kasan: rust: Add KASAN smoke test via UAF

Adds a smoke test to ensure that KASAN in Rust is actually detecting a
Rust-native UAF. There is significant room to expand this test suite,
but this will at least ensure that flags are having the intended effect.

The rename from kasan_test.c to kasan_test_c.c is in order to allow the
single kasan_test.ko test suite to contain both a .o file produced
by the C compiler and one produced by rustc.

Signed-off-by: Matthew Maurer <mmaurer@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Link: https://lore.kernel.org/r/20240820194910.187826-5-mmaurer@google.com
[ Applied empty line nit, removed double empty line,
  applied `rustfmt` and formatted crate comment. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

mm/kasan/Makefile

@@ -44,7 +44,8 @@ ifndef CONFIG_CC_HAS_KASAN_MEMINTRINSIC_PREFIX
 CFLAGS_KASAN_TEST += -fno-builtin
 endif
 
-CFLAGS_kasan_test.o := $(CFLAGS_KASAN_TEST)
+CFLAGS_kasan_test_c.o := $(CFLAGS_KASAN_TEST)
+RUSTFLAGS_kasan_test_rust.o := $(RUSTFLAGS_KASAN)
 CFLAGS_kasan_test_module.o := $(CFLAGS_KASAN_TEST)
 
 obj-y := common.o report.o
@@ -52,5 +53,10 @@ obj-$(CONFIG_KASAN_GENERIC) += init.o generic.o report_generic.o shadow.o quaran
 obj-$(CONFIG_KASAN_HW_TAGS) += hw_tags.o report_hw_tags.o tags.o report_tags.o
 obj-$(CONFIG_KASAN_SW_TAGS) += init.o report_sw_tags.o shadow.o sw_tags.o tags.o report_tags.o
 
+kasan_test-objs := kasan_test_c.o
+ifdef CONFIG_RUST
+	kasan_test-objs += kasan_test_rust.o
+endif
+
 obj-$(CONFIG_KASAN_KUNIT_TEST) += kasan_test.o
 obj-$(CONFIG_KASAN_MODULE_TEST) += kasan_test_module.o

mm/kasan/kasan.h

@@ -555,6 +555,12 @@ static inline bool kasan_arch_is_ready(void)	{ return true; }
 void kasan_kunit_test_suite_start(void);
 void kasan_kunit_test_suite_end(void);
 
+#ifdef CONFIG_RUST
+char kasan_test_rust_uaf(void);
+#else
+static inline char kasan_test_rust_uaf(void) { return '\0'; }
+#endif
+
 #else /* CONFIG_KASAN_KUNIT_TEST */
 
 static inline void kasan_kunit_test_suite_start(void) { }

mm/kasan/kasan_test_c.c

@@ -1899,6 +1899,16 @@ static void match_all_mem_tag(struct kunit *test)
 	kfree(ptr);
 }
 
+/*
+ * Check that Rust performing a use-after-free using `unsafe` is detected.
+ * This is a smoke test to make sure that Rust is being sanitized properly.
+ */
+static void rust_uaf(struct kunit *test)
+{
+	KASAN_TEST_NEEDS_CONFIG_ON(test, CONFIG_RUST);
+	KUNIT_EXPECT_KASAN_FAIL(test, kasan_test_rust_uaf());
+}
+
 static struct kunit_case kasan_kunit_test_cases[] = {
 	KUNIT_CASE(kmalloc_oob_right),
 	KUNIT_CASE(kmalloc_oob_left),
@@ -1971,6 +1981,7 @@ static struct kunit_case kasan_kunit_test_cases[] = {
 	KUNIT_CASE(match_all_not_assigned),
 	KUNIT_CASE(match_all_ptr_tag),
 	KUNIT_CASE(match_all_mem_tag),
+	KUNIT_CASE(rust_uaf),
 	{}
 };
 

mm/kasan/kasan_test_rust.rs

@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Helper crate for KASAN testing.
+//!
+//! Provides behavior to check the sanitization of Rust code.
+
+use core::ptr::addr_of_mut;
+use kernel::prelude::*;
+
+/// Trivial UAF - allocate a big vector, grab a pointer partway through,
+/// drop the vector, and touch it.
+#[no_mangle]
+pub extern "C" fn kasan_test_rust_uaf() -> u8 {
+    let mut v: Vec<u8> = Vec::new();
+    for _ in 0..4096 {
+        v.push(0x42, GFP_KERNEL).unwrap();
+    }
+    let ptr: *mut u8 = addr_of_mut!(v[2048]);
+    drop(v);
+    unsafe { *ptr }
+}