Commit 98589a09 authored Oct 09, 2017 by Shmulik Ladkani Committed by Pablo Neira Ayuso Oct 09, 2017

netfilter: xt_bpf: Fix XT_BPF_MODE_FD_PINNED mode of 'xt_bpf_info_v1'

Commit 2c16d603 ("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 existing 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.

References: [1] https://marc.info/?l=netfilter-devel&m=150564724607440&w=2
            [2] https://marc.info/?l=netfilter-devel&m=150575727129880&w=2



Reported-by: Rafael Buchbinder <rafi@rbk.ms>
Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

parent 49f817d7

include/linux/bpf.h

+5 −0

Original line number	Diff line number	Diff line
		@@ -368,6 +368,11 @@ static inline void __bpf_prog_uncharge(struct user_struct *user, u32 pages)
		{
		}

		static inline int bpf_obj_get_user(const char __user *pathname)
		{
		return -EOPNOTSUPP;
		}

		static inline struct net_device __dev_map_lookup_elem(struct bpf_map map,
		u32 key)
		{

include/uapi/linux/netfilter/xt_bpf.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -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;

kernel/bpf/inode.c

+1 −0

Original line number	Diff line number	Diff line
		@@ -363,6 +363,7 @@ out:
		putname(pname);
		return ret;
		}
		EXPORT_SYMBOL_GPL(bpf_obj_get_user);

		static void bpf_evict_inode(struct inode *inode)
		{

net/netfilter/xt_bpf.c

+20 −2

Original line number	Diff line number	Diff line
		@@ -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;
		}

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