[PATCH net] net/sched: em_text: reject unterminated algo before autoload

[Jamal Hadi Salim <jhs@mojatatu.com>]

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) <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260918133953.12494-1-bernard.ladenthin%40gmail.com
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
---
 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