CVE-2026-98969: net/sched: em_text: reject unterminated algo before autoload

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

net/sched: emtext: reject unterminated algo before autoload

This is a follow-up to commit 9c572a83037a ("net/sched: fix potential stack infoleak in emtextdump()"), which zero-initialised the dump-side struct. Sashiko pointed at a pre-existing bug that exposed the change-side input-validation gap open.

emtextchange() casts the raw netlink attribute payload to struct tcfemtext and passes conf->algo, a 16-byte array with no enforced NUL terminator, to textsearchprepare(). On the TSAUTOLOAD retry textsearchprepare() calls requestmodule("ts%s", algo); vsnprintf() walks %s until it finds a NUL, so an algo[] with no NUL reads past the array into the adjacent struct fields and payload and feeds those bytes to the usermode helper command line. This is an out-of-bounds read and a kernel memory disclosure.

Reject the attribute when no NUL exists within TCEMTEXTALGOSIZ bytes, matching the check xtstring has had since 3ab720881b6e.

Conditions to recreate the bug: - CAPNETADMIN on the target netns (namespace-local via unshare -Urn is enough) - install clsact on an interface, then add a basic filter with one text ematch whose algo[] is 16 bytes with no NUL (e.g. "A"16) and patternlen at least 1 - the add fails with -ENOENT and the kernel invokes modprobe with a module name made of "ts" plus the 16 bytes and the adjacent payload