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@xxxxxxx>
---
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;
}
