diff mbox series

netfilter: xt_bpf: Fix XT_BPF_MODE_FD_PINNED mode of 'xt_bpf_info_v1'

Message ID 20171006160242.4403-1-shmulik@nsof.io
State Changes Requested
Delegated to: Pablo Neira
Headers show
Series netfilter: xt_bpf: Fix XT_BPF_MODE_FD_PINNED mode of 'xt_bpf_info_v1' | expand

Commit Message

Shmulik Ladkani Oct. 6, 2017, 4:02 p.m. UTC 
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>
---
 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(-)

Comments

Willem de Bruijn Oct. 6, 2017, 5:40 p.m. UTC | #1 
On Fri, Oct 6, 2017 at 12:02 PM, Shmulik Ladkani <shmulik@nsof.io> 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.

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@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: Willem de Bruijn <willemb@google.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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Pablo Neira Ayuso Oct. 9, 2017, 11:18 a.m. UTC | #2 
On Fri, Oct 06, 2017 at 01:40:13PM -0400, Willem de Bruijn wrote:
> On Fri, Oct 6, 2017 at 12:02 PM, Shmulik Ladkani <shmulik@nsof.io> 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.
[...]
> >
> > 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: Willem de Bruijn <willemb@google.com>

Applied, thanks.
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Pablo Neira Ayuso Oct. 9, 2017, 11:57 a.m. UTC | #3 
On Mon, Oct 09, 2017 at 01:18:23PM +0200, Pablo Neira Ayuso wrote:
> On Fri, Oct 06, 2017 at 01:40:13PM -0400, Willem de Bruijn wrote:
> > On Fri, Oct 6, 2017 at 12:02 PM, Shmulik Ladkani <shmulik@nsof.io> 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.
> [...]
> > >
> > > 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: Willem de Bruijn <willemb@google.com>
> 
> Applied, thanks.

Hm, I have to keep this back. Compilation breaks here.

net/netfilter/xt_bpf.c: In function ‘__bpf_mt_check_path’:
net/netfilter/xt_bpf.c:59:2: error: implicit declaration of function
‘bpf_obj_get_user’ [-Werror=implicit-function-declaration]
  fd = bpf_obj_get_user(path);
  ^
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Daniel Borkmann Oct. 9, 2017, 12:01 p.m. UTC | #4 
Hi Shmulik,

On 10/09/2017 01:57 PM, Pablo Neira Ayuso wrote:
> On Mon, Oct 09, 2017 at 01:18:23PM +0200, Pablo Neira Ayuso wrote:
>> On Fri, Oct 06, 2017 at 01:40:13PM -0400, Willem de Bruijn wrote:
>>> On Fri, Oct 6, 2017 at 12:02 PM, Shmulik Ladkani <shmulik@nsof.io> 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.
>> [...]
>>>>
>>>> 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: Willem de Bruijn <willemb@google.com>
>>
>> Applied, thanks.
>
> Hm, I have to keep this back. Compilation breaks here.
>
> net/netfilter/xt_bpf.c: In function ‘__bpf_mt_check_path’:
> net/netfilter/xt_bpf.c:59:2: error: implicit declaration of function
> ‘bpf_obj_get_user’ [-Werror=implicit-function-declaration]
>    fd = bpf_obj_get_user(path);
>    ^

Yeah, probably best to just add a dummy bpf_obj_get_user()
returning an error when CONFIG_BPF_SYSCALL is disabled.
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diff mbox series

Patch

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;
 }