On 1 June 2012 10:31, Torne (Richard Coles) <torne@xxxxxxxxxx> wrote: > On 1 June 2012 09:35, Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote: >> On 29/05/12 05:32, Ben Hutchings wrote: >>> On Mon, 2012-05-28 at 18:31 +0100, Torne (Richard Coles) wrote: >>>> From: "Torne (Richard Coles)" <torne@xxxxxxxxxx> >>>> >>>> MMC CSD info can specify very large, ridiculous timeouts, big enough to >>>> overflow timeout_ns on 32-bit machines. This can result in the card >>>> timing out on every operation because the wrapped timeout value is far >>>> too small. >>>> >>>> Fix the overflow by capping the result at 2 seconds. Cards specifying >>>> longer timeouts are almost certainly insane, and host controllers >>>> generally cannot support timeouts that long in any case. >>>> >>>> 2 seconds should be plenty of time for any card to actually function; >>>> the timeout calculation code is already using 1 second as a "worst case" >>>> timeout for cards running in SPI mode. >>> >>> Needs a 'Signed-off-by'. >>> >>>> --- >>>> drivers/mmc/core/core.c | 11 ++++++++++- >>>> 1 files changed, 10 insertions(+), 1 deletions(-) >>>> >>>> diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c >>>> index 0b6141d..3b4a9fc 100644 >>>> --- a/drivers/mmc/core/core.c >>>> +++ b/drivers/mmc/core/core.c >>>> @@ -512,7 +512,16 @@ void mmc_set_data_timeout(struct mmc_data *data, const struct mmc_card *card) >>>> if (data->flags & MMC_DATA_WRITE) >>>> mult <<= card->csd.r2w_factor; >>>> >>>> - data->timeout_ns = card->csd.tacc_ns * mult; >>>> + /* >>>> + * The timeout in nanoseconds may overflow with some cards. Cap it at >>>> + * two seconds both to avoid the overflow and also because host >>>> + * controllers cannot generally generate timeouts that long anyway. >>>> + */ >>>> + if (card->csd.tacc_ns <= (2 * NSEC_PER_SEC) / mult) >>>> + data->timeout_ns = card->csd.tacc_ns * mult; >>>> + else >>>> + data->timeout_ns = 2 * NSEC_PER_SEC; >>> >>> We clearly need to guard against overflow here, and this is the correct >>> way to clamp the multiplication. I can't speak as to whether 2 seconds >>> is the right limit. >> >> The host controllers I have looked at have a limit of around 2.5 seconds. >> >> But why not just use the size of the type as the limit? e.g. >> >> if (card->csd.tacc_ns <= UINT_MAX / mult) >> data->timeout_ns = card->csd.tacc_ns * mult; >> else >> data->timeout_ns = UINT_MAX; > > The host controller drivers don't seem to all do a very good job of > preventing further overflows or handling large values correctly > (though some do). sdhci takes the especially annoying additional step > of printk'ing a warning for *every single MMC command* where > data->timeout_ns is larger than the controller can accommodate. > Capping it to a value with a sensible order of magnitude seems to make > it more likely that cards with obviously bogus CSD parameters will > actually work. I don't object to using a larger number for the limit, > but UINT_MAX on a 64-bit system obviously doesn't limit this at all > and will leave you with timeouts up to 17 minutes, which seems > ridiculous :) Er, not 17 minutes; 102.4 seconds as I used later in my mail. SD cards have their timeouts capped already, so their larger 100x multiplier is not a problem; 102.4 seconds is the longest for an MMC card. > My original motivation for this patch is that I have a device with an > eMMC that specifies a 25.5 second timeout, attached to a sdhci host > whose maximum timeout is 2.8 seconds. Originally I proposed a patch to > just remove the warning in sdhci, but nobody replied, and when I > realised there was actually an overflow happening I opted to fix that > instead. > > So, yeah, we could use UINT_MAX, but then at minimum I also need to > kill the warning in sdhci to make my device work, and probably all the > host controller drivers need to be checked to make sure they don't use > timeout_ns in a way that can overflow. > > I've also just noticed that struct mmc_data's comment for timeout_ns > says /* data timeout (in ns, max 80ms) */ which is not true (the max > is 102.4 seconds if my math is correct), which may have contributed to > the host drivers not being too careful :) > > What do you think? > >>> >>> Ben. >>> >>>> data->timeout_clks = card->csd.tacc_clks * mult; >>>> >>>> /* >>> >> > > > > -- > Torne (Richard Coles) > torne@xxxxxxxxxx -- Torne (Richard Coles) torne@xxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
