[RFC 1/5] security: smack: avoid kmalloc allocations while loading arule string

From: Tomasz Stanislawski
Date: Thu Jun 13 2013 - 11:29:37 EST


The maximal length for a rule line for long format is introduced as
SMK_LOAD2LEN. This allows a buffer for a rule string to be allocated
on a stack instead of a heap (aka kmalloc cache).

Limiting the length of a rule line helps to avoid allocations of a very long
contiguous buffer from a heap if user calls write() for a very long chunk.
Such an allocation often causes a lot swapper/writeback havoc and it is very
likely to fails.

Moreover, stack allocation is slightly faster than from kmalloc.

Signed-off-by: Tomasz Stanislawski <t.stanislaws@xxxxxxxxxxx>
---
security/smack/smackfs.c | 15 ++++++---------
1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/security/smack/smackfs.c b/security/smack/smackfs.c
index 53a08b8..9a3cd0d 100644
--- a/security/smack/smackfs.c
+++ b/security/smack/smackfs.c
@@ -137,6 +137,7 @@ const char *smack_cipso_option = SMACK_CIPSO_OPTION;
* SMK_ACCESS: Maximum possible combination of access permissions
* SMK_ACCESSLEN: Maximum length for a rule access field
* SMK_LOADLEN: Smack rule length
+ * SMK_LOAD2LEN: Smack maximal long rule length excluding \0
*/
#define SMK_OACCESS "rwxa"
#define SMK_ACCESS "rwxat"
@@ -144,6 +145,7 @@ const char *smack_cipso_option = SMACK_CIPSO_OPTION;
#define SMK_ACCESSLEN (sizeof(SMK_ACCESS) - 1)
#define SMK_OLOADLEN (SMK_LABELLEN + SMK_LABELLEN + SMK_OACCESSLEN)
#define SMK_LOADLEN (SMK_LABELLEN + SMK_LABELLEN + SMK_ACCESSLEN)
+#define SMK_LOAD2LEN (2 * SMK_LONGLABEL + SMK_ACCESSLEN + 2)

/*
* Stricly for CIPSO level manipulation.
@@ -447,8 +449,7 @@ static ssize_t smk_write_rules_list(struct file *file, const char __user *buf,
{
struct smack_known *skp;
struct smack_parsed_rule *rule;
- char *data;
- int datalen;
+ char data[SMK_LOAD2LEN + 1];
int rc = -EINVAL;
int load = 0;

@@ -465,13 +466,10 @@ static ssize_t smk_write_rules_list(struct file *file, const char __user *buf,
*/
if (count != SMK_OLOADLEN && count != SMK_LOADLEN)
return -EINVAL;
- datalen = SMK_LOADLEN;
- } else
- datalen = count + 1;
+ }

- data = kzalloc(datalen, GFP_KERNEL);
- if (data == NULL)
- return -ENOMEM;
+ if (count > SMK_LOAD2LEN)
+ count = SMK_LOAD2LEN;

if (copy_from_user(data, buf, count) != 0) {
rc = -EFAULT;
@@ -522,7 +520,6 @@ static ssize_t smk_write_rules_list(struct file *file, const char __user *buf,
out_free_rule:
kfree(rule);
out:
- kfree(data);
return rc;
}

--
1.7.9.5

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