[LTP] [PATCH v2 2/2] kallsyms01: Utilize ksymbol table for unauthorized address access

[Cyril Hrubis]

Hi!
> @@ -0,0 +1,141 @@
> +// SPDX-License-Identifier: GPL-2.0-only

Any reason this is GPL-2.0-only and not 2.0-or-later?

> +/*
> + * Copyright (C) 2024 Red Hat, Inc.
> + */
> +
> +/*\
> + * [Description]
> + *
> + *  Utilize kernel's symbol table for unauthorized address access.
> + *
> + *  Access the system symbols with root permission to test whether it's
> + *  possible to read and write the memory addresses of kernel-space
> + *  from user-space. This helps in identifying potential vulnerabilities
> + *  where user-space processes can inappropriately access kernel memory.
> + *
> + * Steps:
> + *  1. Start a process that reads all symbols and their addresses from
> + *     '/proc/kallsyms' and stores them in a linked list.
> + *
> + *  2. Attempt to write to each kernel address found in the linked list.
> + *     The expectation is that each attempt will fail with a SIGSEGV
> + *     (segmentation fault), indicating that the user-space process
> + *     cannot write to kernel memory.
> + *
> + *  3. Handle each SIGSEGV using a signal handler that sets a flag and
> + *     long jumps out of the faulting context.
> + *
> + *  4. If any write operation does not result in a SIGSEGV, log this as
> + *     a potential security vulnerability.
> + *
> + *  5. Observe and log the behavior and any system responses to these
> + *     unauthorized access attempts.
> + *
> + */
> +
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <assert.h>
> +#include <unistd.h>
> +#include <string.h>
> +#include <setjmp.h>
> +#include <signal.h>
> +
> +#include "tst_test.h"
> +#include "tst_safe_stdio.h"
> +
> +struct kallsym {
> +	unsigned long addr;
> +	char type;
> +	char name[128];
> +};
> +
> +static struct kallsym *sym_table;
> +static unsigned int nr_symbols;
> +static sigjmp_buf jmpbuf;
> +volatile sig_atomic_t segv_caught = 0;
> +
> +static void segv_handler(int sig)
> +{
> +	if (sig == SIGSEGV)
> +		segv_caught++;
> +	else
> +		tst_res(TFAIL, "Unexpected signal %s", strsignal(sig));
> +
> +	siglongjmp(jmpbuf, 1);
> +}
> +
> +static unsigned int read_kallsyms(struct kallsym *table, unsigned int table_size)
> +{
> +	char *line = NULL;
> +	size_t len = 0;
> +	unsigned int nr_syms = 0;
> +	FILE *stream = SAFE_FOPEN("/proc/kallsyms", "r");
> +
> +	while (getline(&line, &len, stream) != -1) {
> +
> +		if (table && nr_syms < table_size) {
> +			sscanf(line, "%lx %c %s",
> +					&table[nr_syms].addr,
> +					&table[nr_syms].type,
> +					table[nr_syms].name);
> +		}
> +
> +		nr_syms++;
> +	}
> +
> +	SAFE_FCLOSE(stream);
> +
> +	return nr_syms;
> +}
> +
> +static void setup(void)
> +{
> +	nr_symbols = read_kallsyms(NULL, 0);
> +	sym_table = SAFE_CALLOC(nr_symbols, sizeof(*sym_table));
> +	unsigned int read_symbols = read_kallsyms(sym_table, nr_symbols);
> +
> +	if (nr_symbols != read_symbols)
> +		tst_res(TWARN, "/proc/kallsyms changed size!?");
> +}
> +
> +static void access_ksymbols_address(struct kallsym *table)
> +{
> +	tst_res(TDEBUG, "Access kernel addr: 0x%lx (%c) (%s)",
> +				table->addr, table->type, table->name);
> +
> +	if (sigsetjmp(jmpbuf, 1) == 0) {
> +		*(volatile unsigned long *)table->addr = 0;
> +
> +		tst_res(TFAIL, "Successfully accessed kernel addr 0x%lx (%c) (%s)",
> +				table->addr, table->type, table->name);
> +	}
> +}
> +
> +static void test_access_kernel_address(void)
> +{
> +	struct sigaction sa;
> +	memset(&sa, 0, sizeof(sa));
> +	sa.sa_handler = segv_handler;
> +	sigaction(SIGSEGV, &sa, NULL);

We can move the sigaction to the test setup.

> +	for (unsigned int i = 0; i < nr_symbols; i++)
> +		access_ksymbols_address(&sym_table[i]);

And we have to set the segv_caught to 0 before this loop, otherwise the
test does not work with -i 2

> +	if (segv_caught == (sig_atomic_t)nr_symbols)
> +		tst_res(TPASS, "Caught %d times SIGSEGV in access ksymbols addr", segv_caught);

And also in a case that we got wrong number on segfaults this does not
produce any results. I guess that we need to do:

	if (...)
		tst_res(TPASS, "...");
	else
		tst_res(TFAIL, "Caught %d SIGSEGV expected %d", ...);


> +}
> +
> +static void cleanup(void)
> +{
> +	if (sym_table)
> +		free(sym_table);
> +}
> +
> +static struct tst_test test = {
> +	.needs_root = 1,
> +	.setup = setup,
> +	.cleanup = cleanup,
> +	.max_runtime = 60,
> +	.test_all = test_access_kernel_address,
> +};
> -- 
> 2.40.1
> 
> 
> -- 
> Mailing list info: https://lists.linux.it/listinfo/ltp

-- 
Cyril Hrubis
chrubis@suse.cz