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Andre Przywara 2a65da5a1e ARM: 9159/1: decompressor: Avoid UNPREDICTABLE NOP encoding 
[ Upstream commit a92882a4d2 ]

In the decompressor's head.S we need to start with an instruction that
is some kind of NOP, but also mimics as the PE/COFF header, when the
kernel is linked as an UEFI application. The clever solution here is
"tstne r0, #0x4d000", which in the worst case just clobbers the
condition flags, and bears the magic "MZ" signature in the lowest 16 bits.

However the encoding used (0x13105a4d) is actually not valid, since bits
[15:12] are supposed to be 0 (written as "(0)" in the ARM ARM).
Violating this is UNPREDICTABLE, and *can* trigger an UNDEFINED
exception. Common Cortex cores seem to ignore those bits, but QEMU
chooses to trap, so the code goes fishing because of a missing exception
handler at this point. We are just saved by the fact that commonly (with
-kernel or when running from U-Boot) the "Z" bit is set, so the
instruction is never executed. See [0] for more details.

To make things more robust and avoid UNPREDICTABLE behaviour in the
kernel code, lets replace this with a "two-instruction NOP":
The first instruction is an exclusive OR, the effect of which the second
instruction reverts. This does not leave any trace, neither in a
register nor in the condition flags. Also it's a perfectly valid
encoding. Kudos to Peter Maydell for coming up with this gem.

[0] https://lore.kernel.org/qemu-devel/YTPIdbUCmwagL5%2FD@os.inf.tu-dresden.de/T/

Link: https://lore.kernel.org/linux-arm-kernel/20210908162617.104962-1-andre.przywara@arm.com/T/

Fixes: 81a0bc39ea ("ARM: add UEFI stub support")
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reported-by: Adam Lackorzynski <adam@l4re.org>
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
2022-01-27 10:53:57 +01:00
arch ARM: 9159/1: decompressor: Avoid UNPREDICTABLE NOP encoding 2022-01-27 10:53:57 +01:00
block iocost: Fix divide-by-zero on donation from low hweight cgroup 2021-12-22 09:30:57 +01:00
certs
crypto crypto: pcrypt - Delay write to padata->info 2021-11-18 14:04:12 +01:00
Documentation bpf: Add kconfig knob for disabling unpriv bpf by default 2022-01-05 12:40:34 +01:00
drivers media: hantro: Fix probe func error path 2022-01-27 10:53:57 +01:00
fs fs: dlm: fix build with CONFIG_IPV6 disabled 2022-01-27 10:53:49 +01:00
include bpf: Disallow BPF_LOG_KERNEL log level for bpf(BPF_BTF_LOAD) 2022-01-27 10:53:54 +01:00
init bpf: Add kconfig knob for disabling unpriv bpf by default 2022-01-05 12:40:34 +01:00
ipc shm: extend forced shm destroy to support objects from several IPC nses 2021-12-01 09:19:10 +01:00
kernel rcu/exp: Mark current CPU as exp-QS in IPI loop second pass 2022-01-27 10:53:55 +01:00
lib siphash: use _unaligned version by default 2021-12-08 09:03:23 +01:00
LICENSES
mm shmem: fix a race between shmem_unused_huge_shrink and shmem_evict_inode 2022-01-27 10:53:44 +01:00
net xfrm: state and policy should fail if XFRMA_IF_ID 0 2022-01-27 10:53:57 +01:00
samples ftrace/samples: Add missing prototypes direct functions 2022-01-11 15:25:00 +01:00
scripts recordmcount.pl: fix typo in s390 mcount regex 2022-01-05 12:40:29 +01:00
security selinux: initialize proto variable in selinux_ip_postroute_compat() 2022-01-05 12:40:30 +01:00
sound ALSA: hda/realtek: Re-order quirk entries for Lenovo 2022-01-20 09:17:51 +01:00
tools selftests: harness: avoid false negatives if test has no ASSERTs 2022-01-27 10:53:55 +01:00
usr
virt KVM: downgrade two BUG_ONs to WARN_ON_ONCE 2021-12-22 09:30:50 +01:00
.clang-format
.cocciconfig
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MAINTAINERS MAINTAINERS: adjust GCC PLUGINS after gcc-plugin.sh removal 2021-12-14 11:32:46 +01:00
Makefile Linux 5.10.93 2022-01-20 09:17:52 +01:00
README

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.