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Subject[PATCH v7 2/5] lkdtm: tests for FORTIFY_SOURCE
Date
From: Daniel Axtens <dja@axtens.net>

Add code to test both:

- runtime detection of the overrun of a structure. This covers the
__builtin_object_size(x, 0) case. This test is called FORTIFY_OBJECT.

- runtime detection of the overrun of a char array within a structure.
This covers the __builtin_object_size(x, 1) case which can be used
for some string functions. This test is called FORTIFY_SUBOBJECT.

Suggested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Francis Laniel <laniel_francis@privacyrequired.com>
---
drivers/misc/lkdtm/bugs.c | 50 ++++++++++++++++++++++++++++++++++++++
drivers/misc/lkdtm/core.c | 2 ++
drivers/misc/lkdtm/lkdtm.h | 2 ++
3 files changed, 54 insertions(+)

diff --git a/drivers/misc/lkdtm/bugs.c b/drivers/misc/lkdtm/bugs.c
index a0675d4154d2..110f5a8538e9 100644
--- a/drivers/misc/lkdtm/bugs.c
+++ b/drivers/misc/lkdtm/bugs.c
@@ -482,3 +482,53 @@ noinline void lkdtm_CORRUPT_PAC(void)
pr_err("XFAIL: this test is arm64-only\n");
#endif
}
+
+void lkdtm_FORTIFY_OBJECT(void)
+{
+ struct target {
+ char a[10];
+ } target[2] = {};
+ int result;
+
+ /*
+ * Using volatile prevents the compiler from determining the value of
+ * 'size' at compile time. Without that, we would get a compile error
+ * rather than a runtime error.
+ */
+ volatile int size = 11;
+
+ pr_info("trying to read past the end of a struct\n");
+
+ result = memcmp(&target[0], &target[1], size);
+
+ /* Print result to prevent the code from being eliminated */
+ pr_err("FAIL: fortify did not catch an object overread!\n"
+ "\"%d\" was the memcmp result.\n", result);
+}
+
+void lkdtm_FORTIFY_SUBOBJECT(void)
+{
+ struct target {
+ char a[10];
+ char b[10];
+ } target;
+ char *src;
+
+ src = kmalloc(20, GFP_KERNEL);
+ strscpy(src, "over ten bytes", 20);
+
+ pr_info("trying to strcpy past the end of a member of a struct\n");
+
+ /*
+ * strncpy(target.a, src, 20); will hit a compile error because the
+ * compiler knows at build time that target.a < 20 bytes. Use strcpy()
+ * to force a runtime error.
+ */
+ strcpy(target.a, src);
+
+ /* Use target.a to prevent the code from being eliminated */
+ pr_err("FAIL: fortify did not catch an sub-object overrun!\n"
+ "\"%s\" was copied.\n", target.a);
+
+ kfree(src);
+}
diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
index 97803f213d9d..b8c51a633fcc 100644
--- a/drivers/misc/lkdtm/core.c
+++ b/drivers/misc/lkdtm/core.c
@@ -117,6 +117,8 @@ static const struct crashtype crashtypes[] = {
CRASHTYPE(UNSET_SMEP),
CRASHTYPE(CORRUPT_PAC),
CRASHTYPE(UNALIGNED_LOAD_STORE_WRITE),
+ CRASHTYPE(FORTIFY_OBJECT),
+ CRASHTYPE(FORTIFY_SUBOBJECT),
CRASHTYPE(OVERWRITE_ALLOCATION),
CRASHTYPE(WRITE_AFTER_FREE),
CRASHTYPE(READ_AFTER_FREE),
diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
index 6dec4c9b442f..49e6b945feb7 100644
--- a/drivers/misc/lkdtm/lkdtm.h
+++ b/drivers/misc/lkdtm/lkdtm.h
@@ -32,6 +32,8 @@ void lkdtm_STACK_GUARD_PAGE_TRAILING(void);
void lkdtm_UNSET_SMEP(void);
void lkdtm_DOUBLE_FAULT(void);
void lkdtm_CORRUPT_PAC(void);
+void lkdtm_FORTIFY_OBJECT(void);
+void lkdtm_FORTIFY_SUBOBJECT(void);

/* lkdtm_heap.c */
void __init lkdtm_heap_init(void);
--
2.20.1
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