arm64: kernel: Don't toggle PAN on systems with UAO

If a CPU supports both Privileged Access Never (PAN) and User Access
Override (UAO), we don't need to disable/re-enable PAN round all
copy_to_user() like calls.

UAO alternatives cause these calls to use the 'unprivileged' load/store
instructions, which are overridden to be the privileged kind when
fs==KERNEL_DS.

This patch changes the copy_to_user() calls to have their PAN toggling
depend on a new composite 'feature' ARM64_ALT_PAN_NOT_UAO.

If both features are detected, PAN will be enabled, but the copy_to_user()
alternatives will not be applied. This means PAN will be enabled all the
time for these functions. If only PAN is detected, the toggling will be
enabled as normal.

This will save the time taken to disable/re-enable PAN, and allow us to
catch copy_to_user() accesses that occur with fs==KERNEL_DS.

Futex and swp-emulation code continue to hang their PAN toggling code on
ARM64_HAS_PAN.

Signed-off-by: James Morse <[e-mail]>
Signed-off-by: Catalin Marinas <[e-mail]>

arch/arm64/include/asm/cpufeature.h

···
 #define ARM64_WORKAROUND_834220			7
 #define ARM64_HAS_NO_HW_PREFETCH		8
 #define ARM64_HAS_UAO				9
+#define ARM64_ALT_PAN_NOT_UAO			10
 
-#define ARM64_NCAPS				10
+#define ARM64_NCAPS				11
 
 #ifndef __ASSEMBLY__
 

arch/arm64/include/asm/uaccess.h

···
 do {									\
 	unsigned long __gu_val;						\
 	__chk_user_ptr(ptr);						\
-	asm(ALTERNATIVE("nop", SET_PSTATE_PAN(0), ARM64_HAS_PAN,	\
+	asm(ALTERNATIVE("nop", SET_PSTATE_PAN(0), ARM64_ALT_PAN_NOT_UAO,\
 			CONFIG_ARM64_PAN));				\
 	switch (sizeof(*(ptr))) {					\
 	case 1:								\
···
 		BUILD_BUG();						\
 	}								\
 	(x) = (__force __typeof__(*(ptr)))__gu_val;			\
-	asm(ALTERNATIVE("nop", SET_PSTATE_PAN(1), ARM64_HAS_PAN,	\
+	asm(ALTERNATIVE("nop", SET_PSTATE_PAN(1), ARM64_ALT_PAN_NOT_UAO,\
 			CONFIG_ARM64_PAN));				\
 } while (0)
 
···
 do {									\
 	__typeof__(*(ptr)) __pu_val = (x);				\
 	__chk_user_ptr(ptr);						\
-	asm(ALTERNATIVE("nop", SET_PSTATE_PAN(0), ARM64_HAS_PAN,	\
+	asm(ALTERNATIVE("nop", SET_PSTATE_PAN(0), ARM64_ALT_PAN_NOT_UAO,\
 			CONFIG_ARM64_PAN));				\
 	switch (sizeof(*(ptr))) {					\
 	case 1:								\
···
 	default:							\
 		BUILD_BUG();						\
 	}								\
-	asm(ALTERNATIVE("nop", SET_PSTATE_PAN(1), ARM64_HAS_PAN,	\
+	asm(ALTERNATIVE("nop", SET_PSTATE_PAN(1), ARM64_ALT_PAN_NOT_UAO,\
 			CONFIG_ARM64_PAN));				\
 } while (0)
 

arch/arm64/kernel/cpufeature.c

···
 		.width = 0,				\
 	}
 
+/* meta feature for alternatives */
+static bool __maybe_unused
+cpufeature_pan_not_uao(const struct arm64_cpu_capabilities *entry);
+
 static struct arm64_ftr_bits ftr_id_aa64isar0[] = {
 	ARM64_FTR_BITS(FTR_STRICT, FTR_EXACT, 32, 32, 0),
 	ARM64_FTR_BITS(FTR_STRICT, FTR_EXACT, ID_AA64ISAR0_RDM_SHIFT, 4, 0),
···
 		.enable = cpu_enable_uao,
 	},
 #endif /* CONFIG_ARM64_UAO */
+#ifdef CONFIG_ARM64_PAN
+	{
+		.capability = ARM64_ALT_PAN_NOT_UAO,
+		.matches = cpufeature_pan_not_uao,
+	},
+#endif /* CONFIG_ARM64_PAN */
 	{},
 };
 
···
 	if (L1_CACHE_BYTES < cls)
 		pr_warn("L1_CACHE_BYTES smaller than the Cache Writeback Granule (%d < %d)\n",
 			L1_CACHE_BYTES, cls);
+}
+
+static bool __maybe_unused
+cpufeature_pan_not_uao(const struct arm64_cpu_capabilities *entry)
+{
+	return (cpus_have_cap(ARM64_HAS_PAN) && !cpus_have_cap(ARM64_HAS_UAO));
 }

arch/arm64/lib/clear_user.S

···
  * Alignment fixed up by hardware.
  */
 ENTRY(__clear_user)
-ALTERNATIVE("nop", __stringify(SET_PSTATE_PAN(0)), ARM64_HAS_PAN, \
+ALTERNATIVE("nop", __stringify(SET_PSTATE_PAN(0)), ARM64_ALT_PAN_NOT_UAO, \
 	    CONFIG_ARM64_PAN)
 	mov	x2, x1			// save the size for fixup return
 	subs	x1, x1, #8
···
 	b.mi	5f
 uao_user_alternative 9f, strb, sttrb, wzr, x0, 0
 5:	mov	x0, #0
-ALTERNATIVE("nop", __stringify(SET_PSTATE_PAN(1)), ARM64_HAS_PAN, \
+ALTERNATIVE("nop", __stringify(SET_PSTATE_PAN(1)), ARM64_ALT_PAN_NOT_UAO, \
 	    CONFIG_ARM64_PAN)
 	ret
 ENDPROC(__clear_user)

arch/arm64/lib/copy_from_user.S

···
 
 end	.req	x5
 ENTRY(__copy_from_user)
-ALTERNATIVE("nop", __stringify(SET_PSTATE_PAN(0)), ARM64_HAS_PAN, \
+ALTERNATIVE("nop", __stringify(SET_PSTATE_PAN(0)), ARM64_ALT_PAN_NOT_UAO, \
 	    CONFIG_ARM64_PAN)
 	add	end, x0, x2
 #include "copy_template.S"
-ALTERNATIVE("nop", __stringify(SET_PSTATE_PAN(1)), ARM64_HAS_PAN, \
+ALTERNATIVE("nop", __stringify(SET_PSTATE_PAN(1)), ARM64_ALT_PAN_NOT_UAO, \
 	    CONFIG_ARM64_PAN)
 	mov	x0, #0				// Nothing to copy
 	ret

arch/arm64/lib/copy_in_user.S

···
 
 end	.req	x5
 ENTRY(__copy_in_user)
-ALTERNATIVE("nop", __stringify(SET_PSTATE_PAN(0)), ARM64_HAS_PAN, \
+ALTERNATIVE("nop", __stringify(SET_PSTATE_PAN(0)), ARM64_ALT_PAN_NOT_UAO, \
 	    CONFIG_ARM64_PAN)
 	add	end, x0, x2
 #include "copy_template.S"
-ALTERNATIVE("nop", __stringify(SET_PSTATE_PAN(1)), ARM64_HAS_PAN, \
+ALTERNATIVE("nop", __stringify(SET_PSTATE_PAN(1)), ARM64_ALT_PAN_NOT_UAO, \
 	    CONFIG_ARM64_PAN)
 	mov	x0, #0
 	ret

arch/arm64/lib/copy_to_user.S

···
 
 end	.req	x5
 ENTRY(__copy_to_user)
-ALTERNATIVE("nop", __stringify(SET_PSTATE_PAN(0)), ARM64_HAS_PAN, \
+ALTERNATIVE("nop", __stringify(SET_PSTATE_PAN(0)), ARM64_ALT_PAN_NOT_UAO, \
 	    CONFIG_ARM64_PAN)
 	add	end, x0, x2
 #include "copy_template.S"
-ALTERNATIVE("nop", __stringify(SET_PSTATE_PAN(1)), ARM64_HAS_PAN, \
+ALTERNATIVE("nop", __stringify(SET_PSTATE_PAN(1)), ARM64_ALT_PAN_NOT_UAO, \
 	    CONFIG_ARM64_PAN)
 	mov	x0, #0
 	ret

arch/arm64/mm/fault.c

···
 	}
 
 	if (permission_fault(esr) && (addr < USER_DS)) {
+		if (get_thread_info(regs->sp)->addr_limit == KERNEL_DS)
+			panic("Accessing user space memory with fs=KERNEL_DS");
+
 		if (!search_exception_tables(regs->pc))
 			panic("Accessing user space memory outside uaccess.h routines");
 	}