[RFC 2/5] security: smack: avoid kmalloc() in smk_parse_long_rule()

[Tomasz Stanislawski]

Function smk_parse_long_rule() allocates a number of temporary strings on heap
(kmalloc cache). Moreover, the sizes of those allocations might be large if
user calls write() for a long chunk. A big kmalloc triggers a heavy reclaim
havoc and it is very likely to fail.

This patch introduces smk_parse_substrings() function that parses a string into
substring separated by whitespaces.  The buffer for substring is preallocated.
It must store substring the worst case scenario which is SMK_LOAD2LEN in case
of long rule parsing.

The buffer is allocated on stack what is slightly faster than kmalloc().

Signed-off-by: Tomasz Stanislawski <t.stanislaws@xxxxxxxxxxx>
---
 security/smack/smackfs.c |   67 +++++++++++++++++++++++-----------------------
 1 file changed, 33 insertions(+), 34 deletions(-)

diff --git a/security/smack/smackfs.c b/security/smack/smackfs.c
index 9a3cd0d..46f111e 100644
--- a/security/smack/smackfs.c
+++ b/security/smack/smackfs.c
@@ -364,6 +364,33 @@ static int smk_parse_rule(const char *data, struct smack_parsed_rule *rule,
 }
 
 /**
+ * smk_parse_strings - parse white-space separated substring from a string
+ * @src: a long string to be parsed, null terminated
+ * @dst: a buffer for substrings, should be at least strlen(src)+1 bytes
+ * @args: table for parsed substring
+ * @size: number of slots in args table
+ *
+ * Returns number of parsed substrings
+ */
+static int smk_parse_substrings(const char *src, char *dst,
+	char *args[], int size)
+{
+	int cnt;
+
+	for (cnt = 0; cnt < size; ++cnt) {
+		src = skip_spaces(src);
+		if (*src == 0)
+			break;
+		args[cnt] = dst;
+		for (; *src && !isspace(*src); ++src, ++dst)
+			*dst = *src;
+		*(dst++) = 0;
+	}
+
+	return cnt;
+}
+
+/**
  * smk_parse_long_rule - parse Smack rule from rule string
  * @data: string to be parsed, null terminated
  * @rule: Will be filled with Smack parsed rule
@@ -375,48 +402,20 @@ static int smk_parse_rule(const char *data, struct smack_parsed_rule *rule,
 static int smk_parse_long_rule(const char *data, struct smack_parsed_rule *rule,
 				int import, int change)
 {
-	char *subject;
-	char *object;
-	char *access1;
-	char *access2;
-	int datalen;
+	char tmp[SMK_LOAD2LEN + 1];
+	char *args[4];
 	int rc = -1;
 
-	/* This is inefficient */
-	datalen = strlen(data);
-
-	/* Our first element can be 64 + \0 with no spaces */
-	subject = kzalloc(datalen + 1, GFP_KERNEL);
-	if (subject == NULL)
-		return -1;
-	object = kzalloc(datalen, GFP_KERNEL);
-	if (object == NULL)
-		goto free_out_s;
-	access1 = kzalloc(datalen, GFP_KERNEL);
-	if (access1 == NULL)
-		goto free_out_o;
-	access2 = kzalloc(datalen, GFP_KERNEL);
-	if (access2 == NULL)
-		goto free_out_a;
-
 	if (change) {
-		if (sscanf(data, "%s %s %s %s",
-			subject, object, access1, access2) == 4)
-			rc = smk_fill_rule(subject, object, access1, access2,
+		if (smk_parse_substrings(data, tmp, args, 4) == 4)
+			rc = smk_fill_rule(args[0], args[1], args[2], args[3],
 				rule, import, 0);
 	} else {
-		if (sscanf(data, "%s %s %s", subject, object, access1) == 3)
-			rc = smk_fill_rule(subject, object, access1, NULL,
+		if (smk_parse_substrings(data, tmp, args, 3) == 3)
+			rc = smk_fill_rule(args[0], args[1], args[2], NULL,
 				rule, import, 0);
 	}
 
-	kfree(access2);
-free_out_a:
-	kfree(access1);
-free_out_o:
-	kfree(object);
-free_out_s:
-	kfree(subject);
 	return rc;
 }
 
-- 
1.7.9.5

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