[PATCH] CIFS: Fix too big maxBuf size for SMB3 mounts

[Pavel Shilovsky <piastry-7qunaywFIewox3rIn2DAYQ@public.gmane.org>]

SMB3 servers can respond with MaxTransactSize of more than 4M
that can cause a memory allocation error returned from kmalloc
in a lock codepath. Also the client doesn't support multicredit
requests now and allows buffer sizes of 65536 bytes only. Set
MaxTransactSize to this maximum supported value.

Cc: stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.org # 3.7+
Signed-off-by: Pavel Shilovsky <piastry-7qunaywFIewox3rIn2DAYQ@public.gmane.org>
---
 fs/cifs/smb2glob.h |    3 +++
 fs/cifs/smb2ops.c  |   14 ++++----------
 fs/cifs/smb2pdu.c  |    4 +++-
 3 files changed, 10 insertions(+), 11 deletions(-)

diff --git a/fs/cifs/smb2glob.h b/fs/cifs/smb2glob.h
index c383508..bc0bb9c 100644
--- a/fs/cifs/smb2glob.h
+++ b/fs/cifs/smb2glob.h
@@ -57,4 +57,7 @@
 #define SMB2_CMACAES_SIZE (16)
 #define SMB3_SIGNKEY_SIZE (16)
 
+/* Maximum buffer size value we can send with 1 credit */
+#define SMB2_MAX_BUFFER_SIZE 65536
+
 #endif	/* _SMB2_GLOB_H */
diff --git a/fs/cifs/smb2ops.c b/fs/cifs/smb2ops.c
index 757da3e..192f51a 100644
--- a/fs/cifs/smb2ops.c
+++ b/fs/cifs/smb2ops.c
@@ -182,11 +182,8 @@ smb2_negotiate_wsize(struct cifs_tcon *tcon, struct smb_vol *volume_info)
 	/* start with specified wsize, or default */
 	wsize = volume_info->wsize ? volume_info->wsize : CIFS_DEFAULT_IOSIZE;
 	wsize = min_t(unsigned int, wsize, server->max_write);
-	/*
-	 * limit write size to 2 ** 16, because we don't support multicredit
-	 * requests now.
-	 */
-	wsize = min_t(unsigned int, wsize, 2 << 15);
+	/* set it to the maximum buffer size value we can send with 1 credit */
+	wsize = min_t(unsigned int, wsize, SMB2_MAX_BUFFER_SIZE);
 
 	return wsize;
 }
@@ -200,11 +197,8 @@ smb2_negotiate_rsize(struct cifs_tcon *tcon, struct smb_vol *volume_info)
 	/* start with specified rsize, or default */
 	rsize = volume_info->rsize ? volume_info->rsize : CIFS_DEFAULT_IOSIZE;
 	rsize = min_t(unsigned int, rsize, server->max_read);
-	/*
-	 * limit write size to 2 ** 16, because we don't support multicredit
-	 * requests now.
-	 */
-	rsize = min_t(unsigned int, rsize, 2 << 15);
+	/* set it to the maximum buffer size value we can send with 1 credit */
+	rsize = min_t(unsigned int, rsize, SMB2_MAX_BUFFER_SIZE);
 
 	return rsize;
 }
diff --git a/fs/cifs/smb2pdu.c b/fs/cifs/smb2pdu.c
index 2013234..787e171 100644
--- a/fs/cifs/smb2pdu.c
+++ b/fs/cifs/smb2pdu.c
@@ -413,7 +413,9 @@ SMB2_negotiate(const unsigned int xid, struct cifs_ses *ses)
 
 	/* SMB2 only has an extended negflavor */
 	server->negflavor = CIFS_NEGFLAVOR_EXTENDED;
-	server->maxBuf = le32_to_cpu(rsp->MaxTransactSize);
+	/* set it to the maximum buffer size value we can send with 1 credit */
+	server->maxBuf = min_t(unsigned int, le32_to_cpu(rsp->MaxTransactSize),
+			       SMB2_MAX_BUFFER_SIZE);
 	server->max_read = le32_to_cpu(rsp->MaxReadSize);
 	server->max_write = le32_to_cpu(rsp->MaxWriteSize);
 	/* BB Do we need to validate the SecurityMode? */
-- 
1.7.10.4