Secure/Non-secure CPU context 전환

QUESTION

EL3가 어느 CPU의 어느 보안 상태로 돌아갈지 어떻게 정하는가?

cm_prepare_el3_exit 1150–1228 · OPTEED runtime 658–725

STRUCTURE

CPU별 보안 상태와 복귀 context

CALL PATH

EL3 복귀 준비와 OPTEED의 SMC 전환을 구분한다

STATE LEDGER

실제 입력·결과·조건

첫 줄은 호출자별 선택 동작이고, 뒤 네 줄은 cm_prepare_el3_exit()의 분기다. 범용 레지스터까지 이 C 함수가 저장한다고 해석하지 않는다.

VISUAL WALKTHROUGH

OPTEED 예시와 world별 context

UPSTREAM SOURCE

원본 코드

아래 코드는 Trusted Firmware-A LTS 2.14.3의 lib/el3_runtime/aarch64/context_mgmt.c에서 1126-1214줄을 그대로 가져온 것이다. 설명을 위해 실제 코드를 가짜 의사 코드로 바꾸지 않았다.

1126	write_hstr_el2(HSTR_EL2_RESET_VAL & ~(HSTR_EL2_T_MASK));
1127
1128	/*
1129	 * Initialise CNTHP_CTL_EL2. All fields are architecturally UNKNOWN on
1130	 * reset.
1131	 *
1132	 * CNTHP_CTL_EL2:ENABLE: Set to zero to disable the EL2 physical timer
1133	 * and prevent timer interrupts.
1134	 */
1135	write_cnthp_ctl_el2(CNTHP_CTL_RESET_VAL & ~(CNTHP_CTL_ENABLE_BIT));
1136
1137	manage_extensions_nonsecure_el2_unused();
1138#endif /* INIT_UNUSED_NS_EL2 */
1139}
1140
1141/*******************************************************************************
1142 * Prepare the CPU system registers for first entry into realm, secure, or
1143 * normal world.
1144 *
1145 * If execution is requested to EL2 or hyp mode, SCTLR_EL2 is initialized
1146 * If execution is requested to non-secure EL1 or svc mode, and the CPU supports
1147 * EL2 then EL2 is disabled by configuring all necessary EL2 registers.
1148 * For all entries, the EL1 registers are initialized from the cpu_context
1149 ******************************************************************************/
1150void cm_prepare_el3_exit(size_t security_state)
1151{
1152	u_register_t sctlr_el2, scr_el3;
1153	cpu_context_t *ctx = cm_get_context(security_state);
1154
1155	assert(ctx != NULL);
1156
1157	if (security_state == NON_SECURE) {
1158		uint64_t el2_implemented = el_implemented(2);
1159
1160		scr_el3 = read_ctx_reg(get_el3state_ctx(ctx),
1161						 CTX_SCR_EL3);
1162
1163		if (el2_implemented != EL_IMPL_NONE) {
1164
1165			/*
1166			 * If context is not being used for EL2, initialize
1167			 * HCRX_EL2 with its init value here.
1168			 */
1169			if (is_feat_hcx_supported()) {
1170				write_hcrx_el2(HCRX_EL2_INIT_VAL);
1171			}
1172
1173			/*
1174			 * Initialize Fine-grained trap registers introduced
1175			 * by FEAT_FGT so all traps are initially disabled when
1176			 * switching to EL2 or a lower EL, preventing undesired
1177			 * behavior.
1178			 */
1179			if (is_feat_fgt_supported()) {
1180				/*
1181				 * Initialize HFG*_EL2 registers with a default
1182				 * value so legacy systems unaware of FEAT_FGT
1183				 * do not get trapped due to their lack of
1184				 * initialization for this feature.
1185				 */
1186				write_hfgitr_el2(HFGITR_EL2_INIT_VAL);
1187				write_hfgrtr_el2(HFGRTR_EL2_INIT_VAL);
1188				write_hfgwtr_el2(HFGWTR_EL2_INIT_VAL);
1189			}
1190
1191			/* Condition to ensure EL2 is being used. */
1192			if ((scr_el3 & SCR_HCE_BIT) != 0U) {
1193				/* Initialize SCTLR_EL2 register with reset value. */
1194				sctlr_el2 = SCTLR_EL2_RES1;
1195
1196				/*
1197				 * If workaround of errata 764081 for Cortex-A75
1198				 * is used then set SCTLR_EL2.IESB to enable
1199				 * Implicit Error Synchronization Barrier.
1200				 */
1201				if (errata_a75_764081_applies()) {
1202					sctlr_el2 |= SCTLR_IESB_BIT;
1203				}
1204
1205				write_sctlr_el2(sctlr_el2);
1206			} else {
1207				/*
1208				 * (scr_el3 & SCR_HCE_BIT==0)
1209				 * EL2 implemented but unused.
1210				 */
1211				init_nonsecure_el2_unused(ctx);
1212			}
1213		}
1214

READING CONTRACT

발췌 범위와 읽기 기준

원문 1126–1214행 발췌는 앞 함수의 끝과 cm_prepare_el3_exit()의 앞부분을 함께 담는다. 함수 전체는 1150–1228행이고, 발췌 뒤에 EL1 restore 빌드 조건과 next ERET context 지정이 이어진다. 아래 관련 원본의 cm_el1_sysregs_context_save/restore()는 별도 함수다.

RELATED SOURCE

함께 읽어야 하는 원본 코드

첫 코드 조각만으로 동작이 완성되지 않는 경우 호출 매크로, 객체 정의와 실제 실행 목록을 같은 페이지에 묶었다. 각 조각은 같은 기준 commit에서 가져왔다.

1954
1955	if (is_feat_ls64_accdata_supported()) {
1956		write_accdata_el1(read_el1_ctx_ls64(ctx, accdata_el1));
1957	}
1958}
1959
1960/*******************************************************************************
1961 * The next couple of functions are used by runtime services to save and restore
1962 * EL1 context on the 'cpu_context' structure for the specified security state.
1963 ******************************************************************************/
1964void cm_el1_sysregs_context_save(uint32_t security_state)
1965{
1966	cpu_context_t *ctx;
1967
1968	ctx = cm_get_context(security_state);
1969	assert(ctx != NULL);
1970
1971	el1_sysregs_context_save(get_el1_sysregs_ctx(ctx));
1972
1973#if IMAGE_BL31
1974	if (security_state == SECURE) {
1975		PUBLISH_EVENT(cm_exited_secure_world);
1976	} else {
1977		PUBLISH_EVENT(cm_exited_normal_world);
1978	}
1979#endif
1980}
1981
1982void cm_el1_sysregs_context_restore(uint32_t security_state)
1983{
1984	cpu_context_t *ctx;
1985
1986	ctx = cm_get_context(security_state);
1987	assert(ctx != NULL);
1988
1989	el1_sysregs_context_restore(get_el1_sysregs_ctx(ctx));
1990
1991#if IMAGE_BL31
1992	if (security_state == SECURE) {
1993		PUBLISH_EVENT(cm_entering_secure_world);
1994	} else {
1995		PUBLISH_EVENT(cm_entering_normal_world);
1996	}
1997#endif
1998}
1999
2000#endif /* ((IMAGE_BL1) || (IMAGE_BL31 && (!CTX_INCLUDE_EL2_REGS))) */
2001
2002/*******************************************************************************
2003 * This function populates ELR_EL3 member of 'cpu_context' pertaining to the
2004 * given security state with the given entrypoint
2005 ******************************************************************************/
2006void cm_set_elr_el3(uint32_t security_state, uintptr_t entrypoint)
2007{
2008	cpu_context_t *ctx;
2009	el3_state_t *state;
2010
2011	ctx = cm_get_context(security_state);
2012	assert(ctx != NULL);
2013
2014	/* Populate EL3 state so that ERET jumps to the correct entry */
2015	state = get_el3state_ctx(ctx);
2016	write_ctx_reg(state, CTX_ELR_EL3, entrypoint);
2017}
2018
2019/*******************************************************************************
2020 * This function populates ELR_EL3 and SPSR_EL3 members of 'cpu_context'
2021 * pertaining to the given security state
2022 ******************************************************************************/
2023void cm_set_elr_spsr_el3(uint32_t security_state,
2024			uintptr_t entrypoint, uint32_t spsr)
2025{
2026	cpu_context_t *ctx;
2027	el3_state_t *state;
2028
2029	ctx = cm_get_context(security_state);
2030	assert(ctx != NULL);
2031
2032	/* Populate EL3 state so that ERET jumps to the correct entry */
2033	state = get_el3state_ctx(ctx);
2034	write_ctx_reg(state, CTX_ELR_EL3, entrypoint);

1972
1973#if IMAGE_BL31
1974	if (security_state == SECURE) {
1975		PUBLISH_EVENT(cm_exited_secure_world);
1976	} else {
1977		PUBLISH_EVENT(cm_exited_normal_world);
1978	}
1979#endif
1980}
1981
1982void cm_el1_sysregs_context_restore(uint32_t security_state)
1983{
1984	cpu_context_t *ctx;
1985
1986	ctx = cm_get_context(security_state);
1987	assert(ctx != NULL);
1988
1989	el1_sysregs_context_restore(get_el1_sysregs_ctx(ctx));
1990
1991#if IMAGE_BL31
1992	if (security_state == SECURE) {
1993		PUBLISH_EVENT(cm_entering_secure_world);
1994	} else {
1995		PUBLISH_EVENT(cm_entering_normal_world);
1996	}
1997#endif
1998}
1999
2000#endif /* ((IMAGE_BL1) || (IMAGE_BL31 && (!CTX_INCLUDE_EL2_REGS))) */
2001
2002/*******************************************************************************
2003 * This function populates ELR_EL3 member of 'cpu_context' pertaining to the
2004 * given security state with the given entrypoint
2005 ******************************************************************************/
2006void cm_set_elr_el3(uint32_t security_state, uintptr_t entrypoint)
2007{
2008	cpu_context_t *ctx;
2009	el3_state_t *state;
2010
2011	ctx = cm_get_context(security_state);
2012	assert(ctx != NULL);
2013
2014	/* Populate EL3 state so that ERET jumps to the correct entry */
2015	state = get_el3state_ctx(ctx);
2016	write_ctx_reg(state, CTX_ELR_EL3, entrypoint);
2017}
2018
2019/*******************************************************************************
2020 * This function populates ELR_EL3 and SPSR_EL3 members of 'cpu_context'
2021 * pertaining to the given security state
2022 ******************************************************************************/
2023void cm_set_elr_spsr_el3(uint32_t security_state,
2024			uintptr_t entrypoint, uint32_t spsr)
2025{
2026	cpu_context_t *ctx;
2027	el3_state_t *state;
2028
2029	ctx = cm_get_context(security_state);
2030	assert(ctx != NULL);
2031
2032	/* Populate EL3 state so that ERET jumps to the correct entry */
2033	state = get_el3state_ctx(ctx);
2034	write_ctx_reg(state, CTX_ELR_EL3, entrypoint);
2035	write_ctx_reg(state, CTX_SPSR_EL3, spsr);
2036}
2037
2038/*******************************************************************************
2039 * This function updates a single bit in the SCR_EL3 member of the 'cpu_context'
2040 * pertaining to the given security state using the value and bit position

DETAILS

저장, 준비, pointer 선택의 경계

IMPLEMENTATION NOTES

조건부 경로를 지우지 않고 읽기

OBJECT LIFETIME

context pointer와 register 묶음

FAILURE PATH

잘못된 복귀를 좁히는 경계

각 증상을 target slot 조회, 조건부 복원, 반환 경계에서 따로 판정한다.

EVIDENCE

빌드와 실행에서 확인할 자료

아래 명령과 계측 항목은 해당 보드의 빌드 결과가 있을 때 사용하는 확인 방법이다. 이 글의 코드 분석을 보드에서 재현한 실행 결과로 해석해서는 안 된다.

LAB

조건을 분리해 대조하기

1. 01
   같은 CPU의 두 slot

   허용된 디버그 환경에서 SECURE와 NON_SECURE context pointer와 target 선택을 비교한다.

2. 02
   EL2 사용 여부

   빌드 옵션·target EL·SCR_HCE_BIT를 기록해 HCE 사용과 unused-EL2 경로를 구분한다.

3. 03
   OPTEED SMC 분기

   Normal 요청, CALL_DONE, FIQ_DONE에서 save/restore 호출 차이를 고정 원문과 대조한다.

PRIMARY REFERENCES

원문과 다음 글