[PATCH] firewire: nosy: don't read packets bigger than requested

[Jann Horn <jannh@google.com>]

In general, accessing userspace memory beyond the length of the supplied
buffer in VFS read/write handlers can lead to both kernel memory corruption
(via kernel_read()/kernel_write(), which can e.g. be triggered via
sys_splice()) and privilege escalation inside userspace.

Fixes: 286468210d83 ("firewire: new driver: nosy - IEEE 1394 traffic sniffer")
Signed-off-by: Jann Horn <jannh@google.com>
---
No CC stable because this device shouldn't be available to unprivileged
code by default and should be pretty rare.

 drivers/firewire/nosy.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/firewire/nosy.c b/drivers/firewire/nosy.c
index a128dd1126ae..732075fc312e 100644
--- a/drivers/firewire/nosy.c
+++ b/drivers/firewire/nosy.c
@@ -161,11 +161,12 @@ packet_buffer_get(struct client *client, char __user *data, size_t user_length)
 	if (atomic_read(&buffer->size) == 0)
 		return -ENODEV;
 
-	/* FIXME: Check length <= user_length. */
-
 	end = buffer->data + buffer->capacity;
 	length = buffer->head->length;
 
+	if (length > user_length)
+		return -EINVAL;
+
 	if (&buffer->head->data[length] < end) {
 		if (copy_to_user(data, buffer->head->data, length))
 			return -EFAULT;
-- 
2.18.0.399.gad0ab374a1-goog