CVE-2026-98302

net: fddi: skfp: fix NULL deref when setting the MAC address while down

Published Oct 6, 2026 CVSS 3.1 · 5.5 Medium EPSS < 1% KEV · Not listed
Description
In the Linux kernel, the following vulnerability has been resolved:

net: fddi: skfp: fix NULL deref when setting the MAC address while down

skfp_ctl_set_mac_address() calls ResetAdapter() unconditionally, without
checking netif_running(). ResetAdapter() first calls card_stop(), which
sets smc->hw.hw_state to STOPPED, and then mac_drv_clear_tx_queue(),
which walks the two transmit queues:

for (i = QUEUE_S; i <= QUEUE_A0; i++) {
queue = smc->hw.fp.tx[i] ;
...
t = queue->tx_curr_get ;

smc->hw.fp.tx[] is only populated by init_tx(), which is reached from
skfp_open() through init_smt() -> init_fddi_driver() -> init_fplus() ->
init_mac() -> init_tx(). The private area is allocated and zeroed by
alloc_fddidev(), so on an interface that has never been brought up both
queue pointers are still NULL. The hw_state test at the top of
mac_drv_clear_tx_queue() does not catch this, because card_stop() has
just set STOPPED; the function proceeds into the loop and dereferences
NULL. ResetAdapter() does call init_smt() itself, but only after the
queues have been cleared.

Setting the MAC address on a down interface therefore oopses:

ip link set dev fddi0 address 02:00:00:00:00:01

BUG: KASAN: null-ptr-deref in mac_drv_clear_tx_queue+0x68/0x2c0 [skfp]
Read of size 8 at addr 0000000000000010 by task ip/302
Call Trace:
<TASK>
mac_drv_clear_tx_queue+0x68/0x2c0 [skfp 6c01d4bab63c36978bd0a7d7e90837adb44cc37b]
ResetAdapter+0x29/0x100 [skfp 6c01d4bab63c36978bd0a7d7e90837adb44cc37b]
skfp_ctl_set_mac_address+0x57/0x80 [skfp 6c01d4bab63c36978bd0a7d7e90837adb44cc37b]
netif_set_mac_address+0x1e4/0x2c0
do_setlink+0x684/0x2680
</TASK>

Address 0x10 is the offset of tx_curr_get, the third pointer in
struct s_smt_tx_queue, on 64-bit. mac_drv_clear_rx_queue(), which
ResetAdapter() calls immediately afterwards, dereferences
smc->hw.fp.rx[QUEUE_R1] in the same way behind the same ineffective
hw_state test; the transmit queue merely crashes first. Both are
covered by the guard below.

Skip the adapter reset when the interface is down. dev_addr_set() is
left unconditional, so the new address is still recorded in
dev->dev_addr. Nothing is lost by not resetting the adapter here:
skfp_open() deliberately re-reads the factory address on every open,

read_address(smc, NULL);
eth_hw_addr_set(dev, smc->hw.fddi_canon_addr.a);

and the comment above it states this is done to discard exactly such an
address override across a close/open cycle. An address set while the
interface is down could not have survived the following open even
before this change, so the guard removes no working behaviour. Guarding
the hardware side of ndo_set_mac_address() with netif_running() is
established practice; skge_set_mac_address() has done so since commit
2eb3e621c4e0 ("skge: set mac address bonding fix").

Guarding the reset as a whole, rather than NULL-checking the queues, is
also what the rest of the driver expects. After a previous open/close
the queue pointers are stale but non-NULL, so there is no crash, yet
ResetAdapter() goes on to call smt_online() and STI_FBI() ("Enable
Board Interrupts") while skfp_close() has already called free_irq() -
the adapter would be brought back online with no handler installed. The
only other ResetAdapter() caller is skfp_interrupt(), which by
construction runs only while the device is open.

Found by automated driver testing against an emulated SysKonnect FDDI
adapter under a KASAN-enabled 7.0.0 kernel. Triggering it requires
CAP_NET_ADMIN.

Remediation

No solution or workaround provided in the CVE record

Tracking

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Advisories

Source ID Title
Debian DLA Debian DLA DLA-4841-1 linux security update
Debian DLA Debian DLA DLA-4850-1 linux-6.12 security update
Debian DSA Debian DSA DSA-6568-1 linux security update
Ubuntu USN Ubuntu USN USN-8961-1 Linux kernel vulnerabilities
Ubuntu USN Ubuntu USN USN-9019-1 Linux kernel (Azure) vulnerabilities
Ubuntu USN Ubuntu USN USN-9020-1 Linux kernel vulnerabilities
Ubuntu USN Ubuntu USN USN-9022-1 Linux kernel vulnerabilities
Ubuntu USN Ubuntu USN USN-9023-1 Linux kernel vulnerabilities
Ubuntu USN Ubuntu USN USN-9024-1 Linux kernel vulnerabilities
Ubuntu USN Ubuntu USN USN-9025-1 Linux kernel vulnerabilities
Ubuntu USN Ubuntu USN USN-9026-1 Linux kernel (Azure) vulnerabilities
Ubuntu USN Ubuntu USN USN-9029-1 Linux kernel (Raspberry Pi) vulnerabilities
Ubuntu USN Ubuntu USN USN-9039-1 Linux kernel (Azure CVM) vulnerabilities
Ubuntu USN Ubuntu USN USN-9040-1 Linux kernel (Low Latency NVIDIA) vulnerabilities
Ubuntu USN Ubuntu USN USN-9066-1 Linux kernel (HWE) vulnerabilities
Ubuntu USN Ubuntu USN USN-9091-1 Linux kernel (NVIDIA Tegra) vulnerabilities
History
Type Values Removed Values Added
References
Metrics threat_severity

None

cvssV3_1

{'score': 5.5, 'vector': 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H'}

threat_severity

Moderate

Type Values Removed Values Added
Weaknesses CWE-476
Type Values Removed Values Added
Weaknesses CWE-690
Type Values Removed Values Added
Weaknesses CWE-690
Type Values Removed Values Added
Description In the Linux kernel, the following vulnerability has been resolved: net: fddi: skfp: fix NULL deref when setting the MAC address while down skfp_ctl_set_mac_address() calls ResetAdapter() unconditionally, without checking netif_running(). ResetAdapter() first calls card_stop(), which sets smc->hw.hw_state to STOPPED, and then mac_drv_clear_tx_queue(), which walks the two transmit queues: for (i = QUEUE_S; i <= QUEUE_A0; i++) { queue = smc->hw.fp.tx[i] ; ... t = queue->tx_curr_get ; smc->hw.fp.tx[] is only populated by init_tx(), which is reached from skfp_open() through init_smt() -> init_fddi_driver() -> init_fplus() -> init_mac() -> init_tx(). The private area is allocated and zeroed by alloc_fddidev(), so on an interface that has never been brought up both queue pointers are still NULL. The hw_state test at the top of mac_drv_clear_tx_queue() does not catch this, because card_stop() has just set STOPPED; the function proceeds into the loop and dereferences NULL. ResetAdapter() does call init_smt() itself, but only after the queues have been cleared. Setting the MAC address on a down interface therefore oopses: ip link set dev fddi0 address 02:00:00:00:00:01 BUG: KASAN: null-ptr-deref in mac_drv_clear_tx_queue+0x68/0x2c0 [skfp] Read of size 8 at addr 0000000000000010 by task ip/302 Call Trace: <TASK> mac_drv_clear_tx_queue+0x68/0x2c0 [skfp 6c01d4bab63c36978bd0a7d7e90837adb44cc37b] ResetAdapter+0x29/0x100 [skfp 6c01d4bab63c36978bd0a7d7e90837adb44cc37b] skfp_ctl_set_mac_address+0x57/0x80 [skfp 6c01d4bab63c36978bd0a7d7e90837adb44cc37b] netif_set_mac_address+0x1e4/0x2c0 do_setlink+0x684/0x2680 </TASK> Address 0x10 is the offset of tx_curr_get, the third pointer in struct s_smt_tx_queue, on 64-bit. mac_drv_clear_rx_queue(), which ResetAdapter() calls immediately afterwards, dereferences smc->hw.fp.rx[QUEUE_R1] in the same way behind the same ineffective hw_state test; the transmit queue merely crashes first. Both are covered by the guard below. Skip the adapter reset when the interface is down. dev_addr_set() is left unconditional, so the new address is still recorded in dev->dev_addr. Nothing is lost by not resetting the adapter here: skfp_open() deliberately re-reads the factory address on every open, read_address(smc, NULL); eth_hw_addr_set(dev, smc->hw.fddi_canon_addr.a); and the comment above it states this is done to discard exactly such an address override across a close/open cycle. An address set while the interface is down could not have survived the following open even before this change, so the guard removes no working behaviour. Guarding the hardware side of ndo_set_mac_address() with netif_running() is established practice; skge_set_mac_address() has done so since commit 2eb3e621c4e0 ("skge: set mac address bonding fix"). Guarding the reset as a whole, rather than NULL-checking the queues, is also what the rest of the driver expects. After a previous open/close the queue pointers are stale but non-NULL, so there is no crash, yet ResetAdapter() goes on to call smt_online() and STI_FBI() ("Enable Board Interrupts") while skfp_close() has already called free_irq() - the adapter would be brought back online with no handler installed. The only other ResetAdapter() caller is skfp_interrupt(), which by construction runs only while the device is open. Found by automated driver testing against an emulated SysKonnect FDDI adapter under a KASAN-enabled 7.0.0 kernel. Triggering it requires CAP_NET_ADMIN.
Title net: fddi: skfp: fix NULL deref when setting the MAC address while down
First Time appeared Linux
Linux linux Kernel
CPEs cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
Vendors & Products Linux
Linux linux Kernel
References
Weaknesses

Sources

Status
PUBLISHED
Assigner
Linux
Published
Updated
Oct 19, 2026, 9:17 AM
Reserved
Sep 25, 2026, 10:25 AM
Status
Received
Published
Oct 6, 2026, 9:18 AM
Modified
Oct 19, 2026, 10:18 AM
Severity
Moderate
Public date
Oct 6, 2026, 12:00 AM