patch: support long (above 14 bytes) HW addresses in arp_ioctl - Kernel

This is a discussion on patch: support long (above 14 bytes) HW addresses in arp_ioctl - Kernel ; While working with OFED infiniband stack that uses 20 byte long HW addresses for IP over IB, I have paid attention to the following arp_ioctl problem. The ioctl uses a data structure that limits a length of HW address to ...

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Thread: patch: support long (above 14 bytes) HW addresses in arp_ioctl

  1. patch: support long (above 14 bytes) HW addresses in arp_ioctl

    While working with OFED infiniband stack that uses 20 byte long HW
    addresses for IP over IB, I have paid attention to the following
    arp_ioctl problem.

    The ioctl uses a data structure that limits a length of HW address to 14
    bytes. The IP stack and the arp cache code do not have that limitation.
    This leads to the following problems:

    * arp_ioctl cannot be used to set, get, or delete arp entries for those
    adapters that have HW addresses longer than 14 bytes
    * arp_ioctl will corrupt the kernel and user memory when this ioctl is
    used on the adapters that have HW addresses longer that 14 bytes. This
    is because when copying the HW address, the arp_ioctl code copies
    dev->addr_len bytes without checking that addr_len is not above 14
    bytes. This is done both for copy_to_user() and memcpy() calls on kernel
    data structures allocated on stack. The memcpy() call in particular,
    will corrupt kernel stack.

    Attached please find the patch that fixes both problems. In addition,
    the patch changes the maximal number of bytes for HW address that will
    be seen in /proc/net/arp from ~10 to ~30. Without the last change,
    output of /proc/net/arp truncates the the large MAC entries, which makes
    the arp utility useless.

    The patch does not change the existing ABI but extends it. The kernel
    structure used in arp_ioctl calls is changed to support larger
    addresses, while the user-space structure is extended by appending
    extra-space to the end of the structure if ATF_NEWARPCTL -- a new flag
    -- is set in arp_flags of existing user-space structure. This allows
    avoiding big changes to the existing code while preserving the ABI
    compatibility.

    --
    ----------------------------------------
    Constantine Gavrilov
    Kernel Developer
    Platform Group
    XIV, an IBM global brand
    1 Azrieli Center, Tel-Aviv
    Phone: +972-3-6074672
    Fax: +972-3-6959749
    ----------------------------------------




  2. Re: patch: support long (above 14 bytes) HW addresses in arp_ioctl

    Updated version of the patch uses MAX_ADDR_LEN from netdevice.h as the
    maximal length of MAC address.

    Constantine Gavrilov wrote:
    > While working with OFED infiniband stack that uses 20 byte long HW
    > addresses for IP over IB, I have paid attention to the following
    > arp_ioctl problem.
    >
    > The ioctl uses a data structure that limits a length of HW address to
    > 14 bytes. The IP stack and the arp cache code do not have that
    > limitation. This leads to the following problems:
    >
    > * arp_ioctl cannot be used to set, get, or delete arp entries for
    > those adapters that have HW addresses longer than 14 bytes
    > * arp_ioctl will corrupt the kernel and user memory when this ioctl is
    > used on the adapters that have HW addresses longer that 14 bytes.
    > This is because when copying the HW address, the arp_ioctl code copies
    > dev->addr_len bytes without checking that addr_len is not above 14
    > bytes. This is done both for copy_to_user() and memcpy() calls on
    > kernel data structures allocated on stack. The memcpy() call in
    > particular, will corrupt kernel stack.
    >
    > Attached please find the patch that fixes both problems. In addition,
    > the patch changes the maximal number of bytes for HW address that will
    > be seen in /proc/net/arp from ~10 to ~30. Without the last change,
    > output of /proc/net/arp truncates the the large MAC entries, which
    > makes the arp utility useless.
    >
    > The patch does not change the existing ABI but extends it. The kernel
    > structure used in arp_ioctl calls is changed to support larger
    > addresses, while the user-space structure is extended by appending
    > extra-space to the end of the structure if ATF_NEWARPCTL -- a new
    > flag -- is set in arp_flags of existing user-space structure. This
    > allows avoiding big changes to the existing code while preserving the
    > ABI compatibility.
    >


    --
    ----------------------------------------
    Constantine Gavrilov
    Kernel Developer
    Platform Group
    XIV, an IBM global brand
    1 Azrieli Center, Tel-Aviv
    Phone: +972-3-6074672
    Fax: +972-3-6959749
    ----------------------------------------




  3. Re: patch: support long (above 14 bytes) HW addresses in arp_ioctl

    > * arp_ioctl will corrupt the kernel and user memory when this ioctl is
    > used on the adapters that have HW addresses longer that 14 bytes.
    > This is because when copying the HW address, the arp_ioctl code copies
    > dev->addr_len bytes without checking that addr_len is not above 14
    > bytes. This is done both for copy_to_user() and memcpy() calls on
    > kernel data structures allocated on stack. The memcpy() call in
    > particular, will corrupt kernel stack.


    It's not obvious to me after a quick glance where this kernel memory
    corruption occurs, but clearly we should at least fix this bug.

    > The patch does not change the existing ABI but extends it. The kernel
    > structure used in arp_ioctl calls is changed to support larger
    > addresses, while the user-space structure is extended by appending
    > extra-space to the end of the structure if ATF_NEWARPCTL -- a new flag
    > -- is set in arp_flags of existing user-space structure. This allows
    > avoiding big changes to the existing code while preserving the ABI
    > compatibility.


    However, given that applications need to be changed to use this,
    wouldn't it make more sense just to change those applications to use
    rtnetlink, which already supports large hardware addresses? ie is there
    much point to extending a legacy ABI to add a feature that the preferred
    modern interface already has?

    - R.
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  4. Re: patch: support long (above 14 bytes) HW addresses in arp_ioctl


    In arp_req_get() in net/arp.c, there is code:

    memcpy(r->arp_ha.sa_data, neigh->ha, dev->addr_len);

    dev->addr_len can be larger than size of r->arp_ha.sa_data. Inititally,
    I thought it would corrupt kernel stack. I was wrong, since r still has
    enough space not to overflow even for the largest HW address (32 bytes).
    It would corrupt the data structure though, and that corrupted reply
    would be propagated to user.

    There is a similar situation in arp_req_set(), where a "junk" arp entry
    will be set if dev->addr_len is larger that 14 bytes.

    At the very minimum, both arp_req_set() and arp_req_get() should return
    error (-EINVAL), and not return junk or set junk. Truncated
    /proc/net/arp output should also be fixed.

    I was not aware that rtnetlink is capable of doing things like arp
    table or interface manipulation (like netdevice ioctls). My applications
    needs to be able to manipulate arp cache for large macs, and I do not
    mind recompiling by adding a flag. I do not mind fixing arp cli to use
    this either (venerable arp does use arp_ioctl). And there are many many
    legacy solutions that use arp_ioctl() in programs and arp utility in
    scripts. Consider porting those to infiniband.

    Will rtnetlink work for any net_device (like netdevice ioctls do) for
    ARP and interface configurations calls or does it require special
    support in net_device itself? Any possible problems with rtnetlink?

    Roland Dreier wrote:
    > > * arp_ioctl will corrupt the kernel and user memory when this ioctl is
    > > used on the adapters that have HW addresses longer that 14 bytes.
    > > This is because when copying the HW address, the arp_ioctl code copies
    > > dev->addr_len bytes without checking that addr_len is not above 14
    > > bytes. This is done both for copy_to_user() and memcpy() calls on
    > > kernel data structures allocated on stack. The memcpy() call in
    > > particular, will corrupt kernel stack.

    >
    > It's not obvious to me after a quick glance where this kernel memory
    > corruption occurs, but clearly we should at least fix this bug.
    >
    > > The patch does not change the existing ABI but extends it. The kernel
    > > structure used in arp_ioctl calls is changed to support larger
    > > addresses, while the user-space structure is extended by appending
    > > extra-space to the end of the structure if ATF_NEWARPCTL -- a new flag
    > > -- is set in arp_flags of existing user-space structure. This allows
    > > avoiding big changes to the existing code while preserving the ABI
    > > compatibility.

    >
    > However, given that applications need to be changed to use this,
    > wouldn't it make more sense just to change those applications to use
    > rtnetlink, which already supports large hardware addresses? ie is there
    > much point to extending a legacy ABI to add a feature that the preferred
    > modern interface already has?
    >
    > - R.
    >


    --
    ----------------------------------------
    Constantine Gavrilov
    Kernel Developer
    Platform Group
    XIV, an IBM global brand
    1 Azrieli Center, Tel-Aviv
    Phone: +972-3-6074672
    Fax: +972-3-6959749
    ----------------------------------------


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  5. Re: patch: support long (above 14 bytes) HW addresses in arp_ioctl

    [netdev added to cc list]

    > In arp_req_get() in net/arp.c, there is code:
    >
    > memcpy(r->arp_ha.sa_data, neigh->ha, dev->addr_len);
    >
    > dev->addr_len can be larger than size of
    > r->arp_ha.sa_data. Inititally, I thought it would corrupt kernel
    > stack. I was wrong, since r still has enough space not to overflow
    > even for the largest HW address (32 bytes). It would corrupt the data
    > structure though, and that corrupted reply would be propagated to
    > user.
    >
    > There is a similar situation in arp_req_set(), where a "junk" arp
    > entry will be set if dev->addr_len is larger that 14 bytes.
    >
    > At the very minimum, both arp_req_set() and arp_req_get() should
    > return error (-EINVAL), and not return junk or set junk. Truncated
    > /proc/net/arp output should also be fixed.


    The EINVAL return makes sense; I'm not sure /proc/net/arp is important
    enough to fix. I guess it depends on the impact of the fix.

    > I was not aware that rtnetlink is capable of doing things like arp
    > table or interface manipulation (like netdevice ioctls). My
    > applications needs to be able to manipulate arp cache for large macs,
    > and I do not mind recompiling by adding a flag. I do not mind fixing
    > arp cli to use this either (venerable arp does use arp_ioctl). And
    > there are many many legacy solutions that use arp_ioctl() in programs
    > and arp utility in scripts. Consider porting those to infiniband.
    >
    > Will rtnetlink work for any net_device (like netdevice ioctls do) for
    > ARP and interface configurations calls or does it require special
    > support in net_device itself? Any possible problems with rtnetlink?


    rtnetlink is the preferred modern interface between userspace and kernel
    for networking information. There is also the "iproute2" package that
    provides a good command line interface that is capable of handling IPoIB
    addresses. For example:

    $ ip addr show dev ib1
    5: ib1: mtu 2044 qdisc pfifo_fast state UP qlen 256
    link/infiniband 80:00:00:48:fe:80:00:00:00:00:00:00:00:02:c9:03:00 :00:01:65 brd 00:ff:ff:ff:ff:12:40:1b:ff:ff:00:00:00:00:00:00:ff :ff:ff:ff
    inet 192.168.145.74/24 brd 192.168.145.255 scope global ib1
    inet6 fe80::202:c903:0:165/64 scope link
    valid_lft forever preferred_lft forever

    $ ip neigh
    192.168.145.73 dev ib1 lladdr 80:00:00:48:fe:80:00:00:00:00:00:00:00:02:c9:03:00 :00:01:30 STALE
    172.29.224.1 dev eth0 lladdr 00:00:0c:07:ac:e0 REACHABLE

    and so on.

    - R.
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