Re: [PATCH v3 3/3] KEYS: Use kvmalloc() to better handle large buffer allocation

On 3/13/20 12:43 PM, Eric Biggers wrote:

On Fri, Mar 13, 2020 at 11:21:02AM -0400, Waiman Long wrote:

quoted

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@redhat.com>
Signed-off-by: Waiman Long <longman@redhat.com>
---
 security/keys/internal.h | 14 ++++++++++++++
 security/keys/keyctl.c   | 10 +++++-----
 2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/security/keys/internal.h b/security/keys/internal.h
index ba3e2da14cef..855b11eb73ee 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((void *)addr, 0, len);
+		vfree(addr);
+	} else {
+		kzfree(addr);
+	}
+}

Since this takes the length as a parameter, it can be simplified to:

static inline void __kvzfree(const void *addr, size_t len)
{
	if (addr) {
		memset((void *)addr, 0, len);
		kvfree(addr);
	}
}

Yes, that will work too.

quoted

 			if (!tmpbuf || unlikely(ret > tmpbuflen)) {
 				if (unlikely(tmpbuf))
-					kzfree(tmpbuf);
+					__kvzfree(tmpbuf, tmpbuflen);

Both kzfree() and __kvzfree() handle a NULL pointer, so there's no need for the
NULL check first.

I would like to keep this one because of the unlikely annotation.

quoted

@@ -920,7 +920,7 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
 				ret = -EFAULT;
 		}
 		if (tmpbuf)
-			kzfree(tmpbuf);
+			__kvzfree(tmpbuf, tmpbuflen);

Likewise here.  No need for the NULL check.

Yes, that tmpbuf check is not really necessary, but it doesn't harm either.

My plan is to send out a mm patch to officially add the kvzfree()
function to mm/util.c. I will remove the tmpbuf check at that time if
you don't mind.

Cheers,
Longman