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_writeable() function is 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_user_nofault() guarantees at least one byte access if the
user address is not faulting.
2. The fault_in_writeable() loop is resumed from the first address that
could not be accessed by copy_to_user_nofault().
If the loop iteration is restarted from an earlier (initial) point, the
loop is repeated with the same conditions and it would live-lock.
Introduce an arch-specific probe_subpage_writeable() and call it from
the newly added fault_in_subpage_writeable() function. The arch code
with sub-page faults will have to implement the specific probing
functionality.
Note that no other fault_in_subpage_*() functions are added since they
have no callers currently susceptible to a live-lock.
Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
arch/Kconfig | 7 +++++++
include/linux/pagemap.h | 1 +
include/linux/uaccess.h | 22 ++++++++++++++++++++++
mm/gup.c | 29 +++++++++++++++++++++++++++++
4 files changed, 59 insertions(+)
diff --git a/arch/Kconfig b/arch/Kconfig
index 29b0167c088b..b34032279926 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -24,6 +24,13 @@ config KEXEC_ELF
config HAVE_IMA_KEXEC
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/pagemap.h b/include/linux/pagemap.h
index 993994cd943a..6165283bdb6f 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -1046,6 +1046,7 @@ void folio_add_wait_queue(struct folio *folio, wait_queue_entry_t *waiter);
* Fault in userspace address range.
*/
size_t fault_in_writeable(char __user *uaddr, size_t size);
+size_t fault_in_subpage_writeable(char __user *uaddr, size_t size);
size_t fault_in_safe_writeable(const char __user *uaddr, size_t size);
size_t fault_in_readable(const char __user *uaddr, size_t size);
diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 546179418ffa..8bbb2dabac19 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -231,6 +231,28 @@ 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;
+}
+
+#endif /* CONFIG_ARCH_HAS_SUBPAGE_FAULTS */
+
#ifndef ARCH_HAS_NOCACHE_UACCESS
static inline __must_check unsigned long
diff --git a/mm/gup.c b/mm/gup.c
index f598a037eb04..501bc150792c 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1648,6 +1648,35 @@ size_t fault_in_writeable(char __user *uaddr, size_t size)
}
EXPORT_SYMBOL(fault_in_writeable);
+/**
+ * fault_in_subpage_writeable - fault in an address range for writing
+ * @uaddr: start of address range
+ * @size: size of address range
+ *
+ * Fault in a user address range for writing while checking for permissions at
+ * sub-page granularity (e.g. arm64 MTE). This function should be used when
+ * the caller cannot guarantee forward progress of a copy_to_user() loop.
+ *
+ * Returns the number of bytes not faulted in (like copy_to_user() and
+ * copy_from_user()).
+ */
+size_t fault_in_subpage_writeable(char __user *uaddr, size_t size)
+{
+ size_t faulted_in;
+
+ /*
+ * Attempt faulting in at page granularity first for page table
+ * permission checking. The arch-specific probe_subpage_writeable()
+ * functions may not check for this.
+ */
+ faulted_in = size - fault_in_writeable(uaddr, size);
+ if (faulted_in)
+ faulted_in -= probe_subpage_writeable(uaddr, faulted_in);
+
+ return size - faulted_in;
+}
+EXPORT_SYMBOL(fault_in_subpage_writeable);
+
/*
* fault_in_safe_writeable - fault in an address range for writing
* @uaddr: start of address range
