Commit c8355785 authored Mar 18, 2020 by Will Deacon

arm64: kpti: Fix "kpti=off" when KASLR is enabled

Enabling KASLR forces the use of non-global page-table entries for kernel
mappings, as this is a decision that we have to make very early on before
mapping the kernel proper. When used in conjunction with the "kpti=off"
command-line option, it is possible to use non-global kernel mappings but
with the kpti trampoline disabled.

Since commit 09e3c22a ("arm64: Use a variable to store non-global
mappings decision"), arm64_kernel_unmapped_at_el0() reflects only the use of
non-global mappings and does not take into account whether the kpti
trampoline is enabled. This breaks context switching of the TPIDRRO_EL0
register for 64-bit tasks, where the clearing of the register is deferred to
the ret-to-user code, but it also breaks the ARM SPE PMU driver which
helpfully recommends passing "kpti=off" on the command line!

Report whether or not KPTI is actually enabled in
arm64_kernel_unmapped_at_el0() and check the 'arm64_use_ng_mappings' global
variable directly when determining the protection flags for kernel mappings.

Cc: Mark Brown <broonie@kernel.org>
Reported-by: Hongbo Yao <yaohongbo@huawei.com>
Tested-by: Hongbo Yao <yaohongbo@huawei.com>
Fixes: 09e3c22a ("arm64: Use a variable to store non-global mappings decision")
Signed-off-by: Will Deacon <will@kernel.org>

parent f50b7dac

arch/arm64/include/asm/mmu.h

+1 −3

Original line number	Diff line number	Diff line
		@@ -29,11 +29,9 @@ typedef struct {
		*/
		#define ASID(mm) ((mm)->context.id.counter & 0xffff)

		extern bool arm64_use_ng_mappings;

		static inline bool arm64_kernel_unmapped_at_el0(void)
		{
		return arm64_use_ng_mappings;
		return cpus_have_const_cap(ARM64_UNMAP_KERNEL_AT_EL0);
		}

		typedef void (*bp_hardening_cb_t)(void);

arch/arm64/include/asm/pgtable-prot.h

+4 −2

Original line number	Diff line number	Diff line
		@@ -23,11 +23,13 @@

		#include <asm/pgtable-types.h>

		extern bool arm64_use_ng_mappings;

		#define _PROT_DEFAULT (PTE_TYPE_PAGE \| PTE_AF \| PTE_SHARED)
		#define _PROT_SECT_DEFAULT (PMD_TYPE_SECT \| PMD_SECT_AF \| PMD_SECT_S)

		#define PTE_MAYBE_NG (arm64_kernel_unmapped_at_el0() ? PTE_NG : 0)
		#define PMD_MAYBE_NG (arm64_kernel_unmapped_at_el0() ? PMD_SECT_NG : 0)
		#define PTE_MAYBE_NG (arm64_use_ng_mappings ? PTE_NG : 0)
		#define PMD_MAYBE_NG (arm64_use_ng_mappings ? PMD_SECT_NG : 0)

		#define PROT_DEFAULT (_PROT_DEFAULT \| PTE_MAYBE_NG)
		#define PROT_SECT_DEFAULT (_PROT_SECT_DEFAULT \| PMD_MAYBE_NG)

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