Re: [PATCH v3] selftests/lam: Test get_user() LAM pointer handling

On Fri, Nov 22, 2024 at 09:55:20AM +0100, Maciej Wieczor-Retman wrote:

Recent change in how get_user() handles pointers [1] has a specific case for LAM. It assigns a different bitmask that's later used to check whether a pointer comes from userland in get_user().

While currently commented out (until LASS [2] is merged into the kernel) it's worth making changes to the LAM selftest ahead of time.

Add test case to LAM that utilizes a ioctl (FIOASYNC) syscall which uses get_user() in its implementation. Execute the syscall with differently tagged pointers to verify that valid user pointers are passing through and invalid kernel/non-canonical pointers are not.

Code was tested on a Sierra Forest Xeon machine that's LAM capable. The test was ran without issues with both the LAM lines from [1] untouched and commented out. The test was also ran without issues with LAM_SUP both enabled and disabled.

4/5 level pagetables code paths were also successfully tested in Simics on a 5-level capable machine.

[1] https://lore.kernel.org/all/20241024013214.129639-1-torvalds@linux-foundatio... [2] https://lore.kernel.org/all/20240710160655.3402786-1-alexander.shishkin@linu...

Signed-off-by: Maciej Wieczor-Retman maciej.wieczor-retman@intel.com

Changelog v3:

• mmap the pointer passed to get_user to high address if 5 level paging is enabled and to low address if 4 level paging is enabled.

Changelog v2:

• Use mmap with HIGH_ADDR to check if we're in 5 or 4 level pagetables.

tools/testing/selftests/x86/lam.c | 110 ++++++++++++++++++++++++++++++ 1 file changed, 110 insertions(+)

diff --git a/tools/testing/selftests/x86/lam.c b/tools/testing/selftests/x86/lam.c index 0ea4f6813930..616a523c3262 100644 --- a/tools/testing/selftests/x86/lam.c +++ b/tools/testing/selftests/x86/lam.c @@ -4,6 +4,7 @@ #include <stdlib.h> #include <string.h> #include <sys/syscall.h> +#include <sys/ioctl.h> #include <time.h> #include <signal.h> #include <setjmp.h> @@ -43,7 +44,15 @@ #define FUNC_INHERITE 0x20 #define FUNC_PASID 0x40 +/* get_user() pointer test cases */ +#define GET_USER_USER 0 +#define GET_USER_KERNEL_TOP 1 +#define GET_USER_KERNEL_BOT 2 +#define GET_USER_KERNEL 3

#define TEST_MASK 0x7f +#define L5_SIGN_EXT_MASK (0xFFUL << 56) +#define L4_SIGN_EXT_MASK (0x1FFFFUL << 47) #define LOW_ADDR (0x1UL << 30) #define HIGH_ADDR (0x3UL << 48) @@ -370,6 +379,80 @@ static int handle_syscall(struct testcases *test) return ret; } +static int get_user_syscall(struct testcases *test) +{

• uint64_t ptr_address, bitmask;
• void *p, *ptr;
• int ret = 0;
• int fd;
• p = mmap((void *)HIGH_ADDR, 1, PROT_READ | PROT_WRITE,

•  MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, -1, 0);
  

• if (p == MAP_FAILED) {

• bitmask = L4_SIGN_EXT_MASK;
  

• ptr_address = LOW_ADDR;
  

• } else {

• bitmask = L5_SIGN_EXT_MASK;
  

• ptr_address = HIGH_ADDR;
  

• }

Hm. Why not use cpu_has_lam() for the paging check?

• munmap(p, 1);
• ptr = mmap((void *)ptr_address, 1, PROT_READ | PROT_WRITE,

•    MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, -1, 0);
  

Mapping sizer of 1 byte looks odd. It is not wrong, but looks strange. Maybe use PAGE_SIZE instead?

• if (ptr == MAP_FAILED) {

• perror("failed to map byte to pass into get_user");
  

• return 1;
  

• }
• if (test->lam != 0) {

It is always true, right?

• if (set_lam(test->lam) != 0) {
  

• 	ret = 2;
  

• 	goto error;
  

• }
  

• }
• fd = memfd_create("lam_ioctl", 0);
• if (fd == -1) {

• munmap(ptr, 1);
  

• exit(EXIT_FAILURE);
  

• }
• switch (test->later) {
• case GET_USER_USER:

• /* Control group - properly tagger user pointer */
  

• ptr = (void *)set_metadata((uint64_t)ptr, test->lam);
  

• break;
  

• case GET_USER_KERNEL_TOP:

• /* Kernel address with top bit cleared */
  

• bitmask &= (bitmask >> 1);
  

• ptr = (void *)((uint64_t)ptr | bitmask);
  

• break;
  

• case GET_USER_KERNEL_BOT:

• /* Kernel address with bottom sign-extension bit cleared */
  

• bitmask &= (bitmask << 1);
  

• ptr = (void *)((uint64_t)ptr | bitmask);
  

• break;
  

• case GET_USER_KERNEL:

• /* Try to pass a kernel address */
  

• ptr = (void *)((uint64_t)ptr | bitmask);
  

• break;
  

• default:

• printf("Invalid test case value passed!\n");
  

• break;
  

• }
• if (ioctl(fd, FIOASYNC, ptr) != 0)

• ret = 1;
  

+error:

• munmap(ptr, 1);

close(fd);

• return ret;

+}

int sys_uring_setup(unsigned int entries, struct io_uring_params *p) { return (int)syscall(__NR_io_uring_setup, entries, p); @@ -883,6 +966,33 @@ static struct testcases syscall_cases[] = { .test_func = handle_syscall, .msg = "SYSCALL:[Negative] Disable LAM. Dereferencing pointer with metadata.\n", },

• {

• .later = GET_USER_USER,
  

• .lam = LAM_U57_BITS,
  

• .test_func = get_user_syscall,
  

• .msg = "GET_USER: get_user() and pass a properly tagged user pointer.\n",
  

• },
• {

• .later = GET_USER_KERNEL_TOP,
  

• .expected = 1,
  

• .lam = LAM_U57_BITS,
  

• .test_func = get_user_syscall,
  

• .msg = "GET_USER:[Negative] get_user() with a kernel pointer and the top bit cleared.\n",
  

• },
• {

• .later = GET_USER_KERNEL_BOT,
  

• .expected = 1,
  

• .lam = LAM_U57_BITS,
  

• .test_func = get_user_syscall,
  

• .msg = "GET_USER:[Negative] get_user() with a kernel pointer and the bottom sign-extension bit cleared.\n",
  

• },
• {

• .later = GET_USER_KERNEL,
  

• .expected = 1,
  

• .lam = LAM_U57_BITS,
  

• .test_func = get_user_syscall,
  

• .msg = "GET_USER:[Negative] get_user() and pass a kernel pointer.\n",
  

• },

}; static struct testcases mmap_cases[] = { -- 2.46.2