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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Christoph Probst <kernel@probst.it>,
        Pavel Shilovsky <pshilov@microsoft.com>,
        Steve French <stfrench@microsoft.com>
Subject: [PATCH 4.14 20/77] cifs: fix strcat buffer overflow and reduce raciness in smb21_set_oplock_level()
Date:   Thu, 23 May 2019 21:05:38 +0200
Message-Id: <20190523181723.075830718@linuxfoundation.org>
In-Reply-To: <20190523181719.982121681@linuxfoundation.org>
References: <20190523181719.982121681@linuxfoundation.org>
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From: Christoph Probst <kernel@probst.it>

commit 6a54b2e002c9d00b398d35724c79f9fe0d9b38fb upstream.

Change strcat to strncpy in the "None" case to fix a buffer overflow
when cinode->oplock is reset to 0 by another thread accessing the same
cinode. It is never valid to append "None" to any other message.

Consolidate multiple writes to cinode->oplock to reduce raciness.

Signed-off-by: Christoph Probst <kernel@probst.it>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
CC: Stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/cifs/smb2ops.c |   14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

--- a/fs/cifs/smb2ops.c
+++ b/fs/cifs/smb2ops.c
@@ -1969,26 +1969,28 @@ smb21_set_oplock_level(struct cifsInodeI
 		       unsigned int epoch, bool *purge_cache)
 {
 	char message[5] = {0};
+	unsigned int new_oplock = 0;
 
 	oplock &= 0xFF;
 	if (oplock == SMB2_OPLOCK_LEVEL_NOCHANGE)
 		return;
 
-	cinode->oplock = 0;
 	if (oplock & SMB2_LEASE_READ_CACHING_HE) {
-		cinode->oplock |= CIFS_CACHE_READ_FLG;
+		new_oplock |= CIFS_CACHE_READ_FLG;
 		strcat(message, "R");
 	}
 	if (oplock & SMB2_LEASE_HANDLE_CACHING_HE) {
-		cinode->oplock |= CIFS_CACHE_HANDLE_FLG;
+		new_oplock |= CIFS_CACHE_HANDLE_FLG;
 		strcat(message, "H");
 	}
 	if (oplock & SMB2_LEASE_WRITE_CACHING_HE) {
-		cinode->oplock |= CIFS_CACHE_WRITE_FLG;
+		new_oplock |= CIFS_CACHE_WRITE_FLG;
 		strcat(message, "W");
 	}
-	if (!cinode->oplock)
-		strcat(message, "None");
+	if (!new_oplock)
+		strncpy(message, "None", sizeof(message));
+
+	cinode->oplock = new_oplock;
 	cifs_dbg(FYI, "%s Lease granted on inode %p\n", message,
 		 &cinode->vfs_inode);
 }