[PATCH v2 2/4] mm: Probe for sub-page faults in fault_in_*()

[Catalin Marinas]

On hardware with features like arm64 MTE or SPARC ADI, an access fault
can be triggered at sub-page granularity. Depending on how the
fault_in_*() functions are used, the caller can get into a live-lock by
continuously retrying the fault-in on an address different from the one
where the uaccess failed.

In the majority of cases progress is ensured by the following
conditions:

1. copy_{to,from}_user_nofault() guarantees at least one byte access if
   the user address is not faulting.

2. The fault_in_*() loop is resumed from the next address that could not
   be accessed by copy_{to,from}_user_nofault().

If the loop iteration is restarted from an earlier point, the loop is
repeated with the same conditions and it would live-lock. The same
problem exists if the fault_in_*() is attempted on the fault address
reported by copy_*_user_nofault() since the latter does not guarantee
the maximum possible bytes are written and fault_in_*() will succeed in
probing a single byte.

Introduce probe_subpage_*() and call them from the corresponding
fault_in_*() functions on the requested 'min_size' range. The arch code
with sub-page faults will have to implement the specific probing
functionality.

Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
---
 arch/Kconfig            |  7 ++++++
 include/linux/uaccess.h | 53 +++++++++++++++++++++++++++++++++++++++++
 mm/gup.c                |  9 ++++---
 3 files changed, 66 insertions(+), 3 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 26b8ed11639d..02502b3362aa 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -27,6 +27,13 @@ config HAVE_IMA_KEXEC
 config SET_FS
 	bool
 
+config ARCH_HAS_SUBPAGE_FAULTS
+	bool
+	help
+	  Select if the architecture can check permissions at sub-page
+	  granularity (e.g. arm64 MTE). The probe_user_*() functions
+	  must be implemented.
+
 config HOTPLUG_SMT
 	bool
 
diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index ac0394087f7d..04ad214c98cd 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -271,6 +271,59 @@ static inline bool pagefault_disabled(void)
  */
 #define faulthandler_disabled() (pagefault_disabled() || in_atomic())
 
+#ifndef CONFIG_ARCH_HAS_SUBPAGE_FAULTS
+
+/**
+ * probe_subpage_writeable: probe the user range for write faults at sub-page
+ *			    granularity (e.g. arm64 MTE)
+ * @uaddr: start of address range
+ * @size: size of address range
+ *
+ * Returns 0 on success, the number of bytes not probed on fault.
+ *
+ * It is expected that the caller checked for the write permission of each
+ * page in the range either by put_user() or GUP. The architecture port can
+ * implement a more efficient get_user() probing if the same sub-page faults
+ * are triggered by either a read or a write.
+ */
+static inline size_t probe_subpage_writeable(void __user *uaddr, size_t size)
+{
+	return 0;
+}
+
+/**
+ * probe_subpage_safe_writeable: probe the user range for write faults at
+ *				 sub-page granularity without corrupting the
+ *				 existing data
+ * @uaddr: start of address range
+ * @size: size of address range
+ *
+ * Returns 0 on success, the number of bytes not probed on fault.
+ *
+ * It is expected that the caller checked for the write permission of each
+ * page in the range either by put_user() or GUP.
+ */
+static inline size_t probe_subpage_safe_writeable(void __user *uaddr,
+						  size_t size)
+{
+	return 0;
+}
+
+/**
+ * probe_subpage_readable: probe the user range for read faults at sub-page
+ *			   granularity
+ * @uaddr: start of address range
+ * @size: size of address range
+ *
+ * Returns 0 on success, the number of bytes not probed on fault.
+ */
+static inline size_t probe_subpage_readable(void __user *uaddr, size_t size)
+{
+	return 0;
+}
+
+#endif
+
 #ifndef ARCH_HAS_NOCACHE_UACCESS
 
 static inline __must_check unsigned long
diff --git a/mm/gup.c b/mm/gup.c
index baa8240615a4..7fa69b0fb859 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1691,7 +1691,8 @@ size_t fault_in_writeable(char __user *uaddr, size_t size, size_t min_size)
 out:
 	if (size > uaddr - start)
 		faulted_in = uaddr - start;
-	if (faulted_in < min_size)
+	if (faulted_in < min_size ||
+	    (min_size && probe_subpage_writeable(start, min_size)))
 		return size;
 	return size - faulted_in;
 }
@@ -1759,7 +1760,8 @@ size_t fault_in_safe_writeable(const char __user *uaddr, size_t size,
 		mmap_read_unlock(mm);
 	if (nstart != end)
 		faulted_in = min_t(size_t, nstart - start, size);
-	if (faulted_in < min_size)
+	if (faulted_in < min_size ||
+	    (min_size && probe_subpage_safe_writeable(uaddr, min_size)))
 		return size;
 	return size - faulted_in;
 }
@@ -1801,7 +1803,8 @@ size_t fault_in_readable(const char __user *uaddr, size_t size,
 	(void)c;
 	if (size > uaddr - start)
 		faulted_in = uaddr - start;
-	if (faulted_in < min_size)
+	if (faulted_in < min_size ||
+	    (min_size && probe_subpage_readable(start, min_size)))
 		return size;
 	return size - faulted_in;
 }