[PATCH net 2/3] netconsole: avoid clobbering userdatum value on truncated write

[Breno Leitao <leitao@debian.org>]

userdatum_value_store() bounds count by MAX_EXTRADATA_VALUE_LEN (200)
and then copies straight into udm->value, which is itself 200 bytes:

	if (count > MAX_EXTRADATA_VALUE_LEN)
		return -EMSGSIZE;
	...
	ret = strscpy(udm->value, buf, sizeof(udm->value));
	if (ret < 0)
		goto out_unlock;

If userspace writes exactly MAX_EXTRADATA_VALUE_LEN bytes with no NUL
within them, strscpy() copies 199 bytes plus a NUL into udm->value and
returns -E2BIG. The function jumps to out_unlock and reports the error
to userspace, but udm->value has already been overwritten with the
truncated string and update_userdata() is skipped, so the corruption
is not yet visible on the wire.

The next successful write to any userdatum entry under the same target
calls update_userdata(), which packs udm->value into the active
netconsole payload. From that point on, every netconsole message
carries the silently truncated value, and userspace has no indication
that a previous, error-returning write left state behind.

Tighten the entry check from "count > MAX_EXTRADATA_VALUE_LEN" to
"count >= MAX_EXTRADATA_VALUE_LEN". With count strictly less than
sizeof(udm->value), strscpy() can no longer return -E2BIG here, so
the corrupting truncation path is removed entirely.

Fixes: 8a6d5fec6c7f ("net: netconsole: add a userdata config_group member to netconsole_target")
Signed-off-by: Breno Leitao <leitao@debian.org>
---
 drivers/net/netconsole.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 5713cb3783ef2..4bef003d9df64 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -1074,7 +1074,7 @@ static ssize_t userdatum_value_store(struct config_item *item, const char *buf,
 	struct userdata *ud;
 	ssize_t ret;
 
-	if (count > MAX_EXTRADATA_VALUE_LEN)
+	if (count >= MAX_EXTRADATA_VALUE_LEN)
 		return -EMSGSIZE;
 
 	mutex_lock(&netconsole_subsys.su_mutex);

-- 
2.52.0