[PATCH net 0/3] netconsole: configfs store callback fixes

[PATCH net 0/3] netconsole: configfs store callback fixes

[Breno Leitao]

This series fixes a small cluster of related issues in netconsole's
configfs store callbacks.

They showed up in sashiko and brought to my attention by Simon:

https://lore.kernel.org/all/20260421162219.GF651125@horms.kernel.org/

None are crashes or security problems, but each is a real correctness
bug that surfaces at boundary conditions and was easy to clean up while
the code was already under the microscope.

All three changes narrow the accepted write size by exactly one byte
at the boundary that was previously buggy, so no well-behaved
userspace should notice the tightening.

Signed-off-by: Breno Leitao <leitao@debian.org>
---
Breno Leitao (3):
      netconsole: return count instead of strnlen(buf, count) from store callbacks
      netconsole: avoid clobbering userdatum value on truncated write
      netconsole: propagate device name truncation in dev_name_store()

 drivers/net/netconsole.c | 31 +++++++++++++++++--------------
 1 file changed, 17 insertions(+), 14 deletions(-)
---
base-commit: 70c8a7ec6715b5fb14e501731b5b9210a16684f7
change-id: 20260422-netconsole_ai_fixes-24599337a79d

Best regards,
--  
Breno Leitao <leitao@debian.org>

[PATCH net 1/3] netconsole: return count instead of strnlen(buf, count) from store callbacks

[Breno Leitao]

Several configfs store callbacks in netconsole end with:

	ret = strnlen(buf, count);

This under-reports the number of bytes consumed when the input
contains an embedded NUL within count, telling the VFS that fewer
bytes were written than userspace actually handed in. A conformant
partial-write loop would then retry the trailing bytes against a
callback that has already accepted them.

Every other configfs driver in the tree returns count directly from
its store callbacks once parsing has succeeded, including
drivers/nvme/target/configfs.c, drivers/gpio/gpio-sim.c,
drivers/most/configfs.c, drivers/block/null_blk/main.c,
drivers/pci/endpoint/pci-ep-cfs.c, and the rest of the configfs
users. netconsole was the outlier (along with
drivers/infiniband/core/cma_configfs.c, which has the same latent
issue).

Align netconsole with the rest of the configfs ecosystem: return
count once the parser/validator has accepted the input. The numeric
and boolean parsers (kstrtobool, kstrtou16, mac_pton,
netpoll_parse_ip_addr) have already validated the meaningful prefix;
any trailing bytes are padding and should simply be reported as
consumed.

Fixes: 0bcc1816188e ("[NET] netconsole: Support dynamic reconfiguration using configfs")
Signed-off-by: Breno Leitao <leitao@debian.org>
---
 drivers/net/netconsole.c | 26 +++++++++++++-------------
 1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 3c9acd6e49e86..5713cb3783ef2 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -750,7 +750,7 @@ static ssize_t enabled_store(struct config_item *item,
 		unregister_netcons_consoles();
 	}
 
-	ret = strnlen(buf, count);
+	ret = count;
 	/* Deferred cleanup */
 	netconsole_process_cleanups();
 out_unlock:
@@ -779,7 +779,7 @@ static ssize_t release_store(struct config_item *item, const char *buf,
 
 	nt->release = release;
 
-	ret = strnlen(buf, count);
+	ret = count;
 out_unlock:
 	dynamic_netconsole_mutex_unlock();
 	return ret;
@@ -805,7 +805,7 @@ static ssize_t extended_store(struct config_item *item, const char *buf,
 		goto out_unlock;
 
 	nt->extended = extended;
-	ret = strnlen(buf, count);
+	ret = count;
 out_unlock:
 	dynamic_netconsole_mutex_unlock();
 	return ret;
@@ -828,7 +828,7 @@ static ssize_t dev_name_store(struct config_item *item, const char *buf,
 	trim_newline(nt->np.dev_name, IFNAMSIZ);
 
 	dynamic_netconsole_mutex_unlock();
-	return strnlen(buf, count);
+	return count;
 }
 
 static ssize_t local_port_store(struct config_item *item, const char *buf,
@@ -847,7 +847,7 @@ static ssize_t local_port_store(struct config_item *item, const char *buf,
 	ret = kstrtou16(buf, 10, &nt->np.local_port);
 	if (ret < 0)
 		goto out_unlock;
-	ret = strnlen(buf, count);
+	ret = count;
 out_unlock:
 	dynamic_netconsole_mutex_unlock();
 	return ret;
@@ -869,7 +869,7 @@ static ssize_t remote_port_store(struct config_item *item,
 	ret = kstrtou16(buf, 10, &nt->np.remote_port);
 	if (ret < 0)
 		goto out_unlock;
-	ret = strnlen(buf, count);
+	ret = count;
 out_unlock:
 	dynamic_netconsole_mutex_unlock();
 	return ret;
@@ -894,7 +894,7 @@ static ssize_t local_ip_store(struct config_item *item, const char *buf,
 		goto out_unlock;
 	nt->np.ipv6 = !!ipv6;
 
-	ret = strnlen(buf, count);
+	ret = count;
 out_unlock:
 	dynamic_netconsole_mutex_unlock();
 	return ret;
@@ -919,7 +919,7 @@ static ssize_t remote_ip_store(struct config_item *item, const char *buf,
 		goto out_unlock;
 	nt->np.ipv6 = !!ipv6;
 
-	ret = strnlen(buf, count);
+	ret = count;
 out_unlock:
 	dynamic_netconsole_mutex_unlock();
 	return ret;
@@ -955,7 +955,7 @@ static ssize_t remote_mac_store(struct config_item *item, const char *buf,
 		goto out_unlock;
 	memcpy(nt->np.remote_mac, remote_mac, ETH_ALEN);
 
-	ret = strnlen(buf, count);
+	ret = count;
 out_unlock:
 	dynamic_netconsole_mutex_unlock();
 	return ret;
@@ -1131,7 +1131,7 @@ static ssize_t sysdata_msgid_enabled_store(struct config_item *item,
 		disable_sysdata_feature(nt, SYSDATA_MSGID);
 
 unlock_ok:
-	ret = strnlen(buf, count);
+	ret = count;
 	dynamic_netconsole_mutex_unlock();
 	mutex_unlock(&netconsole_subsys.su_mutex);
 	return ret;
@@ -1160,7 +1160,7 @@ static ssize_t sysdata_release_enabled_store(struct config_item *item,
 		disable_sysdata_feature(nt, SYSDATA_RELEASE);
 
 unlock_ok:
-	ret = strnlen(buf, count);
+	ret = count;
 	dynamic_netconsole_mutex_unlock();
 	mutex_unlock(&netconsole_subsys.su_mutex);
 	return ret;
@@ -1189,7 +1189,7 @@ static ssize_t sysdata_taskname_enabled_store(struct config_item *item,
 		disable_sysdata_feature(nt, SYSDATA_TASKNAME);
 
 unlock_ok:
-	ret = strnlen(buf, count);
+	ret = count;
 	dynamic_netconsole_mutex_unlock();
 	mutex_unlock(&netconsole_subsys.su_mutex);
 	return ret;
@@ -1223,7 +1223,7 @@ static ssize_t sysdata_cpu_nr_enabled_store(struct config_item *item,
 		disable_sysdata_feature(nt, SYSDATA_CPU_NR);
 
 unlock_ok:
-	ret = strnlen(buf, count);
+	ret = count;
 	dynamic_netconsole_mutex_unlock();
 	mutex_unlock(&netconsole_subsys.su_mutex);
 	return ret;

-- 
2.52.0

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

[Breno Leitao]

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

[PATCH net 3/3] netconsole: propagate device name truncation in dev_name_store()

[Breno Leitao]

dev_name_store() calls strscpy(nt->np.dev_name, buf, IFNAMSIZ) without
checking the return value. If userspace writes an interface name longer
than IFNAMSIZ - 1, strscpy() silently truncates and returns -E2BIG, but
the function ignores it and reports a fully successful write back to
userspace.

If a real interface happens to match the truncated name, netconsole will
bind to the wrong device on the next enable, sending kernel logs and
panic output to an unintended network segment with no indication to
userspace that anything was rewritten.

Reject writes whose length cannot fit in nt->np.dev_name up front:

	if (count >= IFNAMSIZ)
		return -ENAMETOOLONG;

This is not a big deal of a problem, but, it is still the correct
approach.

Fixes: 0bcc1816188e57 ("[NET] netconsole: Support dynamic reconfiguration using configfs")
Signed-off-by: Breno Leitao <leitao@debian.org>
---
 drivers/net/netconsole.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 4bef003d9df64..3914fb90f9afd 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -816,6 +816,9 @@ static ssize_t dev_name_store(struct config_item *item, const char *buf,
 {
 	struct netconsole_target *nt = to_target(item);
 
+	if (count >= IFNAMSIZ)
+		return -ENAMETOOLONG;
+
 	dynamic_netconsole_mutex_lock();
 	if (nt->state == STATE_ENABLED) {
 		pr_err("target (%s) is enabled, disable to update parameters\n",

-- 
2.52.0