arm64: Add support for user sub-page fault probing

With MTE, even if the pte allows an access, a mismatched tag somewhere
within a page can still cause a fault. Select ARCH_HAS_SUBPAGE_FAULTS if
MTE is enabled and implement the probe_subpage_writeable() function.
Note that get_user() is sufficient for the writeable MTE check since the
same tag mismatch fault would be triggered by a read. The caller of
probe_subpage_writeable() will need to check the pte permissions
(put_user, GUP).

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20220423100751.1870771-3-catalin.marinas@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/Kconfig

@@ -1871,6 +1871,7 @@ config ARM64_MTE
 	depends on AS_HAS_LSE_ATOMICS
 	# Required for tag checking in the uaccess routines
 	depends on ARM64_PAN
+	select ARCH_HAS_SUBPAGE_FAULTS
 	select ARCH_USES_HIGH_VMA_FLAGS
 	help
 	  Memory Tagging (part of the ARMv8.5 Extensions) provides

arch/arm64/include/asm/mte.h

@@ -47,6 +47,7 @@ long set_mte_ctrl(struct task_struct *task, unsigned long arg);
 long get_mte_ctrl(struct task_struct *task);
 int mte_ptrace_copy_tags(struct task_struct *child, long request,
 			 unsigned long addr, unsigned long data);
+size_t mte_probe_user_range(const char __user *uaddr, size_t size);
 
 #else /* CONFIG_ARM64_MTE */
 

arch/arm64/include/asm/uaccess.h

@@ -460,4 +460,19 @@ static inline int __copy_from_user_flushcache(void *dst, const void __user *src,
 }
 #endif
 
+#ifdef CONFIG_ARCH_HAS_SUBPAGE_FAULTS
+
+/*
+ * Return 0 on success, the number of bytes not probed otherwise.
+ */
+static inline size_t probe_subpage_writeable(const char __user *uaddr,
+					     size_t size)
+{
+	if (!system_supports_mte())
+		return 0;
+	return mte_probe_user_range(uaddr, size);
+}
+
+#endif /* CONFIG_ARCH_HAS_SUBPAGE_FAULTS */
+
 #endif /* __ASM_UACCESS_H */

arch/arm64/kernel/mte.c

@@ -15,6 +15,7 @@
 #include <linux/swapops.h>
 #include <linux/thread_info.h>
 #include <linux/types.h>
+#include <linux/uaccess.h>
 #include <linux/uio.h>
 
 #include <asm/barrier.h>
@@ -543,3 +544,32 @@ static int register_mte_tcf_preferred_sysctl(void)
 	return 0;
 }
 subsys_initcall(register_mte_tcf_preferred_sysctl);
+
+/*
+ * Return 0 on success, the number of bytes not probed otherwise.
+ */
+size_t mte_probe_user_range(const char __user *uaddr, size_t size)
+{
+	const char __user *end = uaddr + size;
+	int err = 0;
+	char val;
+
+	__raw_get_user(val, uaddr, err);
+	if (err)
+		return size;
+
+	uaddr = PTR_ALIGN(uaddr, MTE_GRANULE_SIZE);
+	while (uaddr < end) {
+		/*
+		 * A read is sufficient for mte, the caller should have probed
+		 * for the pte write permission if required.
+		 */
+		__raw_get_user(val, uaddr, err);
+		if (err)
+			return end - uaddr;
+		uaddr += MTE_GRANULE_SIZE;
+	}
+	(void)val;
+
+	return 0;
+}