ACK/Cmnt: [SRU][P][PATCH 0/1] Creating a VXLAN interface with a Fan mapping causes a NULL pointer dereference

[Wei-Lin Chang]

On Fri, Jun 13, 2025 at 04:54:49PM -0500, Jacob Martin wrote:
> BugLink: https://bugs.launchpad.net/bugs/2113992

In the docs "Reviewing and Applying Mailing List Patches" there
is: "SRU justification must is written in Bug Description (of
the LP bug), not in a comment", but I don't know if this is strictly
enforced, so

Acked-by: Wei-Lin Chang <weilin.chang at canonical.com>

> 
> SRU Justification:
> 
> [Impact]
> 
> Creating a VXLAN link with a Fan map reliably results in a kernel NULL pointer dereference.
> 
> [ 1035.676861] BUG: kernel NULL pointer dereference, address: 0000000000000000
> [ 1035.678459] #PF: supervisor read access in kernel mode
> [ 1035.679321] #PF: error_code(0x0000) - not-present page
> [ 1035.680092] PGD 0 P4D 0 
> [ 1035.680509] Oops: Oops: 0000 [#1] PREEMPT SMP PTI
> [ 1035.681179] CPU: 1 UID: 0 PID: 8470 Comm: ip Not tainted 6.14.0-15-generic #15-Ubuntu
> [ 1035.682291] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009)/LXD, BIOS unknown 2/2/2022
> ...
> 
> This affects 6.14 kernels only.
> 
> [Fix]
> 
> Author: Jacob Martin <jacob.martin at canonical.com>
> Date:   Fri Jun 13 10:33:42 2025 -0500
> 
>     UBUNTU: SAUCE: fan: vxlan: parse fan-map from IFLA_VXLAN_FAN_MAP attribute ID
>     
>     BugLink: https://bugs.launchpad.net/bugs/2113992
>     
>     Before 6c11379b104e ("vxlan: Add an attribute to make VXLAN header
>     validation configurable"), IFLA_IPTUN_FAN_MAP and IFLA_VXLAN_FAN_MAP
>     shared the same integer value, allowing them to be used interchangeably
>     without issue, even though they represented attributes for different
>     link types. The introduction of IFLA_VXLAN_RESERVED_BITS led to
>     IFLA_VXLAN_FAN_MAP's integer value being incremented by 1 (33 to 34).
>     Thus the presence of attribute IFLA_VXLAN_FAN_MAP is checked but parsing
>     of the fan-map is attempted by accessing IFLA_IPTUN_FAN_MAP, causing a
>     NULL pointer dereference when creating a VXLAN device with a Fan
>     mapping.
>     
>     This is resolved by adjusting the vxlan_parse_fan_map() function to
>     access the correct IFLA_VXLAN_FAN_MAP attribute instead of
>     IFLA_IPTUN_FAN_MAP.
>     
>     Fixes: 9ce64bb8afd8 ("UBUNTU: SAUCE: fan: add VXLAN implementation")
>     Signed-off-by: Jacob Martin <jacob.martin at canonical.com>
> 
> [Test Plan]
> 
> The NULL pointer dereference can be reproduced 100% of the time with the
> following:
> # ip link add vxlan0 type vxlan dstport 0 local 192.168.0.1 id 16384000 fan-map 240.0.0.0/8:192.168.0.0/16
> 
> Thus, this can be used to easily verify the issue was resolved.
> 
> I also ran the ubuntu_fan_smoke_test autotest test after patching the
> kernel, and verified that it now passes.
> 
> [Where problems could occur]
> 
> This change affects the vxlan driver, specifically the code that parses
> an optional Ubuntu Fan configuration. Issues could manifest as
> misbehavior of the vxlan driver.
> 
> Jacob Martin (1):
>   UBUNTU: SAUCE: fan: vxlan: parse fan-map from IFLA_VXLAN_FAN_MAP
>     attribute ID
> 
>  drivers/net/vxlan/vxlan_core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> -- 
> 2.43.0
> 
> 
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