KEYS: Avoid false positive ENOMEM error on key read

[ Upstream commit 4f0882491a148059a52480e753b7f07fc550e188 ]

By allocating a kernel buffer with a 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.

Moreover, if the buffer length is larger than the maximum amount of
memory that can be returned by kmalloc() (2^(MAX_ORDER-1) number of
pages), a warning message will also be printed.

To reduce this possibility, we set a threshold (PAGE_SIZE) over which we
do check the actual key length first before allocating a buffer of the
right size to hold it. The threshold is arbitrary, it is just used to
trigger a buffer length check. It does not limit the actual key length
as long as there is enough memory to satisfy the memory request.

To further avoid large buffer allocation failure due to page
fragmentation, kvmalloc() is used to allocate the buffer so that vmapped
pages can be used when there is not a large enough contiguous set of
pages available for allocation.

In the extremely unlikely scenario that the key keeps on being changed
and made longer (still <= buflen) in between 2 __keyctl_read_key()
calls, the __keyctl_read_key() calling loop in keyctl_read_key() may
have to be iterated a large number of times, but definitely not infinite.

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/security/keys/internal.h b/security/keys/internal.h
index e3a573840186667bed09d8e1c790245abec34e67..124273e500cfa7d157b21b089d418cd934ff2222 100644 (file)
--- a/security/keys/internal.h
+++ b/security/keys/internal.h
@@ -20,6 +20,8 @@
 #include <linux/keyctl.h>
 #include <linux/refcount.h>
 #include <linux/compat.h>
+#include <linux/mm.h>
+#include <linux/vmalloc.h>
 
 struct iovec;
 
@@ -305,4 +307,14 @@ static inline void key_check(const struct key *key)
 
 #endif
 
+/*
+ * Helper function to clear and free a kvmalloc'ed memory object.
+ */
+static inline void __kvzfree(const void *addr, size_t len)
+{
+       if (addr) {
+               memset((void *)addr, 0, len);
+               kvfree(addr);
+       }
+}
 #endif /* _INTERNAL_H */

diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index 4b6a084e323b57bd4cdffd30b0e03b73336f98f1..c07c2e2b2478399328d3d8b445adc2d837079257 100644 (file)
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -330,7 +330,7 @@ long keyctl_update_key(key_serial_t id,
        payload = NULL;
        if (plen) {
                ret = -ENOMEM;
-               payload = kmalloc(plen, GFP_KERNEL);
+               payload = kvmalloc(plen, GFP_KERNEL);
                if (!payload)
                        goto error;
 
@@ -351,7 +351,7 @@ long keyctl_update_key(key_serial_t id,
 
        key_ref_put(key_ref);
 error2:
-       kzfree(payload);
+       __kvzfree(payload, plen);
 error:
        return ret;
 }
@@ -772,7 +772,8 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
        struct key *key;
        key_ref_t key_ref;
        long ret;
-       char *key_data;
+       char *key_data = NULL;
+       size_t key_data_len;
 
        /* find the key first */
        key_ref = lookup_user_key(keyid, 0, 0);
@@ -823,24 +824,51 @@ can_read_key:
         * Allocating a temporary buffer to hold the keys before
         * transferring them to user buffer to avoid potential
         * deadlock involving page fault and mmap_sem.
+        *
+        * key_data_len = (buflen <= PAGE_SIZE)
+        *              ? buflen : actual length of key data
+        *
+        * This prevents allocating arbitrary large buffer which can
+        * be much larger than the actual key length. In the latter case,
+        * at least 2 passes of this loop is required.
         */
-       key_data = kmalloc(buflen, GFP_KERNEL);
+       key_data_len = (buflen <= PAGE_SIZE) ? buflen : 0;
+       for (;;) {
+               if (key_data_len) {
+                       key_data = kvmalloc(key_data_len, GFP_KERNEL);
+                       if (!key_data) {
+                               ret = -ENOMEM;
+                               goto key_put_out;
+                       }
+               }
 
-       if (!key_data) {
-               ret = -ENOMEM;
-               goto key_put_out;
-       }
-       ret = __keyctl_read_key(key, key_data, buflen);
+               ret = __keyctl_read_key(key, key_data, key_data_len);
+
+               /*
+                * Read methods will just return the required length without
+                * any copying if the provided length isn't large enough.
+                */
+               if (ret <= 0 || ret > buflen)
+                       break;
+
+               /*
+                * The key may change (unlikely) in between 2 consecutive
+                * __keyctl_read_key() calls. In this case, we reallocate
+                * a larger buffer and redo the key read when
+                * key_data_len < ret <= buflen.
+                */
+               if (ret > key_data_len) {
+                       if (unlikely(key_data))
+                               __kvzfree(key_data, key_data_len);
+                       key_data_len = ret;
+                       continue;       /* Allocate buffer */
+               }
 
-       /*
-        * Read methods will just return the required length without
-        * any copying if the provided length isn't large enough.
-        */
-       if (ret > 0 && ret <= buflen) {
                if (copy_to_user(buffer, key_data, ret))
                        ret = -EFAULT;
+               break;
        }
-       kzfree(key_data);
+       __kvzfree(key_data, key_data_len);
 
 key_put_out:
        key_put(key);