Re: [LTP] [PATCH 2/2] Add test for CVE 2021-38198

[Richard Palethorpe <rpalethorpe@suse.de>]

Hi Martin,

Martin Doucha <mdoucha@suse.cz> writes:

> Signed-off-by: Martin Doucha <mdoucha@suse.cz>
> ---
>  runtest/kvm                            |   1 +
>  testcases/kernel/kvm/.gitignore        |   1 +
>  testcases/kernel/kvm/kvm_pagefault01.c | 235 +++++++++++++++++++++++++
>  3 files changed, 237 insertions(+)
>  create mode 100644 runtest/kvm
>  create mode 100644 testcases/kernel/kvm/.gitignore
>  create mode 100644 testcases/kernel/kvm/kvm_pagefault01.c
>
> diff --git a/runtest/kvm b/runtest/kvm
> new file mode 100644
> index 000000000..16e7c07ff
> --- /dev/null
> +++ b/runtest/kvm
> @@ -0,0 +1 @@
> +kvm_pagefault01 kvm_pagefault01
> diff --git a/testcases/kernel/kvm/.gitignore b/testcases/kernel/kvm/.gitignore
> new file mode 100644
> index 000000000..349260359
> --- /dev/null
> +++ b/testcases/kernel/kvm/.gitignore
> @@ -0,0 +1 @@
> +/kvm_pagefault01
> diff --git a/testcases/kernel/kvm/kvm_pagefault01.c b/testcases/kernel/kvm/kvm_pagefault01.c
> new file mode 100644
> index 000000000..e7d957f3e
> --- /dev/null
> +++ b/testcases/kernel/kvm/kvm_pagefault01.c
> @@ -0,0 +1,235 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * Copyright (C) 2020 SUSE LLC
> + * Author: Nicolai Stange <nstange@suse.de>
> + * LTP port: Martin Doucha <mdoucha@suse.cz>
> + */
> +
> +/*\
> + * CVE 2021-38198
> + *
> + * Check that x86_64 KVM correctly enforces (lack of) write permissions
> + * in 4-level and 5-level memory page table mode. Missing page faults fixed in:
> + *
> + *  commit b1bd5cba3306691c771d558e94baa73e8b0b96b7
> + *  Author: Lai Jiangshan <laijs@linux.alibaba.com>
> + *  Date:   Thu Jun 3 13:24:55 2021 +0800
> + *
> + *  KVM: X86: MMU: Use the correct inherited permissions to get shadow page
> + */
> +
> +#include "kvm_test.h"
> +
> +#ifdef COMPILE_PAYLOAD

Minor nit; I'd prefer guest to payload.

> +#ifdef __x86_64__
> +
> +#include "kvm_x86.h"
> +
> +#define PTE_BITMASK 0x1ff
> +#define PAGESIZE 0x1000
> +
> +int handle_page_fault(void *userdata, struct kvm_interrupt_frame *ifrm,
> +	unsigned long errcode)
> +{
> +	struct kvm_cregs buf;
> +
> +	kvm_read_cregs(&buf);
> +
> +	/* Check that the page fault was caused by write to *readonly below */
> +	if (buf.cr2 == (uintptr_t)userdata) {
> +		tst_res(TPASS, "KVM enforces memory write permissions");
> +		kvm_exit();
> +	}
> +
> +	/* Unexpected page fault, fall back to default handler */
> +	return 0;
> +}
> +
> +void main(void)
> +{
> +	uintptr_t tmp;
> +	struct page_table_entry_pae *subpte, *pte = kvm_pagetable;
> +	int val, *writable, *readonly, *cacher1, *cacher2;
> +
> +	if (!(kvm_rdmsr(MSR_EFER) & EFER_LMA))
> +		tst_brk(TBROK, "Bootstrap did not enable 64bit paging");
> +
> +	/*
> +	 * Find the first page table entry which branches. This entry was
> +	 * configured by bootstrap as follows:
> +	 * 0x00000000 - 0x3fffffff in pte[0] (identity mapped)
> +	 * 0x40000000 - 0x7fffffff in pte[1] (identity mapped)
> +	 * 0x80000000 - 0xbfffffff in pte[2] (unmapped)
> +	 * 0xc0000000 - 0xffffffff in pte[3] (only last page identity mapped)
> +	 */
> +	while (!pte[1].present) {
> +		tmp = kvm_get_page_address_pae(pte);
> +		pte = (struct page_table_entry_pae *)tmp;
> +	}
> +
> +	/*
> +	 * Setup mapping above the 32bit address space. The test needs two
> +	 * different unused 1GB chunks of address space. Remapping part of
> +	 * the lower 4GB address space would make it harder to reproduce
> +	 * the bug because any memory access in the same 1GB chunk (even
> +	 * fetching instructions by the CPU) could evict entries from page
> +	 * table cache and force the bypassable write permission check
> +	 * to happen even on buggy kernels.
> +	 *
> +	 * Allocate 3 pages for page table + 2 pages for data
> +	 */
> +	writable = tst_heap_alloc_aligned(5 * PAGESIZE, PAGESIZE);
> +	memset(writable, 0, 5 * PAGESIZE);
> +	tmp = (uintptr_t)writable;
> +	pte[4].address = tmp >> 12;
> +	pte[4].user_access = 1;
> +	pte[4].writable = 1;
> +	pte[4].present = 1;
> +	pte[5] = pte[4];
> +	pte[5].writable = 0;
> +
> +	subpte = (struct page_table_entry_pae *)tmp;
> +	subpte[0].address = (tmp + PAGESIZE) >> 12;
> +	subpte[0].user_access = 1;
> +	subpte[0].writable = 0;
> +	subpte[0].present = 1;
> +	subpte[1].address = (tmp + 2 * PAGESIZE) >> 12;
> +	subpte[1].user_access = 1;
> +	subpte[1].writable = 1;
> +	subpte[1].present = 1;
> +
> +	subpte = (struct page_table_entry_pae *)(tmp + PAGESIZE);
> +	subpte[0].address = (tmp + 3 * PAGESIZE) >> 12;
> +	subpte[0].user_access = 1;
> +	subpte[0].writable = 1;
> +	subpte[0].present = 1;
> +
> +	subpte = (struct page_table_entry_pae *)(tmp + 2 * PAGESIZE);
> +	subpte[0].address = (tmp + 4 * PAGESIZE) >> 12;
> +	subpte[0].user_access = 1;
> +	subpte[0].writable = 1;
> +	subpte[0].present = 1;
> +
> +	/* Create pointers into the new mapping */
> +	cacher1 = (int *)0x100000000ULL;
> +	writable = (int *)0x100200000ULL;
> +	cacher2 = (int *)0x140000000ULL;
> +	readonly = (int *)0x140200000ULL;
> +	tst_set_interrupt_callback(INTR_PAGE_FAULT, handle_page_fault,
> +		readonly);
> +
> +	/* Fill page table cache */
> +	val = *cacher1;
> +	*writable = val;
> +	val = *cacher2;
> +
> +	/* Trigger page fault (unless the kernel is vulnerable) */
> +	*readonly = val;
> +
> +	/* This line should be unreachable */
> +	tst_res(TFAIL, "Write to read-only address did not page fault");
> +}
> +
> +#else /* __x86_64__ */
> +TST_TEST_TCONF("Test supported only on x86_64");
> +#endif /* __x86_64__ */
> +
> +#else /* COMPILE_PAYLOAD */
> +
> +#include <ctype.h>
> +#include <stdio.h>
> +#include <unistd.h>
> +#include "tst_module.h"
> +
> +#define TDP_MMU_SYSFILE "/sys/module/kvm/parameters/tdp_mmu"
> +#define TDP_AMD_SYSFILE "/sys/module/kvm_amd/parameters/npt"
> +#define TDP_INTEL_SYSFILE "/sys/module/kvm_intel/parameters/ept"
> +
> +#define BUF_SIZE 64
> +
> +static int read_bool_sys_param(const char *filename)
> +{
> +	char buf[BUF_SIZE];
> +	int i, fd, ret;
> +
> +	fd = open(filename, O_RDONLY);
> +
> +	if (fd < 0)
> +		return -1;
> +
> +	ret = read(fd, buf, BUF_SIZE - 1);
> +	SAFE_CLOSE(fd);
> +
> +	if (ret < 1)
> +		return -1;
> +
> +	buf[ret] = '\0';
> +
> +	for (i = 0; buf[i] && !isspace(buf[i]); i++)
> +		;
> +
> +	buf[i] = '\0';
> +
> +	if (isdigit(buf[0])) {
> +		sscanf(buf, "%d", &ret);

checkpatch complains that the return value is not checked. Also it wants
you to use tst_parse_int.

> +		return ret;
> +	}
> +
> +	if (!strcasecmp(buf, "N"))
> +		return 0;
> +
> +	/* Assume that any other value than 0 or N means the param is enabled */
> +	return 1;
> +}
> +
> +static void reload_module(const char *module, char *arg)
> +{
> +	const char *const argv[] = {"modprobe", module, arg, NULL};
> +
> +	tst_res(TINFO, "Reloading module %s with parameter %s", module, arg);
> +	tst_module_unload(module);
> +	tst_cmd(argv, NULL, NULL, 0);
> +}
> +
> +static void disable_tdp(void)
> +{
> +	if (!access(TDP_MMU_SYSFILE, F_OK)) {
> +		/* FIXME: Is this sufficient to disable TDP? */

What happens if this doesn't work and TDP is enabled? I seem to have it
enabled and the test still passes even if I comment out the call
to disable_tdp.

I'm wondering whether it will be easy to tell if a test failure is due
to TDP or if it can result in silent false negatives?

> +		SAFE_FILE_PRINTF(TDP_MMU_SYSFILE, "0");
> +		return;
> +	}
> +
> +	if (read_bool_sys_param(TDP_AMD_SYSFILE) > 0)
> +		reload_module("kvm_amd", "npt=0");
> +
> +	if (read_bool_sys_param(TDP_INTEL_SYSFILE) > 0)
> +		reload_module("kvm_intel", "ept=0");
> +}
> +
> +static void setup(void)
> +{
> +	disable_tdp();
> +	tst_kvm_setup();
> +}
> +
> +static struct tst_test test = {
> +	.test_all = tst_kvm_run,
> +	.setup = setup,
> +	.cleanup = tst_kvm_cleanup,
> +	.needs_root = 1,
> +	.save_restore = (const char *const []) {
> +		"?/sys/module/kvm/parameters/tdp_mmu",

This needs updating to use struct tst_path_val.

> +		NULL
> +	},
> +	.supported_archs = (const char *const []) {
> +		"x86_64",
> +		NULL
> +	},
> +	.tags = (struct tst_tag[]){
> +		{"linux-git", "b1bd5cba3306"},
> +		{"CVE", "2021-38198"},
> +		{}
> +	}
> +};
> +
> +#endif /* COMPILE_PAYLOAD */
> -- 
> 2.35.1


-- 
Thank you,
Richard.

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