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SubjectRe: [PATCH v2 2/2] KEYS: Avoid false positive ENOMEM error on key read
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On 3/10/20 1:12 PM, David Howells wrote:
> Waiman Long <longman@redhat.com> 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@redhat.com>
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@redhat.com>
Signed-off-by: Waiman Long <longman@redhat.com>
---
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
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