linux-kernel - [PATCH BUGFIX -rc4] Smackfs: Do not trust `count' in inodes write()s

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Date:	Sat, 8 Mar 2008 23:38:26 +0200
From:	"Ahmed S. Darwish" <darwish.07@...il.com>
To:	Casey Schaufler <casey@...aufler-ca.com>,
	akpm <akpm@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Linus <torvalds@...ux-foundation.org>
Subject: [PATCH BUGFIX -rc4] Smackfs: Do not trust `count' in inodes
	write()s

Hi all,

Smackfs write() implementation does not put a higher bound on the 
number of bytes to copy from user-space. This may lead to a DOS
attack if a malicious `count' field is given. 

Assure that given `count' is exactly the length needed for a 
/smack/load rule. In case of /smack/cipso where the length is 
relative, assure that `count' does not exceed the size needed for 
a buffer representing maximum possible number of CIPSO 2.2 
categories.

Signed-off-by: Ahmed S. Darwish <darwish.07@...il.com>
---

 smack.h   |    8 --------
 smackfs.c |   31 ++++++++++++++++++++-----------
 2 files changed, 20 insertions(+), 19 deletions(-)

Smack user-space utilities (smackload, smackcipso) still work 
fine after below change.

[
  Who's responsible for forwarding Smack patches to Linus ?
  I've CCed Andrew and Linus just in case.
]

diff --git a/security/smack/smack.h b/security/smack/smack.h
index c444f48..4a4477f 100644
--- a/security/smack/smack.h
+++ b/security/smack/smack.h
@@ -27,14 +27,6 @@
 #define SMK_MAXLEN	23
 #define SMK_LABELLEN	(SMK_MAXLEN+1)
 
-/*
- * How many kinds of access are there?
- * Here's your answer.
- */
-#define SMK_ACCESSDASH	'-'
-#define SMK_ACCESSLOW	"rwxa"
-#define SMK_ACCESSKINDS	(sizeof(SMK_ACCESSLOW) - 1)
-
 struct superblock_smack {
 	char		*smk_root;
 	char		*smk_floor;
diff --git a/security/smack/smackfs.c b/security/smack/smackfs.c
index effd3de..3fc3a07 100644
--- a/security/smack/smackfs.c
+++ b/security/smack/smackfs.c
@@ -81,10 +81,23 @@ static struct semaphore smack_write_sem;
 /*
  * Values for parsing cipso rules
  * SMK_DIGITLEN: Length of a digit field in a rule.
- * SMK_CIPSOMEN: Minimum possible cipso rule length.
+ * SMK_CIPSOMIN: Minimum possible cipso rule length.
+ * SMK_CIPSOMAX: Maximum possible cipso rule length.
  */
 #define SMK_DIGITLEN 4
-#define SMK_CIPSOMIN (SMK_MAXLEN + 2 * SMK_DIGITLEN)
+#define SMK_CIPSOMIN (SMK_LABELLEN + 2 * SMK_DIGITLEN)
+#define SMK_CIPSOMAX (SMK_CIPSOMIN + SMACK_CIPSO_MAXCATNUM * SMK_DIGITLEN)
+
+/*
+ * Values for parsing MAC rules
+ * SMK_ACCESS: Maximum possible combination of access permissions
+ * SMK_ACCESSLEN: Maximum length for a rule access field
+ * SMK_LOADLEN: Smack rule length
+ */
+#define SMK_ACCESS    "rwxa"
+#define SMK_ACCESSLEN (sizeof(SMK_ACCESS) - 1)
+#define SMK_LOADLEN   (SMK_LABELLEN + SMK_LABELLEN + SMK_ACCESSLEN)
+
 
 /*
  * Seq_file read operations for /smack/load
@@ -229,14 +242,10 @@ static void smk_set_access(struct smack_rule *srp)
  * The format is exactly:
  *     char subject[SMK_LABELLEN]
  *     char object[SMK_LABELLEN]
- *     char access[SMK_ACCESSKINDS]
- *
- *     Anything following is commentary and ignored.
+ *     char access[SMK_ACCESSLEN]
  *
- * writes must be SMK_LABELLEN+SMK_LABELLEN+4 bytes.
+ * writes must be SMK_LABELLEN+SMK_LABELLEN+SMK_ACCESSLEN bytes.
  */
-#define MINIMUM_LOAD (SMK_LABELLEN + SMK_LABELLEN + SMK_ACCESSKINDS)
-
 static ssize_t smk_write_load(struct file *file, const char __user *buf,
 			      size_t count, loff_t *ppos)
 {
@@ -253,7 +262,7 @@ static ssize_t smk_write_load(struct file *file, const char __user *buf,
 		return -EPERM;
 	if (*ppos != 0)
 		return -EINVAL;
-	if (count < MINIMUM_LOAD)
+	if (count != SMK_LOADLEN)
 		return -EINVAL;
 
 	data = kzalloc(count, GFP_KERNEL);
@@ -513,7 +522,7 @@ static ssize_t smk_write_cipso(struct file *file, const char __user *buf,
 		return -EPERM;
 	if (*ppos != 0)
 		return -EINVAL;
-	if (count <= SMK_CIPSOMIN)
+	if (count < SMK_CIPSOMIN || count > SMK_CIPSOMAX)
 		return -EINVAL;
 
 	data = kzalloc(count + 1, GFP_KERNEL);
@@ -547,7 +556,7 @@ static ssize_t smk_write_cipso(struct file *file, const char __user *buf,
 	if (ret != 1 || catlen > SMACK_CIPSO_MAXCATNUM)
 		goto out;
 
-	if (count <= (SMK_CIPSOMIN + catlen * SMK_DIGITLEN))
+	if (count != (SMK_CIPSOMIN + catlen * SMK_DIGITLEN))
 		goto out;
 
 	memset(mapcatset, 0, sizeof(mapcatset));

Regards,

-- 

"Better to light a candle, than curse the darkness"

Ahmed S. Darwish
Homepage: http://darwish.07.googlepages.com
Blog: http://darwish-07.blogspot.com

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