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Subject[PATCH v2 2/2] 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 | 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
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