize the structure used to perform epoll file descriptor
> + * inclusion loops checks.
> + */
> + ep_nested_calls_init(&poll_loop_ncalls);
> +
> /* Initialize the structure used to perform safe poll wait head wake ups */
> ep_nested_calls_init(&poll_safewake_ncalls);
>
> --
> 1.7.2.43.g68ef4
>
> --
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>
- Davide
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