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Subject[PATCH 1/4] lkdtm: Avoid more compiler optimizations for bad writes
Date
It seems at least Clang is able to throw away writes it knows are
destined for read-only memory, which makes things like the WRITE_RO test
fail, as the write gets elided. Instead, force the variable to be
volatile, and make similar changes through-out other tests in an effort
to avoid needing to repeat fixing these kinds of problems. Also includes
pr_err() calls in failure paths so that kernel logs are more clear in
the failure case.

Reported-by: Prasad Sodagudi <psodagud@codeaurora.org>
Suggested-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
drivers/misc/lkdtm/bugs.c | 11 +++++------
drivers/misc/lkdtm/perms.c | 22 +++++++++++++++-------
drivers/misc/lkdtm/usercopy.c | 7 +++++--
3 files changed, 25 insertions(+), 15 deletions(-)

diff --git a/drivers/misc/lkdtm/bugs.c b/drivers/misc/lkdtm/bugs.c
index 886459e0ddd9..e1b43f615549 100644
--- a/drivers/misc/lkdtm/bugs.c
+++ b/drivers/misc/lkdtm/bugs.c
@@ -118,9 +118,8 @@ noinline void lkdtm_CORRUPT_STACK(void)
/* Use default char array length that triggers stack protection. */
char data[8] __aligned(sizeof(void *));

- __lkdtm_CORRUPT_STACK(&data);
-
- pr_info("Corrupted stack containing char array ...\n");
+ pr_info("Corrupting stack containing char array ...\n");
+ __lkdtm_CORRUPT_STACK((void *)&data);
}

/* Same as above but will only get a canary with -fstack-protector-strong */
@@ -131,9 +130,8 @@ noinline void lkdtm_CORRUPT_STACK_STRONG(void)
unsigned long *ptr;
} data __aligned(sizeof(void *));

- __lkdtm_CORRUPT_STACK(&data);
-
- pr_info("Corrupted stack containing union ...\n");
+ pr_info("Corrupting stack containing union ...\n");
+ __lkdtm_CORRUPT_STACK((void *)&data);
}

void lkdtm_UNALIGNED_LOAD_STORE_WRITE(void)
@@ -248,6 +246,7 @@ void lkdtm_ARRAY_BOUNDS(void)

kfree(not_checked);
kfree(checked);
+ pr_err("FAIL: survived array bounds overflow!\n");
}

void lkdtm_CORRUPT_LIST_ADD(void)
diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
index 62f76d506f04..2dede2ef658f 100644
--- a/drivers/misc/lkdtm/perms.c
+++ b/drivers/misc/lkdtm/perms.c
@@ -57,6 +57,7 @@ static noinline void execute_location(void *dst, bool write)
}
pr_info("attempting bad execution at %px\n", func);
func();
+ pr_err("FAIL: func returned\n");
}

static void execute_user_location(void *dst)
@@ -75,20 +76,22 @@ static void execute_user_location(void *dst)
return;
pr_info("attempting bad execution at %px\n", func);
func();
+ pr_err("FAIL: func returned\n");
}

void lkdtm_WRITE_RO(void)
{
- /* Explicitly cast away "const" for the test. */
- unsigned long *ptr = (unsigned long *)&rodata;
+ /* Explicitly cast away "const" for the test and make volatile. */
+ volatile unsigned long *ptr = (unsigned long *)&rodata;

pr_info("attempting bad rodata write at %px\n", ptr);
*ptr ^= 0xabcd1234;
+ pr_err("FAIL: survived bad write\n");
}

void lkdtm_WRITE_RO_AFTER_INIT(void)
{
- unsigned long *ptr = &ro_after_init;
+ volatile unsigned long *ptr = &ro_after_init;

/*
* Verify we were written to during init. Since an Oops
@@ -102,19 +105,21 @@ void lkdtm_WRITE_RO_AFTER_INIT(void)

pr_info("attempting bad ro_after_init write at %px\n", ptr);
*ptr ^= 0xabcd1234;
+ pr_err("FAIL: survived bad write\n");
}

void lkdtm_WRITE_KERN(void)
{
size_t size;
- unsigned char *ptr;
+ volatile unsigned char *ptr;

size = (unsigned long)do_overwritten - (unsigned long)do_nothing;
ptr = (unsigned char *)do_overwritten;

pr_info("attempting bad %zu byte write at %px\n", size, ptr);
- memcpy(ptr, (unsigned char *)do_nothing, size);
+ memcpy((void *)ptr, (unsigned char *)do_nothing, size);
flush_icache_range((unsigned long)ptr, (unsigned long)(ptr + size));
+ pr_err("FAIL: survived bad write\n");

do_overwritten();
}
@@ -193,9 +198,11 @@ void lkdtm_ACCESS_USERSPACE(void)
pr_info("attempting bad read at %px\n", ptr);
tmp = *ptr;
tmp += 0xc0dec0de;
+ pr_err("FAIL: survived bad read\n");

pr_info("attempting bad write at %px\n", ptr);
*ptr = tmp;
+ pr_err("FAIL: survived bad write\n");

vm_munmap(user_addr, PAGE_SIZE);
}
@@ -203,19 +210,20 @@ void lkdtm_ACCESS_USERSPACE(void)
void lkdtm_ACCESS_NULL(void)
{
unsigned long tmp;
- unsigned long *ptr = (unsigned long *)NULL;
+ volatile unsigned long *ptr = (unsigned long *)NULL;

pr_info("attempting bad read at %px\n", ptr);
tmp = *ptr;
tmp += 0xc0dec0de;
+ pr_err("FAIL: survived bad read\n");

pr_info("attempting bad write at %px\n", ptr);
*ptr = tmp;
+ pr_err("FAIL: survived bad write\n");
}

void __init lkdtm_perms_init(void)
{
/* Make sure we can write to __ro_after_init values during __init */
ro_after_init |= 0xAA;
-
}
diff --git a/drivers/misc/lkdtm/usercopy.c b/drivers/misc/lkdtm/usercopy.c
index e172719dd86d..b833367a45d0 100644
--- a/drivers/misc/lkdtm/usercopy.c
+++ b/drivers/misc/lkdtm/usercopy.c
@@ -304,19 +304,22 @@ void lkdtm_USERCOPY_KERNEL(void)
return;
}

- pr_info("attempting good copy_to_user from kernel rodata\n");
+ pr_info("attempting good copy_to_user from kernel rodata: %px\n",
+ test_text);
if (copy_to_user((void __user *)user_addr, test_text,
unconst + sizeof(test_text))) {
pr_warn("copy_to_user failed unexpectedly?!\n");
goto free_user;
}

- pr_info("attempting bad copy_to_user from kernel text\n");
+ pr_info("attempting bad copy_to_user from kernel text: %px\n",
+ vm_mmap);
if (copy_to_user((void __user *)user_addr, vm_mmap,
unconst + PAGE_SIZE)) {
pr_warn("copy_to_user failed, but lacked Oops\n");
goto free_user;
}
+ pr_err("FAIL: survived bad copy_to_user()\n");

free_user:
vm_munmap(user_addr, PAGE_SIZE);
--
2.25.1
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