[PATCH] mtk-sd: Fix a pagefault in dma_unmap_sg() for not prepared data

[Masami Hiramatsu (Google)]

From: Masami Hiramatsu (Google) <mhiramat at kernel.org>

mtk-msdc driver causes a kernel crash after swiotlb buffer is full.

---
mtk-msdc 11240000.mmc: swiotlb buffer is full (sz: 16384 bytes), total 32768 (slots), used 32732 (slots)
mtk-msdc 11240000.mmc: msdc_track_cmd_data: cmd=18 arg=0397A6F8; host->error=0x00000004
Unable to handle kernel paging request at virtual address ffffffffc0001fc0
---

When swiotlb buffer is full, the dma_map_sg() returns 0 to
msdc_prepare_data(), but it does not check it and sets the
MSDC_PREPARE_FLAG.

swiotlb_tbl_map_single() /* prints "swiotlb buffer is full" */
  <-swiotlb_map()
    <-dma_direct_map_page()
      <-dma_direct_map_sg()
        <-__dma_map_sg_attrs()
          <-dma_map_sg_attrs()
            <-dma_map_sg()  /* returns 0 (pages mapped) */
              <-msdc_prepare_data()

Then, the msdc_unprepare_data() checks MSDC_PREPARE_FLAG and calls
dma_unmap_sg() with unmapped pages. It causes a page fault.

To fix this problem, Do not set MSDC_PREPARE_FLAG if dma_map_sg()
fails because this is not prepared.

Fixes: 208489032bdd ("mmc: mediatek: Add Mediatek MMC driver")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat at kernel.org>
---
 drivers/mmc/host/mtk-sd.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/mmc/host/mtk-sd.c b/drivers/mmc/host/mtk-sd.c
index 345ea91629e0..3594010bc229 100644
--- a/drivers/mmc/host/mtk-sd.c
+++ b/drivers/mmc/host/mtk-sd.c
@@ -827,9 +827,10 @@ static inline void msdc_dma_setup(struct msdc_host *host, struct msdc_dma *dma,
 static void msdc_prepare_data(struct msdc_host *host, struct mmc_data *data)
 {
 	if (!(data->host_cookie & MSDC_PREPARE_FLAG)) {
-		data->host_cookie |= MSDC_PREPARE_FLAG;
 		data->sg_count = dma_map_sg(host->dev, data->sg, data->sg_len,
 					    mmc_get_dma_dir(data));
+		if (data->sg_count)
+			data->host_cookie |= MSDC_PREPARE_FLAG;
 	}
 }