[PATCH 6.12] netconsole: avoid OOB reads, msg is not nul-terminated

[Markus Boehme <markus.boehme@mailbox.org>]

From: Jakub Kicinski <kuba@kernel.org>

[ Upstream commit 82aec772fca2223bc5774bd9af486fd95766e578 ]

msg passed to netconsole from the console subsystem is not guaranteed
to be nul-terminated. Before recent
commit 7eab73b18630 ("netconsole: convert to NBCON console infrastructure")
the message would be placed in printk_shared_pbufs, a static global
buffer, so KASAN had harder time catching OOB accesses. Now we see:

    printk: console [netcon_ext0] enabled
    BUG: KASAN: slab-out-of-bounds in string+0x1f7/0x240
    Read of size 1 at addr ffff88813b6d4c00 by task pr/netcon_ext0/594

    CPU: 65 UID: 0 PID: 594 Comm: pr/netcon_ext0 Not tainted 6.19.0-11754-g4246fd6547c9
    Call Trace:
     kasan_report+0xe4/0x120
     string+0x1f7/0x240
     vsnprintf+0x655/0xba0
     scnprintf+0xba/0x120
     netconsole_write+0x3fe/0xa10
     nbcon_emit_next_record+0x46e/0x860
     nbcon_kthread_func+0x623/0x750

    Allocated by task 1:
     nbcon_alloc+0x1ea/0x450
     register_console+0x26b/0xe10
     init_netconsole+0xbb0/0xda0

    The buggy address belongs to the object at ffff88813b6d4000
                which belongs to the cache kmalloc-4k of size 4096
    The buggy address is located 0 bytes to the right of
                allocated 3072-byte region [ffff88813b6d4000, ffff88813b6d4c00)

Fixes: c62c0a17f9b7 ("netconsole: Append kernel version to message")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260219195021.2099699-1-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
[ mb: adjusted context; in 6.12 the affected code lives in
  send_ext_msg_udp() and writes to a static buffer instead of nt->buf ]
Signed-off-by: Markus Boehme <markus.boehme@mailbox.org>
---
 drivers/net/netconsole.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 60375bb814a1..3b76ec3cd49b 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -1103,7 +1103,8 @@ static void send_ext_msg_udp(struct netconsole_target *nt, const char *msg,
 	if (msg_len + release_len + userdata_len <= MAX_PRINT_CHUNK) {
 		/* No fragmentation needed */
 		if (nt->release) {
-			scnprintf(buf, MAX_PRINT_CHUNK, "%s,%s", release, msg);
+			scnprintf(buf, MAX_PRINT_CHUNK, "%s,%.*s", release,
+				  msg_len, msg);
 			msg_len += release_len;
 		} else {
 			memcpy(buf, msg, msg_len);
-- 
2.55.0