arm64: Remove useless UAO IPI and describe how this gets enabled (c8b06e3f) · Commits · 戴 / test

Since its introduction, the UAO enable call was broken, and useless. commit ("arm64: cpufeature: Schedule enable() calls instead of calling them via IPI"), fixed the framework so that these calls are scheduled, so that they can modify PSTATE. Now it is just useless. Remove it. UAO is enabled by the code patching which causes get_user() and friends to use the 'ldtr' family of instructions. This relies on the PSTATE.UAO bit being set to match addr_limit, which we do in uao_thread_switch() called via __switch_to(). All that is needed to enable UAO is patch the code, and call schedule(). __apply_alternatives_multi_stop() calls stop_machine() when it modifies the kernel text to enable the alternatives, (including the UAO code in uao_thread_switch()). Once stop_machine() has finished __switch_to() is called to reschedule the original task, this causes PSTATE.UAO to be set appropriately. An explicit enable() call is not needed. Reported-by:

Vladimir Murzin <vladimir.murzin@arm.com> Signed-off-by:

James Morse <james.morse@arm.com>

arch/arm64/include/asm/processor.h

+0 −1

Original line number	Diff line number	Diff line
		@@ -187,7 +187,6 @@ static inline void spin_lock_prefetch(const void *ptr)
		#endif

		int cpu_enable_pan(void *__unused);
		int cpu_enable_uao(void *__unused);
		int cpu_enable_cache_maint_trap(void *__unused);

		#endif /* __ASM_PROCESSOR_H */

arch/arm64/kernel/cpufeature.c

+4 −1

Original line number	Diff line number	Diff line
		@@ -806,7 +806,10 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
		.sys_reg = SYS_ID_AA64MMFR2_EL1,
		.field_pos = ID_AA64MMFR2_UAO_SHIFT,
		.min_field_value = 1,
		.enable = cpu_enable_uao,
		/*
		* We rely on stop_machine() calling uao_thread_switch() to set
		* UAO immediately after patching.
		*/
		},
		#endif /* CONFIG_ARM64_UAO */
		#ifdef CONFIG_ARM64_PAN

arch/arm64/mm/fault.c

+0 −14

Original line number	Diff line number	Diff line
		@@ -691,17 +691,3 @@ int cpu_enable_pan(void *__unused)
		return 0;
		}
		#endif /* CONFIG_ARM64_PAN */

		#ifdef CONFIG_ARM64_UAO
		/*
		* Kernel threads have fs=KERNEL_DS by default, and don't need to call
		* set_fs(), devtmpfs in particular relies on this behaviour.
		* We need to enable the feature at runtime (instead of adding it to
		* PSR_MODE_EL1h) as the feature may not be implemented by the cpu.
		*/
		int cpu_enable_uao(void *__unused)
		{
		asm(SET_PSTATE_UAO(1));
		return 0;
		}
		#endif /* CONFIG_ARM64_UAO */

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