Le 31/01/2020 à 06:31, Russell Currey a écrit :
Kernel Userspace Access Prevention (KUAP) on powerpc supports
allowing only one access direction (Read or Write) when allowing access
to or from user memory.
A bug was recently found that showed that these one-way unlocks never
worked, and allowing Read *or* Write would actually unlock Read *and*
Write. We should have a test case for this so we can make sure this
doesn't happen again.
Like ACCESS_USERSPACE, the correct result is for the test to fault.
At the time of writing this, the upstream kernel still has this bug
present, so the test will allow both accesses whereas ACCESS_USERSPACE
will correctly fault.
Signed-off-by: Russell Currey <ruscur@xxxxxxxxxx>
---
drivers/misc/lkdtm/core.c | 3 +++
drivers/misc/lkdtm/lkdtm.h | 3 +++
drivers/misc/lkdtm/perms.c | 43 ++++++++++++++++++++++++++++++++++++++
3 files changed, 49 insertions(+)
diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
index ee0d6e721441..baef3c6f48d6 100644
--- a/drivers/misc/lkdtm/core.c
+++ b/drivers/misc/lkdtm/core.c
@@ -137,6 +137,9 @@ static const struct crashtype crashtypes[] = {
CRASHTYPE(EXEC_USERSPACE),
CRASHTYPE(EXEC_NULL),
CRASHTYPE(ACCESS_USERSPACE),
+#ifdef CONFIG_PPC_KUAP
+ CRASHTYPE(ACCESS_USERSPACE_KUAP),
+#endif
I'm not sure it is a good idea to build this test as a specific test for
powerpc, more comments below.
CRASHTYPE(ACCESS_NULL),
CRASHTYPE(WRITE_RO),
CRASHTYPE(WRITE_RO_AFTER_INIT),
diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
index c56d23e37643..406a3fb32e6f 100644
--- a/drivers/misc/lkdtm/lkdtm.h
+++ b/drivers/misc/lkdtm/lkdtm.h
@@ -57,6 +57,9 @@ void lkdtm_EXEC_RODATA(void);
void lkdtm_EXEC_USERSPACE(void);
void lkdtm_EXEC_NULL(void);
void lkdtm_ACCESS_USERSPACE(void);
+#ifdef CONFIG_PPC_KUAP
+void lkdtm_ACCESS_USERSPACE_KUAP(void);
+#endif
void lkdtm_ACCESS_NULL(void);
/* lkdtm_refcount.c */
diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
index 62f76d506f04..2c9aa0114333 100644
--- a/drivers/misc/lkdtm/perms.c
+++ b/drivers/misc/lkdtm/perms.c
@@ -10,6 +10,9 @@
#include <linux/mman.h>
#include <linux/uaccess.h>
#include <asm/cacheflush.h>
+#ifdef CONFIG_PPC_KUAP
+#include <asm/uaccess.h>
+#endif
asm/uaccess.h is already included by linux/uaccess.h
/* Whether or not to fill the target memory area with do_nothing(). */
#define CODE_WRITE true
@@ -200,6 +203,46 @@ void lkdtm_ACCESS_USERSPACE(void)
vm_munmap(user_addr, PAGE_SIZE);
}
+/* Test that KUAP's directional user access unlocks work as intended */
+#ifdef CONFIG_PPC_KUAP
+void lkdtm_ACCESS_USERSPACE_KUAP(void)
+{
+ unsigned long user_addr, tmp = 0;
+ unsigned long *ptr;
Should be a __user ptr because allow_write_to_user() and friends takes
__user pointers.
+
+ user_addr = vm_mmap(NULL, 0, PAGE_SIZE,
+ PROT_READ | PROT_WRITE | PROT_EXEC,
+ MAP_ANONYMOUS | MAP_PRIVATE, 0);
+ if (user_addr >= TASK_SIZE) {
Should use IS_ERR_VALUE() here.
+ pr_warn("Failed to allocate user memory\n");
+ return;
+ }
+
+ if (copy_to_user((void __user *)user_addr, &tmp, sizeof(tmp))) {
Should use ptr instead of casted user_addr.
Why using copy_to_user() for writing an unsigned long ? put_user()
should be enough.
+ pr_warn("copy_to_user failed\n");
+ vm_munmap(user_addr, PAGE_SIZE);
+ return;
+ }
+
+ ptr = (unsigned long *)user_addr;
move before copy_to_user() and use there.
+
+ /* Allowing "write to" should not allow "read from" */
+ allow_write_to_user(ptr, sizeof(unsigned long));
This is powerpc specific. I think we should build this around the
user_access_begin()/user_access_end() generic fonctions.
I'm about to propose an enhancement to this in order to allow unlocking
only read or write. See discussion at
https://patchwork.ozlabs.org/patch/1227926/.
My plan is to propose my enhancement once powerpc implementation of
user_access_begin stuff is merged. I don't know if Michael is still
planning to merge the series for 5.6
(https://patchwork.ozlabs.org/patch/1228801/ - patch 1 of the series has
already been merged by Linus in 5.5)
+ pr_info("attempting bad read at %px with write allowed\n", ptr);
+ tmp = *ptr;
+ tmp += 0xc0dec0de;
+ prevent_write_to_user(ptr, sizeof(unsigned long));
Does it work ? I would have thought that if the read fails the process
will die and the following test won't be performed.
+
+ /* Allowing "read from" should not allow "write to" */
+ allow_read_from_user(ptr, sizeof(unsigned long));
+ pr_info("attempting bad write at %px with read allowed\n", ptr);
+ *ptr = tmp;
+ prevent_read_from_user(ptr, sizeof(unsigned long));
+
+ vm_munmap(user_addr, PAGE_SIZE);
+}
+#endif
+
void lkdtm_ACCESS_NULL(void)
{
unsigned long tmp;
Christophe
