arm64: kpti: Fix the interaction between ASID switching and software PAN

With ARM64_SW_TTBR0_PAN enabled, the exception entry code checks the
active ASID to decide whether user access was enabled (non-zero ASID)
when the exception was taken. On return from exception, if user access
was previously disabled, it re-instates TTBR0_EL1 from the per-thread
saved value (updated in switch_mm() or efi_set_pgd()).

Commit 7655abb95386 ("arm64: mm: Move ASID from TTBR0 to TTBR1") makes a
TTBR0_EL1 + ASID switching non-atomic. Subsequently, commit 27a921e75711
("arm64: mm: Fix and re-enable ARM64_SW_TTBR0_PAN") changes the
__uaccess_ttbr0_disable() function and asm macro to first write the
reserved TTBR0_EL1 followed by the ASID=0 update in TTBR1_EL1. If an
exception occurs between these two, the exception return code will
re-instate a valid TTBR0_EL1. Similar scenario can happen in
cpu_switch_mm() between setting the reserved TTBR0_EL1 and the ASID
update in cpu_do_switch_mm().

This patch reverts the entry.S check for ASID == 0 to TTBR0_EL1 and
disables the interrupts around the TTBR0_EL1 and ASID switching code in
__uaccess_ttbr0_disable(). It also ensures that, when returning from the
EFI runtime services, efi_set_pgd() doesn't leave a non-zero ASID in
TTBR1_EL1 by using uaccess_ttbr0_{enable,disable}.

The accesses to current_thread_info()->ttbr0 are updated to use
READ_ONCE/WRITE_ONCE.

As a safety measure, __uaccess_ttbr0_enable() always masks out any
existing non-zero ASID TTBR1_EL1 before writing in the new ASID.

Fixes: 27a921e75711 ("arm64: mm: Fix and re-enable ARM64_SW_TTBR0_PAN")
Acked-by: Will Deacon <will.deacon@arm.com>
Reported-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: James Morse <james.morse@arm.com>
Tested-by: James Morse <james.morse@arm.com>
Co-developed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/asm-uaccess.h

@@ -14,11 +14,11 @@
 #ifdef CONFIG_ARM64_SW_TTBR0_PAN
 	.macro	__uaccess_ttbr0_disable, tmp1
 	mrs	\tmp1, ttbr1_el1			// swapper_pg_dir
+	bic	\tmp1, \tmp1, #TTBR_ASID_MASK
 	sub	\tmp1, \tmp1, #RESERVED_TTBR0_SIZE	// reserved_ttbr0 just before swapper_pg_dir
 	msr	ttbr0_el1, \tmp1			// set reserved TTBR0_EL1
 	isb
 	add	\tmp1, \tmp1, #RESERVED_TTBR0_SIZE
-	bic	\tmp1, \tmp1, #TTBR_ASID_MASK
 	msr	ttbr1_el1, \tmp1		// set reserved ASID
 	isb
 	.endm
@@ -35,9 +35,11 @@
 	isb
 	.endm
 
-	.macro	uaccess_ttbr0_disable, tmp1
+	.macro	uaccess_ttbr0_disable, tmp1, tmp2
 alternative_if_not ARM64_HAS_PAN
+	save_and_disable_irq \tmp2		// avoid preemption
 	__uaccess_ttbr0_disable \tmp1
+	restore_irq \tmp2
 alternative_else_nop_endif
 	.endm
 
@@ -49,7 +51,7 @@ alternative_if_not ARM64_HAS_PAN
 alternative_else_nop_endif
 	.endm
 #else
-	.macro	uaccess_ttbr0_disable, tmp1
+	.macro	uaccess_ttbr0_disable, tmp1, tmp2
 	.endm
 
 	.macro	uaccess_ttbr0_enable, tmp1, tmp2, tmp3
@@ -59,8 +61,8 @@ alternative_else_nop_endif
 /*
  * These macros are no-ops when UAO is present.
  */
-	.macro	uaccess_disable_not_uao, tmp1
-	uaccess_ttbr0_disable \tmp1
+	.macro	uaccess_disable_not_uao, tmp1, tmp2
+	uaccess_ttbr0_disable \tmp1, \tmp2
 alternative_if ARM64_ALT_PAN_NOT_UAO
 	SET_PSTATE_PAN(1)
 alternative_else_nop_endif

arch/arm64/include/asm/efi.h

@@ -121,19 +121,21 @@ static inline void efi_set_pgd(struct mm_struct *mm)
 		if (mm != current->active_mm) {
 			/*
 			 * Update the current thread's saved ttbr0 since it is
-			 * restored as part of a return from exception. Set
-			 * the hardware TTBR0_EL1 using cpu_switch_mm()
-			 * directly to enable potential errata workarounds.
+			 * restored as part of a return from exception. Enable
+			 * access to the valid TTBR0_EL1 and invoke the errata
+			 * workaround directly since there is no return from
+			 * exception when invoking the EFI run-time services.
 			 */
 			update_saved_ttbr0(current, mm);
-			cpu_switch_mm(mm->pgd, mm);
+			uaccess_ttbr0_enable();
+			post_ttbr_update_workaround();
 		} else {
 			/*
 			 * Defer the switch to the current thread's TTBR0_EL1
 			 * until uaccess_enable(). Restore the current
 			 * thread's saved ttbr0 corresponding to its active_mm
 			 */
-			cpu_set_reserved_ttbr0();
+			uaccess_ttbr0_disable();
 			update_saved_ttbr0(current, current->active_mm);
 		}
 	}

arch/arm64/include/asm/mmu_context.h

@@ -184,7 +184,7 @@ static inline void update_saved_ttbr0(struct task_struct *tsk,
 	else
 		ttbr = virt_to_phys(mm->pgd) | ASID(mm) << 48;
 
-	task_thread_info(tsk)->ttbr0 = ttbr;
+	WRITE_ONCE(task_thread_info(tsk)->ttbr0, ttbr);
 }
 #else
 static inline void update_saved_ttbr0(struct task_struct *tsk,
@@ -239,6 +239,7 @@ switch_mm(struct mm_struct *prev, struct mm_struct *next,
 #define activate_mm(prev,next)	switch_mm(prev, next, current)
 
 void verify_cpu_asid_bits(void);
+void post_ttbr_update_workaround(void);
 
 #endif /* !__ASSEMBLY__ */
 

arch/arm64/include/asm/uaccess.h

@@ -105,16 +105,18 @@ static inline void set_fs(mm_segment_t fs)
 #ifdef CONFIG_ARM64_SW_TTBR0_PAN
 static inline void __uaccess_ttbr0_disable(void)
 {
-	unsigned long ttbr;
+	unsigned long flags, ttbr;
 
+	local_irq_save(flags);
 	ttbr = read_sysreg(ttbr1_el1);
+	ttbr &= ~TTBR_ASID_MASK;
 	/* reserved_ttbr0 placed before swapper_pg_dir */
 	write_sysreg(ttbr - RESERVED_TTBR0_SIZE, ttbr0_el1);
 	isb();
 	/* Set reserved ASID */
-	ttbr &= ~TTBR_ASID_MASK;
 	write_sysreg(ttbr, ttbr1_el1);
 	isb();
+	local_irq_restore(flags);
 }
 
 static inline void __uaccess_ttbr0_enable(void)
@@ -127,10 +129,11 @@ static inline void __uaccess_ttbr0_enable(void)
 	 * roll-over and an update of 'ttbr0'.
 	 */
 	local_irq_save(flags);
-	ttbr0 = current_thread_info()->ttbr0;
+	ttbr0 = READ_ONCE(current_thread_info()->ttbr0);
 
 	/* Restore active ASID */
 	ttbr1 = read_sysreg(ttbr1_el1);
+	ttbr1 &= ~TTBR_ASID_MASK;		/* safety measure */
 	ttbr1 |= ttbr0 & TTBR_ASID_MASK;
 	write_sysreg(ttbr1, ttbr1_el1);
 	isb();

arch/arm64/kernel/entry.S

@@ -204,7 +204,7 @@ alternative_if ARM64_HAS_PAN
 alternative_else_nop_endif
 
 	.if	\el != 0
-	mrs	x21, ttbr1_el1
+	mrs	x21, ttbr0_el1
 	tst	x21, #TTBR_ASID_MASK		// Check for the reserved ASID
 	orr	x23, x23, #PSR_PAN_BIT		// Set the emulated PAN in the saved SPSR
 	b.eq	1f				// TTBR0 access already disabled

arch/arm64/lib/clear_user.S

@@ -50,7 +50,7 @@ uao_user_alternative 9f, strh, sttrh, wzr, x0, 2
 	b.mi	5f
 uao_user_alternative 9f, strb, sttrb, wzr, x0, 0
 5:	mov	x0, #0
-	uaccess_disable_not_uao x2
+	uaccess_disable_not_uao x2, x3
 	ret
 ENDPROC(__clear_user)
 

arch/arm64/lib/copy_from_user.S

@@ -67,7 +67,7 @@ ENTRY(__arch_copy_from_user)
 	uaccess_enable_not_uao x3, x4, x5
 	add	end, x0, x2
 #include "copy_template.S"
-	uaccess_disable_not_uao x3
+	uaccess_disable_not_uao x3, x4
 	mov	x0, #0				// Nothing to copy
 	ret
 ENDPROC(__arch_copy_from_user)

arch/arm64/lib/copy_in_user.S

@@ -68,7 +68,7 @@ ENTRY(raw_copy_in_user)
 	uaccess_enable_not_uao x3, x4, x5
 	add	end, x0, x2
 #include "copy_template.S"
-	uaccess_disable_not_uao x3
+	uaccess_disable_not_uao x3, x4
 	mov	x0, #0
 	ret
 ENDPROC(raw_copy_in_user)

arch/arm64/lib/copy_to_user.S

@@ -66,7 +66,7 @@ ENTRY(__arch_copy_to_user)
 	uaccess_enable_not_uao x3, x4, x5
 	add	end, x0, x2
 #include "copy_template.S"
-	uaccess_disable_not_uao x3
+	uaccess_disable_not_uao x3, x4
 	mov	x0, #0
 	ret
 ENDPROC(__arch_copy_to_user)

arch/arm64/mm/cache.S

@@ -72,7 +72,7 @@ USER(9f, ic	ivau, x4	)		// invalidate I line PoU
 	isb
 	mov	x0, #0
 1:
-	uaccess_ttbr0_disable x1
+	uaccess_ttbr0_disable x1, x2
 	ret
 9:
 	mov	x0, #-EFAULT

arch/arm64/mm/proc.S

@@ -153,6 +153,9 @@ ENDPROC(cpu_do_resume)
 ENTRY(cpu_do_switch_mm)
 	mrs	x2, ttbr1_el1
 	mmid	x1, x1				// get mm->context.id
+#ifdef CONFIG_ARM64_SW_TTBR0_PAN
+	bfi	x0, x1, #48, #16		// set the ASID field in TTBR0
+#endif
 	bfi	x2, x1, #48, #16		// set the ASID
 	msr	ttbr1_el1, x2			// in TTBR1 (since TCR.A1 is set)
 	isb

arch/arm64/xen/hypercall.S

@@ -107,6 +107,6 @@ ENTRY(privcmd_call)
 	/*
 	 * Disable userspace access from kernel once the hyp call completed.
 	 */
-	uaccess_ttbr0_disable x6
+	uaccess_ttbr0_disable x6, x7
 	ret
 ENDPROC(privcmd_call);