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>

1 files changed, 30 insertions, 0 deletions

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 78b3e0f8e997..35697a09926f 100644
--- a/arch/arm64/kernel/mte.c
+++ b/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;
+}