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Message-ID: <1345876989.19483.83.camel@edumazet-glaptop>
Date: Sat, 25 Aug 2012 08:43:09 +0200
From: Eric Dumazet <eric.dumazet@...il.com>
To: Xi Wang <xi.wang@...il.com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH] af_unix: fix unix_nr_socks check in unix_create1()
On Sat, 2012-08-25 at 02:05 -0400, Xi Wang wrote:
> This patch complements 518de9b3 ("fs: allow for more than 2^31 files").
>
> get_max_files() returns files_stat.max_files, which can be set to any
> value from user space via /proc/sys/fs/file-max. A large file-max will
> cause an integer overflow in the following check:
>
> if (atomic_long_read(&unix_nr_socks) > 2 * get_max_files())
> goto out;
>
> The kernel will then fail to create a unix domain socket.
>
> Rewrite the check using "/ 2" on the left-hand side.
>
> Signed-off-by: Xi Wang <xi.wang@...il.com>
> Cc: Eric Dumazet <edumazet@...gle.com>
> ---
> net/unix/af_unix.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index c5ee4ff..99a8bc9 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -630,9 +630,9 @@ static struct sock *unix_create1(struct net *net, struct socket *sock)
> {
> struct sock *sk = NULL;
> struct unix_sock *u;
> + unsigned long nr_socks = atomic_long_inc_return(&unix_nr_socks);
>
> - atomic_long_inc(&unix_nr_socks);
> - if (atomic_long_read(&unix_nr_socks) > 2 * get_max_files())
> + if (nr_socks / 2 > get_max_files())
> goto out;
>
> sk = sk_alloc(net, PF_UNIX, GFP_KERNEL, &unix_proto);
I dont think this patch is right.
atomic_long_inc_return() is expensive, more than atomic_long_inc()
And setting a 2^63 limit for max number of files is plain wrong,
as there is no way a kernel can allocate 2^63 files structures.
(same apply on 32bit arches, but for 2^31)
If you feel you must warn a sysadmin of stupid settings, add sane
boundaries in kernel/sysctl.c, and send your patch to lkml instead.
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