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Message-ID: <jqvqvnx4shpubflb3xrkplefkmjicaqrg2mut3kc7ijgq3i3op@poafl6jo3t7p>
Date: Fri, 22 Nov 2024 12:09:10 +0100
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
To: "Kirill A. Shutemov" <kirill@...temov.name>
CC: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
<x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, Shuah Khan
<shuah@...nel.org>, <linux-kernel@...r.kernel.org>,
<linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH v3] selftests/lam: Test get_user() LAM pointer handling
On 2024-11-22 at 11:13:44 +0200, Kirill A. Shutemov wrote:
>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-foundation.org/
>> [2] https://lore.kernel.org/all/20240710160655.3402786-1-alexander.shishkin@linux.intel.com/
>>
>> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.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?
cpu_has_lam() seems to return what the cpuid reports about LAM being available
on the system.
The problem I was trying to solve here was to determine what pagetable level is
used currently so I can setup the bitmask to create fake kernel pointers below.
Can cpu_has_lam() achieve that? I didn't see any correlation between the cpuid
and active paging mode.
>
>> +
>> + 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?
Okay, I'll try that.
>
>> +
>> + 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?
Right, I forgot to remove it.
>
>> + 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);
Thanks :)
>
>> + 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
>>
>
>--
> Kiryl Shutsemau / Kirill A. Shutemov
--
Kind regards
Maciej Wieczór-Retman
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