Re: [PATCH v2 2/2] KEYS: Avoid false positive ENOMEM error on key read

[Waiman Long <longman@xxxxxxxxxx>]

On 3/10/20 1:12 PM, David Howells wrote:
> Waiman Long <longman@xxxxxxxxxx> wrote:
>
>> That is not as simple as I thought. First of that, there is not an
>> equivalent kzvfree() helper to clear the buffer first before clearing.
>> Of course, I can do that manually.
> Yeah, the actual substance of vfree() may get deferred.  It may be worth
> adding a kvzfree() that switches between kzfree() and memset(),vfree().
>
>> With patch 2, the allocated buffer length will be max(1024, keylen). The
>> security code uses kmalloc() for allocation. If we use kvalloc() here,
>> perhaps we should also use that for allocation that can be potentially
>> large like that in big_key. What do you think?
> Not for big_key: if it's larger than BIG_KEY_FILE_THRESHOLD (~1KiB) it gets
> written encrypted into shmem so that it can be swapped out to disk when not in
> use.
>
> However, other cases, sure - just be aware that on a 32-bit system,
> vmalloc/vmap space is a strictly limited resource.

Attached is an additional patch to make the transition from kmalloc() to
kvmalloc(). I put the __kvzfree() helper in internal.h for now. I plan
to send a patch later to add a kvzfree() API once there is a use case in
the kernel.

I am not going to touch other places for now to make thing simpler.

Cheers,
Longman

>From e2e73e2bc0c5cd168de273b0fe9df1e5c48cd232 Mon Sep 17 00:00:00 2001
From: Waiman Long <longman@xxxxxxxxxx>
Date: Wed, 11 Mar 2020 11:01:59 -0400
Subject: [PATCH v2 3/3] KEYS: Use kvmalloc() to better handle large buffer
 allocation

For large multi-page temporary buffer allocation, the security/keys
subsystem don't need contiguous physical pages. It will work perfectly
fine with virtually mapped pages.

Replace the kmalloc() call by kvmalloc() and provide a __kvzfree()
helper function to clear and free the kvmalloc'ed buffer. This will
reduce the chance of memory allocation failure just because of highly
fragmented pages.

Suggested-by: David Howells <dhowells@xxxxxxxxxx>
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 security/keys/internal.h | 14 ++++++++++++++
 security/keys/keyctl.c   | 12 ++++++------
 2 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/security/keys/internal.h b/security/keys/internal.h
index ba3e2da14cef..1b6e2d66e378 100644
--- a/security/keys/internal.h
+++ b/security/keys/internal.h
@@ -16,6 +16,8 @@
 #include <linux/keyctl.h>
 #include <linux/refcount.h>
 #include <linux/compat.h>
+#include <linux/mm.h>
+#include <linux/vmalloc.h>
 
 struct iovec;
 
@@ -349,4 +351,16 @@ 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 (is_vmalloc_addr(addr)) {
+		memset((char *)addr, 0, len);
+		vfree(addr);
+	} else {
+		kzfree(addr);
+	}
+}
 #endif /* _INTERNAL_H */
diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index 662a638a680d..ca05604bc9c0 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -339,7 +339,7 @@ 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;
 
@@ -360,7 +360,7 @@ long keyctl_update_key(key_serial_t id,
 
 	key_ref_put(key_ref);
 error2:
-	kzfree(payload);
+	__kvzfree(payload, plen);
 error:
 	return ret;
 }
@@ -870,7 +870,7 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
 		 */
 		if (buflen && buffer && (buflen <= 0x400)) {
 allocbuf:
-			tmpbuf = kmalloc(tbuflen, GFP_KERNEL);
+			tmpbuf = kvmalloc(tbuflen, GFP_KERNEL);
 			if (!tmpbuf) {
 				ret = -ENOMEM;
 				goto error2;
@@ -892,9 +892,9 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
 			 * again.
 			 */
 			if (!tmpbuf || unlikely(ret > tbuflen)) {
-				tbuflen = ret;
 				if (unlikely(tmpbuf))
-					kzfree(tmpbuf);
+					__kvzfree(tmpbuf, tbuflen);
+				tbuflen = ret;
 				goto allocbuf;
 			}
 
@@ -903,7 +903,7 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
 		}
 
 		if (tmpbuf)
-			kzfree(tmpbuf);
+			__kvzfree(tmpbuf, tbuflen);
 	}
 
 error2:
-- 
2.18.1