This is a follow-up to commit 9c572a83037a ("net/sched: fix potential stack infoleak in em_text_dump()"), which zero-initialised the dump-side struct. Sashiko pointed at a pre-existing bug that exposed the change-side input-validation gap open. em_text_change() casts the raw netlink attribute payload to struct tcf_em_text and passes conf->algo, a 16-byte array with no enforced NUL terminator, to textsearch_prepare(). On the TS_AUTOLOAD retry textsearch_prepare() calls request_module("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 TC_EM_TEXT_ALGOSIZ bytes, matching the check xt_string has had since 3ab720881b6e. Conditions to recreate the bug: - CAP_NET_ADMIN 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 pattern_len 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 Fixes: d675c989ed2d4 ("[PKT_SCHED]: Packet classification based on textsearch (ematch)") Reported-by: Sashiko (gemini) Closes: https://sashiko.dev/#/patchset/20260918133953.12494-1-bernard.ladenthin%40gmail.com Signed-off-by: Jamal Hadi Salim --- net/sched/em_text.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/net/sched/em_text.c b/net/sched/em_text.c index 4132f8c3c5fc..bc45edb8c03b 100644 --- a/net/sched/em_text.c +++ b/net/sched/em_text.c @@ -58,6 +58,9 @@ static int em_text_change(struct net *net, void *data, int len, if (len < sizeof(*conf) || len < (sizeof(*conf) + conf->pattern_len)) return -EINVAL; + if (!memchr(conf->algo, '\0', sizeof(conf->algo))) + return -EINVAL; + if (conf->from_layer > conf->to_layer) return -EINVAL; -- 2.43.0