[PATCH 5.15.y] of: module: add buffer overflow check in of_modalias()

["Uwe Kleine-König" <ukleinek@debian.org>]

From: Sergey Shtylyov <s.shtylyov@omp.ru>

commit cf7385cb26ac4f0ee6c7385960525ad534323252 upstream.

In of_modalias(), if the buffer happens to be too small even for the 1st
snprintf() call, the len parameter will become negative and str parameter
(if not NULL initially) will point beyond the buffer's end. Add the buffer
overflow check after the 1st snprintf() call and fix such check after the
strlen() call (accounting for the terminating NUL char).

Fixes: bc575064d688 ("of/device: use of_property_for_each_string to parse compatible strings")
Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Link: https://lore.kernel.org/r/bbfc6be0-c687-62b6-d015-5141b93f313e@omp.ru
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Uwe Kleine-König <ukleinek@debian.org>
---
 drivers/of/device.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/of/device.c b/drivers/of/device.c
index 19c42a9dcba9..f503bb10b10b 100644
--- a/drivers/of/device.c
+++ b/drivers/of/device.c
@@ -257,14 +257,15 @@ static ssize_t of_device_get_modalias(struct device *dev, char *str, ssize_t len
 	csize = snprintf(str, len, "of:N%pOFn%c%s", dev->of_node, 'T',
 			 of_node_get_device_type(dev->of_node));
 	tsize = csize;
+	if (csize >= len)
+		csize = len > 0 ? len - 1 : 0;
 	len -= csize;
-	if (str)
-		str += csize;
+	str += csize;
 
 	of_property_for_each_string(dev->of_node, "compatible", p, compat) {
 		csize = strlen(compat) + 1;
 		tsize += csize;
-		if (csize > len)
+		if (csize >= len)
 			continue;
 
 		csize = snprintf(str, len, "C%s", compat);

base-commit: 16fdf2c7111bd6927f16c3e811f5086fecebbf00
-- 
2.47.2