From: Waiman Long <longman@redhat.com>
To: David Howells <dhowells@redhat.com>,
Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
James Morris <jmorris@namei.org>,
"Serge E. Hallyn" <serge@hallyn.com>,
Mimi Zohar <zohar@linux.ibm.com>
Cc: keyrings@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-security-module@vger.kernel.org,
linux-integrity@vger.kernel.org,
Sumit Garg <sumit.garg@linaro.org>,
Jerry Snitselaar <jsnitsel@redhat.com>,
Roberto Sassu <roberto.sassu@huawei.com>,
Eric Biggers <ebiggers@google.com>,
Chris von Recklinghausen <crecklin@redhat.com>,
Waiman Long <longman@redhat.com>
Subject: [PATCH v2 2/2] KEYS: Avoid false positive ENOMEM error on key read
Date: Sun, 8 Mar 2020 13:04:10 -0400 [thread overview]
Message-ID: <20200308170410.14166-3-longman@redhat.com> (raw)
In-Reply-To: <20200308170410.14166-1-longman@redhat.com>
By allocating a kernel buffer with an user-supplied buffer length, it
is possible that a false positive ENOMEM error may be returned because
the user-supplied length is just too large even if the system do have
enough memory to hold the actual key data.
To reduce this possibility, we set a threshold (1024) over which we
do check the actual key length first before allocating a buffer of the
right size to hold it.
Signed-off-by: Waiman Long <longman@redhat.com>
---
security/keys/keyctl.c | 46 ++++++++++++++++++++++++++++++++----------
1 file changed, 35 insertions(+), 11 deletions(-)
diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index 89a14e71eb0a..662a638a680d 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -855,28 +855,52 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
* deadlock involving page fault and mmap_sem.
*/
char *tmpbuf = NULL;
+ size_t tbuflen = buflen;
- if (buffer && buflen) {
- tmpbuf = kmalloc(buflen, GFP_KERNEL);
+ /*
+ * We don't want an erronous -ENOMEM error due to an
+ * arbitrary large user-supplied buflen. So if buflen
+ * exceeds a threshold (1024 bytes in this case), we call
+ * the read method twice. The first time to get the buffer
+ * length and the second time to read out the key data.
+ *
+ * N.B. All the read methods will return the required
+ * buffer length with a NULL input buffer or when
+ * the input buffer length isn't large enough.
+ */
+ if (buflen && buffer && (buflen <= 0x400)) {
+allocbuf:
+ tmpbuf = kmalloc(tbuflen, GFP_KERNEL);
if (!tmpbuf) {
ret = -ENOMEM;
goto error2;
}
}
+
down_read(&key->sem);
ret = key_validate(key);
if (ret == 0)
- ret = key->type->read(key, tmpbuf, buflen);
+ ret = key->type->read(key, tmpbuf, tbuflen);
up_read(&key->sem);
- /*
- * Read methods will just return the required length
- * without any copying if the provided length isn't big
- * enough.
- */
- if ((ret > 0) && (ret <= buflen) && buffer &&
- copy_to_user(buffer, tmpbuf, ret))
- ret = -EFAULT;
+ if ((ret > 0) && (ret <= buflen) && buffer) {
+ /*
+ * It is possible, though unlikely, that the key
+ * changes in between the up_read->down_read period.
+ * If the key becomes longer, we will have to
+ * allocate a larger buffer and redo the key read
+ * again.
+ */
+ if (!tmpbuf || unlikely(ret > tbuflen)) {
+ tbuflen = ret;
+ if (unlikely(tmpbuf))
+ kzfree(tmpbuf);
+ goto allocbuf;
+ }
+
+ if (copy_to_user(buffer, tmpbuf, ret))
+ ret = -EFAULT;
+ }
if (tmpbuf)
kzfree(tmpbuf);
--
2.18.1
next prev parent reply other threads:[~2020-03-08 17:04 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-08 17:04 [PATCH v2 0/2] KEYS: Read keys to internal buffer & then copy to userspace Waiman Long
2020-03-08 17:04 ` [PATCH v2 1/2] KEYS: Don't write out to userspace while holding key semaphore Waiman Long
2020-03-13 1:04 ` Jarkko Sakkinen
2020-03-13 13:29 ` Waiman Long
2020-03-13 15:28 ` Jarkko Sakkinen
2020-03-13 16:57 ` Waiman Long
2020-03-08 17:04 ` Waiman Long [this message]
2020-03-09 16:32 ` [PATCH v2 2/2] KEYS: Avoid false positive ENOMEM error on key read David Howells
2020-03-10 15:45 ` Waiman Long
2020-03-10 15:58 ` Waiman Long
2020-03-10 17:12 ` David Howells
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