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

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

[Jakub Kicinski]

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>
---
CC: leitao@debian.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 d144787b2947..1b6a4135ec08 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -1679,7 +1679,8 @@ static void send_msg_no_fragmentation(struct netconsole_target *nt,
 	if (release_len) {
 		release = init_utsname()->release;
 
-		scnprintf(nt->buf, MAX_PRINT_CHUNK, "%s,%s", release, msg);
+		scnprintf(nt->buf, MAX_PRINT_CHUNK, "%s,%.*s", release,
+			  msg_len, msg);
 		msg_len += release_len;
 	} else {
 		memcpy(nt->buf, msg, msg_len);
-- 
2.53.0

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

[Simon Horman]

On Thu, Feb 19, 2026 at 11:50:21AM -0800, Jakub Kicinski wrote:
> 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>

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

[Breno Leitao]

On Sun, Feb 22, 2026 at 04:13:19PM +0000, Simon Horman wrote:
> On Thu, Feb 19, 2026 at 11:50:21AM -0800, Jakub Kicinski wrote:
> > 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>

Thanks for the fix and review while I was out.

--breno

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

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Paolo Abeni <pabeni@redhat.com>:

On Thu, 19 Feb 2026 11:50:21 -0800 you wrote:
> 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
> 
> [...]

Here is the summary with links:
  - [net] netconsole: avoid OOB reads, msg is not nul-terminated
    https://git.kernel.org/netdev/net/c/82aec772fca2

You are awesome, thank you!
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