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Message-ID: <CAF=yD-JbZ_ViWsBWK7Ye=4RYF9BgnMKvD04xE5P26RNRpnmuiQ@mail.gmail.com>
Date: Fri, 6 Oct 2017 13:40:13 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Shmulik Ladkani <shmulik@...f.io>
Cc: Pablo Neira Ayuso <pablo@...filter.org>,
netfilter-devel <netfilter-devel@...r.kernel.org>,
Willem de Bruijn <willemb@...gle.com>,
Network Development <netdev@...r.kernel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Rafael Buchbinder <rafi@....ms>,
Shmulik Ladkani <shmulik.ladkani@...il.com>
Subject: Re: [PATCH] netfilter: xt_bpf: Fix XT_BPF_MODE_FD_PINNED mode of 'xt_bpf_info_v1'
On Fri, Oct 6, 2017 at 12:02 PM, Shmulik Ladkani <shmulik@...f.io> wrote:
> From: Shmulik Ladkani <shmulik.ladkani@...il.com>
>
> Commit 2c16d6033264 ("netfilter: xt_bpf: support ebpf") introduced
> support for attaching an eBPF object by an fd, with the
> 'bpf_mt_check_v1' ABI expecting the '.fd' to be specified upon each
> IPT_SO_SET_REPLACE call.
>
> However this breaks subsequent iptables calls:
>
> # iptables -A INPUT -m bpf --object-pinned /sys/fs/bpf/xxx -j ACCEPT
> # iptables -A INPUT -s 5.6.7.8 -j ACCEPT
> iptables: Invalid argument. Run `dmesg' for more information.
>
> That's because iptables works by loading exising rules using
> IPT_SO_GET_ENTRIES to userspace, then issuing IPT_SO_SET_REPLACE with
> the replacement set.
>
> However, the loaded 'xt_bpf_info_v1' has an arbitrary '.fd' number
> (from the initial "iptables -m bpf" invocation) - so when 2nd invocation
> occurs, userspace passes a bogus fd number, which leads to
> 'bpf_mt_check_v1' to fail.
>
> One suggested solution [1] was to hack iptables userspace, to perform a
> "entries fixup" immediatley after IPT_SO_GET_ENTRIES, by opening a new,
> process-local fd per every 'xt_bpf_info_v1' entry seen.
>
> However, in [2] both Pablo Neira Ayuso and Willem de Bruijn suggested to
> depricate the xt_bpf_info_v1 ABI dealing with pinned ebpf objects.
>
> This fix changes the XT_BPF_MODE_FD_PINNED behavior to ignore the given
> '.fd' and instead perform an in-kernel lookup for the bpf object given
> the provided '.path'.
>
> It also defines an alias for the XT_BPF_MODE_FD_PINNED mode, named
> XT_BPF_MODE_PATH_PINNED, to better reflect the fact that the user is
> expected to provide the path of the pinned object.
>
> Existing XT_BPF_MODE_FD_ELF behavior (non-pinned fd mode) is preserved.
I suppose that we have the same problem here. As a matter of fact, the
implementation is the same for both FD_ELF and FD_PINNED, and checks
f.file->f_op == &bpf_prog_fops
so a file descriptor to a random open ELF file outside a bpf fs would not be
accepted as is.
Anyway, that is outside the scope of this fix.
>
> References: [1] https://marc.info/?l=netfilter-devel&m=150564724607440&w=2
> [2] https://marc.info/?l=netfilter-devel&m=150575727129880&w=2
>
> Cc: Pablo Neira Ayuso <pablo@...filter.org>
> Cc: Willem de Bruijn <willemb@...gle.com>
> Reported-by: Rafael Buchbinder <rafi@....ms>
> Signed-off-by: Shmulik Ladkani <shmulik.ladkani@...il.com>
Acked-by: Willem de Bruijn <willemb@...gle.com>
Thanks a lot for fixing this.
> ---
> include/uapi/linux/netfilter/xt_bpf.h | 1 +
> kernel/bpf/inode.c | 1 +
> net/netfilter/xt_bpf.c | 22 ++++++++++++++++++++--
> 3 files changed, 22 insertions(+), 2 deletions(-)
>
> diff --git a/include/uapi/linux/netfilter/xt_bpf.h b/include/uapi/linux/netfilter/xt_bpf.h
> index b97725af2ac0..da161b56c79e 100644
> --- a/include/uapi/linux/netfilter/xt_bpf.h
> +++ b/include/uapi/linux/netfilter/xt_bpf.h
> @@ -23,6 +23,7 @@ enum xt_bpf_modes {
> XT_BPF_MODE_FD_PINNED,
> XT_BPF_MODE_FD_ELF,
> };
> +#define XT_BPF_MODE_PATH_PINNED XT_BPF_MODE_FD_PINNED
>
> struct xt_bpf_info_v1 {
> __u16 mode;
> diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
> index e833ed914358..be1dde967208 100644
> --- a/kernel/bpf/inode.c
> +++ b/kernel/bpf/inode.c
> @@ -363,6 +363,7 @@ int bpf_obj_get_user(const char __user *pathname)
> putname(pname);
> return ret;
> }
> +EXPORT_SYMBOL_GPL(bpf_obj_get_user);
>
> static void bpf_evict_inode(struct inode *inode)
> {
> diff --git a/net/netfilter/xt_bpf.c b/net/netfilter/xt_bpf.c
> index 38986a95216c..29123934887b 100644
> --- a/net/netfilter/xt_bpf.c
> +++ b/net/netfilter/xt_bpf.c
> @@ -8,6 +8,7 @@
> */
>
> #include <linux/module.h>
> +#include <linux/syscalls.h>
> #include <linux/skbuff.h>
> #include <linux/filter.h>
> #include <linux/bpf.h>
> @@ -49,6 +50,22 @@ static int __bpf_mt_check_fd(int fd, struct bpf_prog **ret)
> return 0;
> }
>
> +static int __bpf_mt_check_path(const char *path, struct bpf_prog **ret)
> +{
> + mm_segment_t oldfs = get_fs();
> + int retval, fd;
> +
> + set_fs(KERNEL_DS);
> + fd = bpf_obj_get_user(path);
> + set_fs(oldfs);
> + if (fd < 0)
> + return fd;
> +
> + retval = __bpf_mt_check_fd(fd, ret);
> + sys_close(fd);
> + return retval;
> +}
> +
> static int bpf_mt_check(const struct xt_mtchk_param *par)
> {
> struct xt_bpf_info *info = par->matchinfo;
> @@ -66,9 +83,10 @@ static int bpf_mt_check_v1(const struct xt_mtchk_param *par)
> return __bpf_mt_check_bytecode(info->bpf_program,
> info->bpf_program_num_elem,
> &info->filter);
> - else if (info->mode == XT_BPF_MODE_FD_PINNED ||
> - info->mode == XT_BPF_MODE_FD_ELF)
> + else if (info->mode == XT_BPF_MODE_FD_ELF)
> return __bpf_mt_check_fd(info->fd, &info->filter);
> + else if (info->mode == XT_BPF_MODE_PATH_PINNED)
> + return __bpf_mt_check_path(info->path, &info->filter);
> else
> return -EINVAL;
> }
> --
> 2.14.2
>
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