[PATCH] net/sched: fix potential stack infoleak in em_text_dump()

[PATCH] net/sched: fix potential stack infoleak in em_text_dump()

[Bernard Ladenthin]

em_text_dump() allocates struct tcf_em_text on the stack without zeroing
it.  strscpy() writes the algorithm name and a NUL terminator into
conf.algo[], leaving the remaining bytes uninitialised.  nla_put_nohdr()
then copies the full struct to the netlink response.

KMSAN on Linux 7.2-rc6 reports two kernel-infoleak splats from this path,
one triggered via "tc filter show" and one via a raw RTM_GETTFILTER dump:

  BUG: KMSAN: kernel-infoleak in _copy_to_iter+0x1c9/0x2620
    nla_put_nohdr+0x83/0x130
    em_text_dump+0x291/0x550
  Local variable conf created at: em_text_dump+0x5d/0x550
  Bytes 168-179 of 199 are uninitialized

I am not certain whether this constitutes a real security problem in
practice: the test was conducted in a controlled KMSAN environment and
the leaked stack bytes may or may not carry sensitive data on actual
production kernels.  I am reporting it because KMSAN flagged it as a
kernel-infoleak and the fix is straightforward.  I can provide a
userspace reproducer on request.

The original code used strncpy() which zero-pads to the destination size.
Commit b04202d6065c ("net/sched: replace strncpy with strscpy") replaced
it with strscpy(), which does not pad, creating this condition.
Zero-initialising the struct closes it.

Fixes: b04202d6065c ("net/sched: replace strncpy with strscpy")
Link: https://lore.kernel.org/netdev/20250327143733.187438-1-richard120310@gmail.com/
Assisted-by: Claude:claude-sonnet-4-6 [KMSAN]
Signed-off-by: Bernard Ladenthin <bernard.ladenthin@gmail.com>
---
 net/sched/em_text.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sched/em_text.c b/net/sched/em_text.c
index 343f1aebeec2..4132f8c3c5fc 100644
--- a/net/sched/em_text.c
+++ b/net/sched/em_text.c
@@ -113,7 +113,7 @@ static void em_text_destroy(struct tcf_ematch *m)
 static int em_text_dump(struct sk_buff *skb, struct tcf_ematch *m)
 {
 	struct text_match *tm = EM_TEXT_PRIV(m);
-	struct tcf_em_text conf;
+	struct tcf_em_text conf = {};
 
 	strscpy(conf.algo, tm->config->ops->name);
 	conf.from_offset = tm->from_offset;
-- 
2.49.0.windows.1