[PATCH v2] netfilter: xt_bpf: Fix XT_BPF_MODE_FD_PINNED mode of 'xt_bpf_info_v1'

[PATCH v2] netfilter: xt_bpf: Fix XT_BPF_MODE_FD_PINNED mode of 'xt_bpf_info_v1'

[Shmulik Ladkani]

From: Shmulik Ladkani <shmulik.ladkani@gmail.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.

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@netfilter.org>
Cc: Willem de Bruijn <willemb@google.com>
Reported-by: Rafael Buchbinder <rafi@rbk.ms>
Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
---
v2: Fix xt_bpf.c compilation if CONFIG_BPF_SYSCALL=n
---
 include/linux/bpf.h                   |  5 +++++
 include/uapi/linux/netfilter/xt_bpf.h |  1 +
 kernel/bpf/inode.c                    |  1 +
 net/netfilter/xt_bpf.c                | 22 ++++++++++++++++++++--
 4 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index a67daea731ab..a49b321a1262 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -407,6 +407,11 @@ 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)
 {
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

Re: [PATCH v2] netfilter: xt_bpf: Fix XT_BPF_MODE_FD_PINNED mode of 'xt_bpf_info_v1'

[Daniel Borkmann]

On 10/09/2017 02:27 PM, Shmulik Ladkani wrote:
> From: Shmulik Ladkani <shmulik.ladkani@gmail.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.
>
> 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@netfilter.org>
> Cc: Willem de Bruijn <willemb@google.com>
> Reported-by: Rafael Buchbinder <rafi@rbk.ms>
> Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>

Acked-by: Daniel Borkmann <daniel@iogearbox.net>

Re: [PATCH v2] netfilter: xt_bpf: Fix XT_BPF_MODE_FD_PINNED mode of 'xt_bpf_info_v1'

[Pablo Neira Ayuso]

On Mon, Oct 09, 2017 at 02:35:46PM +0200, Daniel Borkmann wrote:
> On 10/09/2017 02:27 PM, Shmulik Ladkani wrote:
> >From: Shmulik Ladkani <shmulik.ladkani@gmail.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.
> >
> >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@netfilter.org>
> >Cc: Willem de Bruijn <willemb@google.com>
> >Reported-by: Rafael Buchbinder <rafi@rbk.ms>
> >Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
> 
> Acked-by: Daniel Borkmann <daniel@iogearbox.net>

Applied, thanks everyone.