cifs: protect against server returning invalid file system block size

For a network file system we generally prefer large i/o, but
if the server returns invalid file system block/sector sizes
in cifs (vers=1.0) QFSInfo then set block size to a default
of a reasonable minimum (4K).
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>

fs/cifs/cifssmb.c

@@ -5027,6 +5027,13 @@ SMBOldQFSInfo(const unsigned int xid, struct cifs_tcon *tcon,
 				le16_to_cpu(response_data->BytesPerSector) *
 				le32_to_cpu(response_data->
 					SectorsPerAllocationUnit);
+			/*
+			 * much prefer larger but if server doesn't report
+			 * a valid size than 4K is a reasonable minimum
+			 */
+			if (FSData->f_bsize < 512)
+				FSData->f_bsize = 4096;
+
 			FSData->f_blocks =
 			       le32_to_cpu(response_data->TotalAllocationUnits);
 			FSData->f_bfree = FSData->f_bavail =
@@ -5107,6 +5114,13 @@ CIFSSMBQFSInfo(const unsigned int xid, struct cifs_tcon *tcon,
 			    le32_to_cpu(response_data->BytesPerSector) *
 			    le32_to_cpu(response_data->
 					SectorsPerAllocationUnit);
+			/*
+			 * much prefer larger but if server doesn't report
+			 * a valid size than 4K is a reasonable minimum
+			 */
+			if (FSData->f_bsize < 512)
+				FSData->f_bsize = 4096;
+
 			FSData->f_blocks =
 			    le64_to_cpu(response_data->TotalAllocationUnits);
 			FSData->f_bfree = FSData->f_bavail =
@@ -5470,6 +5484,13 @@ CIFSSMBQFSPosixInfo(const unsigned int xid, struct cifs_tcon *tcon,
 				 data_offset);
 			FSData->f_bsize =
 					le32_to_cpu(response_data->BlockSize);
+			/*
+			 * much prefer larger but if server doesn't report
+			 * a valid size than 4K is a reasonable minimum
+			 */
+			if (FSData->f_bsize < 512)
+				FSData->f_bsize = 4096;
+
 			FSData->f_blocks =
 					le64_to_cpu(response_data->TotalBlocks);
 			FSData->f_bfree =