staging:rts_pstor:Fix possible panic by NULL pointer dereference

rtsx_transport.c (rtsx_transfer_sglist_adma_partial):
pointer struct scatterlist *sg, which is mapped in dma_map_sg,
is used as an iterator in later transfer operation. It is corrupted and
passed to dma_unmap_sg, thus causing fatal unmap of some erroneous address.
Fix it by duplicating *sg_ptr for iterating.
Signed-off-by: Nwwang <wei_wang@realsil.com.cn>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rts_pstor/rtsx_transport.c

@@ -335,6 +335,7 @@ static int rtsx_transfer_sglist_adma_partial(struct rtsx_chip *chip, u8 card,
 	int sg_cnt, i, resid;
 	int err = 0;
 	long timeleft;
+	struct scatterlist *sg_ptr;
 	u32 val = TRIG_DMA;
 
 	if ((sg == NULL) || (num_sg <= 0) || !offset || !index)
@@ -371,7 +372,7 @@ static int rtsx_transfer_sglist_adma_partial(struct rtsx_chip *chip, u8 card,
 	sg_cnt = dma_map_sg(&(rtsx->pci->dev), sg, num_sg, dma_dir);
 
 	resid = size;
-
+	sg_ptr = sg;
 	chip->sgi = 0;
 	/* Usually the next entry will be @sg@ + 1, but if this sg element
 	 * is part of a chained scatterlist, it could jump to the start of
@@ -379,14 +380,14 @@ static int rtsx_transfer_sglist_adma_partial(struct rtsx_chip *chip, u8 card,
 	 * the proper sg
 	 */
 	for (i = 0; i < *index; i++)
-		sg = sg_next(sg);
+		sg_ptr = sg_next(sg_ptr);
 	for (i = *index; i < sg_cnt; i++) {
 		dma_addr_t addr;
 		unsigned int len;
 		u8 option;
 
-		addr = sg_dma_address(sg);
-		len = sg_dma_len(sg);
+		addr = sg_dma_address(sg_ptr);
+		len = sg_dma_len(sg_ptr);
 
 		RTSX_DEBUGP("DMA addr: 0x%x, Len: 0x%x\n",
 			     (unsigned int)addr, len);
@@ -415,7 +416,7 @@ static int rtsx_transfer_sglist_adma_partial(struct rtsx_chip *chip, u8 card,
 		if (!resid)
 			break;
 
-		sg = sg_next(sg);
+		sg_ptr = sg_next(sg_ptr);
 	}
 
 	RTSX_DEBUGP("SG table count = %d\n", chip->sgi);