[PATCH 1/1] mmc: Avoid buffer overrun in mmc_startup()

[Heinrich Schuchardt]

If the CSD register contains a reserved value (4 - 7) in bits 0:2 of the
TRAN_SPEED field, a buffer overrun occurs. Resize the mapping table.

According to the original report
https://lore.kernel.org/u-boot/20180826231332.2491-11-erosca@de.adit-jv.com/
reserved values have been observed resulting in a buffer overrun.

Reported-by: Eugeniu Rosca <erosca at de.adit-jv.com>
Fixes: 272cc70b211e ("Add MMC Framework")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt at canonical.com>
---
 drivers/mmc/mmc.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/mmc/mmc.c b/drivers/mmc/mmc.c
index d96db7a0f8..00f4964aae 100644
--- a/drivers/mmc/mmc.c
+++ b/drivers/mmc/mmc.c
@@ -1570,13 +1570,20 @@ static int sd_read_ssr(struct mmc *mmc)
 	return 0;
 }
 #endif
-/* frequency bases */
-/* divided by 10 to be nice to platforms without floating point */
+/*
+ * TRAN_SPEED bits 0:2 encode the frequency unit:
+ * 0 = 100KHz, 1 = 1MHz, 2 = 10MHz, 3 = 100MHz, values 4 - 7 are reserved.
+ * The values in fbase[] are divided by 10 to avoid floats in multiplier[].
+ */
 static const int fbase[] = {
 	10000,
 	100000,
 	1000000,
 	10000000,
+	0,	/* reserved */
+	0,	/* reserved */
+	0,	/* reserved */
+	0,	/* reserved */
 };
 
 /* Multiplier values for TRAN_SPEED.  Multiplied by 10 to be nice
@@ -2560,6 +2567,8 @@ static int mmc_startup(struct mmc *mmc)
 	mult = multipliers[((cmd.response[0] >> 3) & 0xf)];
 
 	mmc->legacy_speed = freq * mult;
+	if (!mmc->legacy_speed)
+		log_debug("TRAN_SPEED: reserved value");
 	mmc_select_mode(mmc, MMC_LEGACY);
 
 	mmc->dsr_imp = ((cmd.response[1] >> 12) & 0x1);
-- 
2.43.0