[PATCH] params: Fix an overflow in param_attr_show

[Jean Delvare <jdelvare@suse.de>]

Function param_attr_show could overflow the buffer it is operating
on. The buffer size is PAGE_SIZE, and the string returned by
attribute->param->ops->get is generated by scnprintf(buffer,
PAGE_SIZE, ...) so it could be PAGE_SIZE - 1 long, with the
terminating '\0' at the very end of the buffer. Calling
strcat(..., "\n") on this isn't safe, as the '\0' will be replaced
by '\n' (OK) and then another '\0' will be added past the end of
the buffer (not OK.)

Simply add the trailing '\n' when writing the attribute contents to
the buffer originally. This is safe, and also faster.

Credits to Teradata for discovering this issue.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
---
 kernel/params.c |   22 +++++++++-------------
 1 file changed, 9 insertions(+), 13 deletions(-)

--- linux-4.13.orig/kernel/params.c	2017-09-19 16:07:18.794254776 +0200
+++ linux-4.13/kernel/params.c	2017-09-19 16:12:57.398426205 +0200
@@ -236,14 +236,14 @@ char *parse_args(const char *doing,
 	EXPORT_SYMBOL(param_ops_##name)
 
 
-STANDARD_PARAM_DEF(byte, unsigned char, "%hhu", kstrtou8);
-STANDARD_PARAM_DEF(short, short, "%hi", kstrtos16);
-STANDARD_PARAM_DEF(ushort, unsigned short, "%hu", kstrtou16);
-STANDARD_PARAM_DEF(int, int, "%i", kstrtoint);
-STANDARD_PARAM_DEF(uint, unsigned int, "%u", kstrtouint);
-STANDARD_PARAM_DEF(long, long, "%li", kstrtol);
-STANDARD_PARAM_DEF(ulong, unsigned long, "%lu", kstrtoul);
-STANDARD_PARAM_DEF(ullong, unsigned long long, "%llu", kstrtoull);
+STANDARD_PARAM_DEF(byte, unsigned char, "%hhu\n", kstrtou8);
+STANDARD_PARAM_DEF(short, short, "%hi\n", kstrtos16);
+STANDARD_PARAM_DEF(ushort, unsigned short, "%hu\n", kstrtou16);
+STANDARD_PARAM_DEF(int, int, "%i\n", kstrtoint);
+STANDARD_PARAM_DEF(uint, unsigned int, "%u\n", kstrtouint);
+STANDARD_PARAM_DEF(long, long, "%li\n", kstrtol);
+STANDARD_PARAM_DEF(ulong, unsigned long, "%lu\n", kstrtoul);
+STANDARD_PARAM_DEF(ullong, unsigned long long, "%llu\n", kstrtoull);
 
 int param_set_charp(const char *val, const struct kernel_param *kp)
 {
@@ -270,7 +270,7 @@ EXPORT_SYMBOL(param_set_charp);
 
 int param_get_charp(char *buffer, const struct kernel_param *kp)
 {
-	return scnprintf(buffer, PAGE_SIZE, "%s", *((char **)kp->arg));
+	return scnprintf(buffer, PAGE_SIZE, "%s\n", *((char **)kp->arg));
 }
 EXPORT_SYMBOL(param_get_charp);
 
@@ -549,10 +549,6 @@ static ssize_t param_attr_show(struct mo
 	kernel_param_lock(mk->mod);
 	count = attribute->param->ops->get(buf, attribute->param);
 	kernel_param_unlock(mk->mod);
-	if (count > 0) {
-		strcat(buf, "\n");
-		++count;
-	}
 	return count;
 }
 


-- 
Jean Delvare
SUSE L3 Support