CVE-2026-64190: net: team: fix NULL pointer dereference in team_xmit during mode change

In the Linux kernel, the following vulnerability has been resolved:

net: team: fix NULL pointer dereference in teamxmit during mode change

teamchangemode() clears team->ops with memset() before restoring safe dummy handlers via teamadjustops(). A concurrent teamxmit() running under RCU on another CPU can read team->ops.transmit during this window and call a NULL function pointer, crashing the kernel.

The race requires a mode change (CAPNETADMIN) concurrent with transmit on the team device.

BUG: kernel NULL pointer dereference, address: 0000000000000000 Oops: 0010 [#1] SMP KASAN NOPTI RIP: 0010:0x0 Call Trace: teamxmit (drivers/net/team/teamcore.c:1853) devhardstartxmit (net/core/dev.c:3904) devqueuexmit (net/core/dev.c:4871) packetsendmsg (net/packet/afpacket.c:3109) syssendto (net/socket.c:2265)

The original code assumed that no ports means no traffic, so mode changes could freely memset()/memcpy() the ops. AFPACKET with forced carrier breaks that assumption.

Prevent the race instead of making it safe: replace memset()/memcpy() with per-field updates that never touch transmit or receive. Those two handlers are managed solely by teamadjustops(), which already installs dummies when txenportcount == 0 (always true during mode change since no ports are present). WRITEONCE/READONCE prevent store/load tearing on the handler pointers.

synchronizenet() before exitop() drains in-flight readers that may still reference old mode state from before port removal switched the handlers to dummies.