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Thomas Gleixner 7c7075c88d rtc: mc146818: Prevent reading garbage 
[ Upstream commit 05a0302c35 ]

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
Stable-dep-of: cd17420ebe ("rtc: cmos: avoid UIP when writing alarm time")
Signed-off-by: Sasha Levin <sashal@kernel.org>
2022-12-14 11:31:55 +01:00
arch ARM: dts: rockchip: disable arm_global_timer on rk3066 and rk3188 2022-12-14 11:31:53 +01:00
block block: unhash blkdev part inode when the part is deleted 2022-12-08 11:23:59 +01:00
certs certs/blacklist_hashes.c: fix const confusion in certs blacklist 2022-06-22 14:13:17 +02:00
crypto crypto: akcipher - default implementation for setting a private key 2022-10-26 13:25:42 +02:00
Documentation docs: update mediator contact information in CoC doc 2022-11-25 17:45:53 +01:00
drivers rtc: mc146818: Prevent reading garbage 2022-12-14 11:31:55 +01:00
fs btrfs: send: avoid unaligned encoded writes when attempting to clone range 2022-12-14 11:31:53 +01:00
include mm/khugepaged: fix GUP-fast interaction by sending IPI 2022-12-14 11:31:55 +01:00
init init/Kconfig: fix CC_HAS_ASM_GOTO_TIED_OUTPUT test with dash 2022-12-02 17:40:03 +01:00
ipc ipc/sem: Fix dangling sem_array access in semtimedop race 2022-12-08 11:24:00 +01:00
kernel proc: proc_skip_spaces() shouldn't think it is working on C strings 2022-12-08 11:24:00 +01:00
lib Kconfig.debug: provide a little extra FRAME_WARN leeway when KASAN is enabled 2022-12-08 11:23:58 +01:00
LICENSES LICENSES/deprecated: add Zlib license text 2020-09-16 14:33:49 +02:00
mm mm/khugepaged: invoke MMU notifiers in shmem/file collapse paths 2022-12-14 11:31:55 +01:00
net 9p/xen: check logical size for buffer size 2022-12-14 11:31:54 +01:00
samples x86: Prepare inline-asm for straight-line-speculation 2022-07-25 11:26:29 +02:00
scripts scripts/faddr2line: Fix regression in name resolution on ppc64le 2022-12-08 11:23:54 +01:00
security capabilities: fix potential memleak on error path from vfs_getxattr_alloc() 2022-11-10 18:14:26 +01:00
sound ASoC: soc-pcm: Add NULL check in BE reparenting 2022-12-14 11:31:54 +01:00
tools ipv4: Fix route deletion when nexthop info is not specified 2022-12-08 11:23:59 +01:00
usr usr/include/Makefile: add linux/nfc.h to the compile-test coverage 2022-02-01 17:25:48 +01:00
virt kvm: Add support for arch compat vm ioctls 2022-10-30 09:41:15 +01:00
.clang-format RDMA 5.10 pull request 2020-10-17 11:18:18 -07:00
.cocciconfig
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore kbuild: generate Module.symvers only when vmlinux exists 2021-05-19 10:12:59 +02:00
.mailmap mailmap: add two more addresses of Uwe Kleine-König 2020-12-06 10:19:07 -08:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Move Jason Cooper to CREDITS 2020-11-30 10:20:34 +01:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS MAINTAINERS: add Amir as xfs maintainer for 5.10.y 2022-07-02 16:39:22 +02:00
Makefile Linux 5.10.158 2022-12-08 11:24:00 +01:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.