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arm64: warn on incorrect placement of the kernel by the bootloader

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Commit cfa7ede20f ("arm64: set TEXT_OFFSET to 0x0 in preparation for
removing it entirely") results in boot failures when booting kernels that
are built without KASLR support on broken bootloaders that ignore the
TEXT_OFFSET value passed via the header, and use the default of 0x80000
instead.

To work around this, turn CONFIG_RELOCATABLE on by default, even if KASLR
itself (CONFIG_RANDOMIZE_BASE) is turned off, and require CONFIG_EXPERT
to be enabled to deviate from this. Then, emit a warning into the kernel
log if we are not booting via the EFI stub (which is permitted to deviate
from the placement restrictions) and the kernel base address is not placed
according to the rules as laid out in Documentation/arm64/booting.rst.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20200611124330.252163-1-ardb@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>
This commit is contained in:
Ard Biesheuvel 2020-06-11 14:43:30 +02:00 committed by Will Deacon
parent a194c33f45
commit dd4bc60765
2 changed files with 6 additions and 1 deletions
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3
arch/arm64/Kconfig
View file

@ -1746,8 +1746,9 @@ config ARM64_DEBUG_PRIORITY_MASKING
endif
config RELOCATABLE
bool
bool "Build a relocatable kernel image" if EXPERT
select ARCH_HAS_RELR
default y
help
This builds the kernel as a Position Independent Executable (PIE),
which retains all relocation metadata required to relocate the

4
arch/arm64/kernel/setup.c
View file

@ -319,6 +319,10 @@ void __init setup_arch(char **cmdline_p)
xen_early_init();
efi_init();
if (!efi_enabled(EFI_BOOT) && ((u64)_text % MIN_KIMG_ALIGN) != 0)
pr_warn(FW_BUG "Kernel image misaligned at boot, please fix your bootloader!");
arm64_memblock_init();
paging_init();