[PATCH v4 2/4] uaccess: always export _copy_[from|to]_user with CONFIG_RUST

[Alice Ryhl]

From: Arnd Bergmann <arnd@xxxxxxxx>

Rust code needs to be able to access _copy_from_user and _copy_to_user
so that it can skip the check_copy_size check in cases where the length
is known at compile-time, mirroring the logic for when C code will skip
check_copy_size. To do this, we ensure that exported versions of these
methods are available when CONFIG_RUST is enabled.

Alice has verified that this patch passes the CONFIG_TEST_USER_COPY test
on x86 using the Android cuttlefish emulator.

Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
Tested-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
Reviewed-by: Boqun Feng <boqun.feng@xxxxxxxxx>
Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
---
 include/linux/uaccess.h | 38 ++++++++++++++++++++++++--------------
 lib/usercopy.c          | 30 ++++--------------------------
 2 files changed, 28 insertions(+), 40 deletions(-)

diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 3064314f4832..2ebfce98b5cc 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -5,6 +5,7 @@
 #include <linux/fault-inject-usercopy.h>
 #include <linux/instrumented.h>
 #include <linux/minmax.h>
+#include <linux/nospec.h>
 #include <linux/sched.h>
 #include <linux/thread_info.h>
 
@@ -138,13 +139,18 @@ __copy_to_user(void __user *to, const void *from, unsigned long n)
 	return raw_copy_to_user(to, from, n);
 }
 
-#ifdef INLINE_COPY_FROM_USER
 static inline __must_check unsigned long
-_copy_from_user(void *to, const void __user *from, unsigned long n)
+_inline_copy_from_user(void *to, const void __user *from, unsigned long n)
 {
 	unsigned long res = n;
 	might_fault();
 	if (!should_fail_usercopy() && likely(access_ok(from, n))) {
+		/*
+		 * Ensure that bad access_ok() speculation will not
+		 * lead to nasty side effects *after* the copy is
+		 * finished:
+		 */
+		barrier_nospec();
 		instrument_copy_from_user_before(to, from, n);
 		res = raw_copy_from_user(to, from, n);
 		instrument_copy_from_user_after(to, from, n, res);
@@ -153,14 +159,11 @@ _copy_from_user(void *to, const void __user *from, unsigned long n)
 		memset(to + (n - res), 0, res);
 	return res;
 }
-#else
 extern __must_check unsigned long
 _copy_from_user(void *, const void __user *, unsigned long);
-#endif
 
-#ifdef INLINE_COPY_TO_USER
 static inline __must_check unsigned long
-_copy_to_user(void __user *to, const void *from, unsigned long n)
+_inline_copy_to_user(void __user *to, const void *from, unsigned long n)
 {
 	might_fault();
 	if (should_fail_usercopy())
@@ -171,25 +174,32 @@ _copy_to_user(void __user *to, const void *from, unsigned long n)
 	}
 	return n;
 }
-#else
 extern __must_check unsigned long
 _copy_to_user(void __user *, const void *, unsigned long);
-#endif
 
 static __always_inline unsigned long __must_check
 copy_from_user(void *to, const void __user *from, unsigned long n)
 {
-	if (check_copy_size(to, n, false))
-		n = _copy_from_user(to, from, n);
-	return n;
+	if (!check_copy_size(to, n, false))
+		return n;
+#ifdef INLINE_COPY_FROM_USER
+	return _inline_copy_from_user(to, from, n);
+#else
+	return _copy_from_user(to, from, n);
+#endif
 }
 
 static __always_inline unsigned long __must_check
 copy_to_user(void __user *to, const void *from, unsigned long n)
 {
-	if (check_copy_size(from, n, true))
-		n = _copy_to_user(to, from, n);
-	return n;
+	if (!check_copy_size(from, n, true))
+		return n;
+
+#ifdef INLINE_COPY_TO_USER
+	return _inline_copy_to_user(to, from, n);
+#else
+	return _copy_to_user(to, from, n);
+#endif
 }
 
 #ifndef copy_mc_to_kernel
diff --git a/lib/usercopy.c b/lib/usercopy.c
index d29fe29c6849..de7f30618293 100644
--- a/lib/usercopy.c
+++ b/lib/usercopy.c
@@ -7,40 +7,18 @@
 
 /* out-of-line parts */
 
-#ifndef INLINE_COPY_FROM_USER
+#if !defined(INLINE_COPY_FROM_USER) || defined(CONFIG_RUST)
 unsigned long _copy_from_user(void *to, const void __user *from, unsigned long n)
 {
-	unsigned long res = n;
-	might_fault();
-	if (!should_fail_usercopy() && likely(access_ok(from, n))) {
-		/*
-		 * Ensure that bad access_ok() speculation will not
-		 * lead to nasty side effects *after* the copy is
-		 * finished:
-		 */
-		barrier_nospec();
-		instrument_copy_from_user_before(to, from, n);
-		res = raw_copy_from_user(to, from, n);
-		instrument_copy_from_user_after(to, from, n, res);
-	}
-	if (unlikely(res))
-		memset(to + (n - res), 0, res);
-	return res;
+	return _inline_copy_from_user(to, from, n);
 }
 EXPORT_SYMBOL(_copy_from_user);
 #endif
 
-#ifndef INLINE_COPY_TO_USER
+#if !defined(INLINE_COPY_TO_USER) || defined(CONFIG_RUST)
 unsigned long _copy_to_user(void __user *to, const void *from, unsigned long n)
 {
-	might_fault();
-	if (should_fail_usercopy())
-		return n;
-	if (likely(access_ok(to, n))) {
-		instrument_copy_to_user(to, from, n);
-		n = raw_copy_to_user(to, from, n);
-	}
-	return n;
+	return _inline_copy_to_user(to, from, n);
 }
 EXPORT_SYMBOL(_copy_to_user);
 #endif

-- 
2.44.0.478.gd926399ef9-goog