[PATCH] ksmbd: Compare MACs in constant time

[PATCH] ksmbd: Compare MACs in constant time

[Eric Biggers]

To prevent timing attacks, MAC comparisons need to be constant-time.
Replace the memcmp() with the correct function, crypto_memneq().

Fixes: e2f34481b24d ("cifsd: add server-side procedures for SMB3")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
---
 fs/smb/server/Kconfig   | 1 +
 fs/smb/server/auth.c    | 4 +++-
 fs/smb/server/smb2pdu.c | 5 +++--
 3 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/fs/smb/server/Kconfig b/fs/smb/server/Kconfig
index 2775162c535c..12594879cb64 100644
--- a/fs/smb/server/Kconfig
+++ b/fs/smb/server/Kconfig
@@ -11,10 +11,11 @@ config SMB_SERVER
 	select CRYPTO_LIB_ARC4
 	select CRYPTO_LIB_DES
 	select CRYPTO_LIB_MD5
 	select CRYPTO_LIB_SHA256
 	select CRYPTO_LIB_SHA512
+	select CRYPTO_LIB_UTILS
 	select CRYPTO_CMAC
 	select CRYPTO_AEAD2
 	select CRYPTO_CCM
 	select CRYPTO_GCM
 	select ASN1
diff --git a/fs/smb/server/auth.c b/fs/smb/server/auth.c
index 09af55b71153..a69e8694605a 100644
--- a/fs/smb/server/auth.c
+++ b/fs/smb/server/auth.c
@@ -13,10 +13,11 @@
 #include <linux/xattr.h>
 #include <crypto/hash.h>
 #include <crypto/aead.h>
 #include <crypto/md5.h>
 #include <crypto/sha2.h>
+#include <crypto/utils.h>
 #include <linux/random.h>
 #include <linux/scatterlist.h>
 
 #include "auth.h"
 #include "glob.h"
@@ -163,11 +164,12 @@ int ksmbd_auth_ntlmv2(struct ksmbd_conn *conn, struct ksmbd_session *sess,
 	/* Generate the session key */
 	hmac_md5_usingrawkey(ntlmv2_hash, CIFS_HMAC_MD5_HASH_SIZE,
 			     ntlmv2_rsp, CIFS_HMAC_MD5_HASH_SIZE,
 			     sess->sess_key);
 
-	if (memcmp(ntlmv2->ntlmv2_hash, ntlmv2_rsp, CIFS_HMAC_MD5_HASH_SIZE) != 0)
+	if (crypto_memneq(ntlmv2->ntlmv2_hash, ntlmv2_rsp,
+			  CIFS_HMAC_MD5_HASH_SIZE))
 		return -EINVAL;
 	return 0;
 }
 
 /**
diff --git a/fs/smb/server/smb2pdu.c b/fs/smb/server/smb2pdu.c
index cbb31efdbaa2..7a88cf3bd29e 100644
--- a/fs/smb/server/smb2pdu.c
+++ b/fs/smb/server/smb2pdu.c
@@ -2,10 +2,11 @@
 /*
  *   Copyright (C) 2016 Namjae Jeon <linkinjeon@kernel.org>
  *   Copyright (C) 2018 Samsung Electronics Co., Ltd.
  */
 
+#include <crypto/utils.h>
 #include <linux/inetdevice.h>
 #include <net/addrconf.h>
 #include <linux/syscalls.h>
 #include <linux/namei.h>
 #include <linux/statfs.h>
@@ -8878,11 +8879,11 @@ int smb2_check_sign_req(struct ksmbd_work *work)
 	iov[0].iov_len = len;
 
 	ksmbd_sign_smb2_pdu(work->conn, work->sess->sess_key, iov, 1,
 			    signature);
 
-	if (memcmp(signature, signature_req, SMB2_SIGNATURE_SIZE)) {
+	if (crypto_memneq(signature, signature_req, SMB2_SIGNATURE_SIZE)) {
 		pr_err("bad smb2 signature\n");
 		return 0;
 	}
 
 	return 1;
@@ -8966,11 +8967,11 @@ int smb3_check_sign_req(struct ksmbd_work *work)
 	iov[0].iov_len = len;
 
 	if (ksmbd_sign_smb3_pdu(conn, signing_key, iov, 1, signature))
 		return 0;
 
-	if (memcmp(signature, signature_req, SMB2_SIGNATURE_SIZE)) {
+	if (crypto_memneq(signature, signature_req, SMB2_SIGNATURE_SIZE)) {
 		pr_err("bad smb2 signature\n");
 		return 0;
 	}
 
 	return 1;

base-commit: 2961f841b025fb234860bac26dfb7fa7cb0fb122
-- 
2.53.0

Re: [PATCH] ksmbd: Compare MACs in constant time

[Namjae Jeon]

On Wed, Feb 18, 2026 at 1:29 PM Eric Biggers <ebiggers@kernel.org> wrote:
>
> To prevent timing attacks, MAC comparisons need to be constant-time.
> Replace the memcmp() with the correct function, crypto_memneq().
>
> Fixes: e2f34481b24d ("cifsd: add server-side procedures for SMB3")
> Cc: stable@vger.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Applied it to #ksmbd-for-next-next.
Thanks!