of: module: prevent NULL pointer dereference in vsnprintf()

In of_modalias(), we can get passed the str and len parameters which would
cause a kernel oops in vsnprintf() since it only allows passing a NULL ptr
when the length is also 0. Also, we need to filter out the negative values
of the len parameter as these will result in a really huge buffer since
snprintf() takes size_t parameter while ours is ssize_t...

Found by Linux Verification Center (linuxtesting.org) with the Svace static
analysis tool.

Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/1d211023-3923-685b-20f0-f3f90ea56e1f@omp.ru
Signed-off-by: Rob Herring <robh@kernel.org>

Author: Sergey Shtylyov

drivers/of/module.c

@@ -16,6 +16,14 @@ ssize_t of_modalias(const struct device_node *np, char *str, ssize_t len)
 	ssize_t csize;
 	ssize_t tsize;
 
+	/*
+	 * Prevent a kernel oops in vsnprintf() -- it only allows passing a
+	 * NULL ptr when the length is also 0. Also filter out the negative
+	 * lengths...
+	 */
+	if ((len > 0 && !str) || len < 0)
+		return -EINVAL;
+
 	/* Name & Type */
 	/* %p eats all alphanum characters, so %c must be used here */
 	csize = snprintf(str, len, "of:N%pOFn%c%s", np, 'T',