Subject: Re: [PATCH net-next v1] hinic: fix strncpy output truncated compile warnings

From: Kees Cook <[email-masked]>
Date: Fri, 7 Aug 2020 23:44:51 -0700

On Fri, Aug 07, 2020 at 08:42:43PM -0700, David Miller wrote:
> From: "luobin (L)" <[email-masked]>
> Date: Sat, 8 Aug 2020 11:36:42 +0800
> 
> > On 2020/8/7 17:32, David Laight wrote:
> >>> diff --git a/drivers/net/ethernet/huawei/hinic/hinic_devlink.c
> >>> b/drivers/net/ethernet/huawei/hinic/hinic_devlink.c
> >>> index c6adc776f3c8..1ec88ebf81d6 100644
> >>> --- a/drivers/net/ethernet/huawei/hinic/hinic_devlink.c
> >>> +++ b/drivers/net/ethernet/huawei/hinic/hinic_devlink.c
> >>> @@ -342,9 +342,9 @@ static int chip_fault_show(struct devlink_fmsg *fmsg,
> >>>
> >>>  	level = event->event.chip.err_level;
> >>>  	if (level < FAULT_LEVEL_MAX)
> >>> -		strncpy(level_str, fault_level[level], strlen(fault_level[level]));
> >>> +		strncpy(level_str, fault_level[level], strlen(fault_level[level]) + 1);
> >> 
> >> Have you even considered what that code is actually doing?
>  ...
> > I'm sorry that I haven't got what you mean and I haven't found any defects in that code. Can you explain more to me?
> 
> David is trying to express the same thing I was trying to explain to
> you, you should use sizeof(level_str) as the third argument because
> the code is trying to make sure that the destination buffer is not
> overrun.
> 
> If you use the strlen() of the source buffer, the strncpy() can still
> overflow the destination buffer.
> 
> Now do you understand?

Agh, please never use strncpy() on NUL-terminated strings[1]. (You can
see this ultimately gets passed down into devlink_fmsg_string_put()
which expects NUL-terminated strings and does not require trailing
NUL-padding (which if it did, should still never use strncpy(), but
rather strscpy_pad()).

But, as David Laight hints, none of this is needed. The entire buffer
can be avoided (just point into the existing array of strings -- which
should also be const). Add I see that one of the array sizes is wrong.
Both use FAULT_TYPE_MAX, but one should be FAULT_LEVEL_MAX. And since
"Unknown" can just be added to the array, do that and clamp the value
since it's only used for finding the strings in the array.

I would suggest this (totally untested):

diff --git a/drivers/net/ethernet/huawei/hinic/hinic_devlink.c b/drivers/net/ethernet/huawei/hinic/hinic_devlink.c
index c6adc776f3c8..20bfb05896e5 100644
--- a/drivers/net/ethernet/huawei/hinic/hinic_devlink.c
+++ b/drivers/net/ethernet/huawei/hinic/hinic_devlink.c
@@ -334,18 +334,12 @@ void hinic_devlink_unregister(struct hinic_devlink_priv *priv)
 static int chip_fault_show(struct devlink_fmsg *fmsg,
 			   struct hinic_fault_event *event)
 {
-	char fault_level[FAULT_TYPE_MAX][FAULT_SHOW_STR_LEN + 1] = {
-		"fatal", "reset", "flr", "general", "suggestion"};
-	char level_str[FAULT_SHOW_STR_LEN + 1] = {0};
-	u8 level;
+	const char * const level_str[FAULT_LEVEL_MAX + 1] = {
+		"fatal", "reset", "flr", "general", "suggestion",
+		[FAULT_LEVEL_MAX] = "Unknown"};
+	u8 fault_level;
 	int err;
 
-	level = event->event.chip.err_level;
-	if (level < FAULT_LEVEL_MAX)
-		strncpy(level_str, fault_level[level], strlen(fault_level[level]));
-	else
-		strncpy(level_str, "Unknown", strlen("Unknown"));
-
 	if (level == FAULT_LEVEL_SERIOUS_FLR) {
 		err = devlink_fmsg_u32_pair_put(fmsg, "Function level err func_id",
 						(u32)event->event.chip.func_id);
@@ -361,7 +355,8 @@ static int chip_fault_show(struct devlink_fmsg *fmsg,
 	if (err)
 		return err;
 
-	err = devlink_fmsg_string_pair_put(fmsg, "err_level", level_str);
+	fault_level = clamp(event->event.chip.err_level, FAULT_LEVEL_MAX);
+	err = devlink_fmsg_string_pair_put(fmsg, "err_level", fault_str[fault_level]);
 	if (err)
 		return err;
 
@@ -381,18 +376,15 @@ static int chip_fault_show(struct devlink_fmsg *fmsg,
 static int fault_report_show(struct devlink_fmsg *fmsg,
 			     struct hinic_fault_event *event)
 {
-	char fault_type[FAULT_TYPE_MAX][FAULT_SHOW_STR_LEN + 1] = {
+	const char * const type_str[FAULT_TYPE_MAX + 1] = {
 		"chip", "ucode", "mem rd timeout", "mem wr timeout",
-		"reg rd timeout", "reg wr timeout", "phy fault"};
-	char type_str[FAULT_SHOW_STR_LEN + 1] = {0};
+		"reg rd timeout", "reg wr timeout", "phy fault",
+		[FAULT_TYPE_MAX] = "Unknown"};
+	u8 fault_type;
 	int err;
 
-	if (event->type < FAULT_TYPE_MAX)
-		strncpy(type_str, fault_type[event->type], strlen(fault_type[event->type]));
-	else
-		strncpy(type_str, "Unknown", strlen("Unknown"));
-
-	err = devlink_fmsg_string_pair_put(fmsg, "Fault type", type_str);
+	fault_type = clamp(event->type, FAULT_TYPE_MAX);
+	err = devlink_fmsg_string_pair_put(fmsg, "Fault type", type_str[fault_type]);
 	if (err)
 		return err;

-Kees

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings

-- 
Kees Cook