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Subject[PATCH v3 2/3] KEYS: Avoid false positive ENOMEM error on key read
Date
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 | 48 ++++++++++++++++++++++++++++++++----------
1 file changed, 37 insertions(+), 11 deletions(-)

diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index 81f68e434b9f..a05a4dd2f9ce 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -877,24 +877,50 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
* transferring them to user buffer to avoid potential
* deadlock involving page fault and mmap_sem.
*/
- char *tmpbuf = kmalloc(buflen, GFP_KERNEL);
-
- if (!tmpbuf) {
- ret = -ENOMEM;
- goto error2;
- }
- ret = __keyctl_read_key(key, tmpbuf, buflen);
+ char *tmpbuf = NULL;
+ size_t tmpbuflen = buflen;

/*
- * Read methods will just return the required length
- * without any copying if the provided length isn't big
- * enough.
+ * 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 <= 0x400) {
+allocbuf:
+ tmpbuf = kmalloc(tmpbuflen, GFP_KERNEL);
+ if (!tmpbuf) {
+ ret = -ENOMEM;
+ goto error2;
+ }
+ }
+
+ ret = __keyctl_read_key(key, tmpbuf, tmpbuflen);
if ((ret > 0) && (ret <= buflen)) {
+ /*
+ * 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 > tmpbuflen)) {
+ if (unlikely(tmpbuf))
+ kzfree(tmpbuf);
+ tmpbuflen = ret;
+ goto allocbuf;
+ }
+
if (copy_to_user(buffer, tmpbuf, ret))
ret = -EFAULT;
}
- kzfree(tmpbuf);
+ if (tmpbuf)
+ kzfree(tmpbuf);
}

error2:
--
2.18.1
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