diff options
author | Thomas Gleixner <tglx@linutronix.de> | 2020-12-06 22:46:14 +0100 |
---|---|---|
committer | Thomas Gleixner <tglx@linutronix.de> | 2020-12-11 10:40:52 +0100 |
commit | 05a0302c35481e9b47fb90ba40922b0a4cae40d8 (patch) | |
tree | 7db10222ad1336406d49830cd504e2767164f227 /Makefile | |
parent | fef92cd2bc04c64bb3743d40c0b4be47aedf9e23 (diff) | |
download | linux-05a0302c35481e9b47fb90ba40922b0a4cae40d8.tar.bz2 |
rtc: mc146818: Prevent reading garbage
The MC146818 driver is prone to read garbage from the RTC. There are
several issues all related to the update cycle of the MC146818. The chip
increments seconds obviously once per second and indicates that by a bit in
a register. The bit goes high 244us before the actual update starts. During
the update the readout of the time values is undefined.
The code just checks whether the update in progress bit (UIP) is set before
reading the clock. If it's set it waits arbitrary 20ms before retrying,
which is ample because the maximum update time is ~2ms.
But this check does not guarantee that the UIP bit goes high and the actual
update happens during the readout. So the following can happen
0.997 UIP = False
-> Interrupt/NMI/preemption
0.998 UIP -> True
0.999 Readout <- Undefined
To prevent this rework the code so it checks UIP before and after the
readout and if set after the readout try again.
But that's not enough to cover the following:
0.997 UIP = False
Readout seconds
-> NMI (or vCPU scheduled out)
0.998 UIP -> True
update completes
UIP -> False
1.000 Readout minutes,....
UIP check succeeds
That can make the readout wrong up to 59 seconds.
To prevent this, read the seconds value before the first UIP check,
validate it after checking UIP and after reading out the rest.
It's amazing that the original i386 code had this actually correct and
the generic implementation of the MC146818 driver got it wrong in 2002 and
it stayed that way until today.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201206220541.594826678@linutronix.de
Diffstat (limited to 'Makefile')
0 files changed, 0 insertions, 0 deletions