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s390/mm: Pin identity mapping base to zero

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SIE instruction performs faster when the virtual address of
SIE block matches the physical one. Pin the identity mapping
base to zero for the benefit of SIE and other instructions
that have similar performance impact. Still, randomize the
base when DEBUG_VM kernel configuration option is enabled.

Suggested-by: Vasily Gorbik <gor@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
This commit is contained in:
Alexander Gordeev 2024-08-09 08:47:16 +02:00 committed by Vasily Gorbik
parent a3ca27c405
commit 32db401965
2 changed files with 15 additions and 1 deletions
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13
arch/s390/Kconfig
View File

@ -604,6 +604,19 @@ config RANDOMIZE_BASE
as a security feature that deters exploit attempts relying on
knowledge of the location of kernel internals.
config RANDOMIZE_IDENTITY_BASE
bool "Randomize the address of the identity mapping base"
depends on RANDOMIZE_BASE
default DEBUG_VM
help
The identity mapping base address is pinned to zero by default.
Allow randomization of that base to expose otherwise missed
notion of physical and virtual addresses of data structures.
That does not have any impact on the base address at which the
kernel image is loaded.
If unsure, say N
config KERNEL_IMAGE_BASE
hex "Kernel image base address"
range 0x100000 0x1FFFFFE0000000 if !KASAN

3
arch/s390/boot/startup.c
View File

@ -341,7 +341,8 @@ static unsigned long setup_kernel_memory_layout(unsigned long kernel_size)
BUILD_BUG_ON(MAX_DCSS_ADDR > (1UL << MAX_PHYSMEM_BITS));
max_mappable = max(ident_map_size, MAX_DCSS_ADDR);
max_mappable = min(max_mappable, vmemmap_start);
__identity_base = round_down(vmemmap_start - max_mappable, rte_size);
if (IS_ENABLED(CONFIG_RANDOMIZE_IDENTITY_BASE))
__identity_base = round_down(vmemmap_start - max_mappable, rte_size);
return asce_limit;
}