Re: [PATCH 4/4] selftests/rseq: Add test for rseq+pkeys

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2025-02-17 06:07, Dmitry Vyukov wrote:
> Add a test that ensures that PKEY-protected struct rseq works
> and does not lead to process kills.

Rather than depend on the glibc rseq tunable to disable glibc
rseq support, you could simply setup a rseq_cs in a separate
page and get the kernel to hit _that_ page through the
rseq_cs pointer.

Thanks,

Mathieu

> 
> Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Aruna Ramakrishna <aruna.ramakrishna@oracle.com>
> Cc: x86@kernel.org
> Cc: linux-kernel@vger.kernel.org
> ---
>   tools/testing/selftests/rseq/Makefile    |  2 +-
>   tools/testing/selftests/rseq/pkey_test.c | 61 ++++++++++++++++++++++++
>   2 files changed, 62 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/rseq/Makefile b/tools/testing/selftests/rseq/Makefile
> index 5a3432fceb586..9111d25fea3af 100644
> --- a/tools/testing/selftests/rseq/Makefile
> +++ b/tools/testing/selftests/rseq/Makefile
> @@ -16,7 +16,7 @@ OVERRIDE_TARGETS = 1
>   
>   TEST_GEN_PROGS = basic_test basic_percpu_ops_test basic_percpu_ops_mm_cid_test param_test \
>   		param_test_benchmark param_test_compare_twice param_test_mm_cid \
> -		param_test_mm_cid_benchmark param_test_mm_cid_compare_twice
> +		param_test_mm_cid_benchmark param_test_mm_cid_compare_twice pkey_test
>   
>   TEST_GEN_PROGS_EXTENDED = librseq.so
>   
> diff --git a/tools/testing/selftests/rseq/pkey_test.c b/tools/testing/selftests/rseq/pkey_test.c
> new file mode 100644
> index 0000000000000..ba5c1f6e99ab5
> --- /dev/null
> +++ b/tools/testing/selftests/rseq/pkey_test.c
> @@ -0,0 +1,61 @@
> +// SPDX-License-Identifier: LGPL-2.1
> +/*
> + * Ensure that rseq works when rseq data is protected with PKEYs.
> + */
> +
> +#define _GNU_SOURCE
> +#include <err.h>
> +#include <errno.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <string.h>
> +#include <sys/mman.h>
> +#include <sys/syscall.h>
> +#include <unistd.h>
> +
> +int main(int argc, char **argv)
> +{
> +	void *rseq;
> +	unsigned long page_size;
> +	int pkey, i;
> +
> +	pkey = pkey_alloc(0, 0);
> +	if (pkey == -1) {
> +		printf("[SKIP]\tKernel does not support PKEYs: %s\n",
> +			strerror(errno));
> +		return 0;
> +	}
> +
> +	/*
> +	 * Prevent glibc from registering own struct rseq.
> +	 * We need to know the rseq address to protect it, but also we need
> +	 * it to be placed on own page that does not contain other data
> +	 * (e.g. errno).
> +	 */
> +	if (!getenv("RSEQ_TEST_REEXECED")) {
> +		setenv("RSEQ_TEST_REEXECED", "1", 1);
> +		setenv("GLIBC_TUNABLES", "glibc.pthread.rseq=0", 1);
> +		if (execvpe(argv[0], argv, environ))
> +			err(1, "execvpe failed");
> +	}
> +
> +	page_size = getpagesize();
> +	rseq = mmap(NULL, page_size, PROT_READ | PROT_WRITE,
> +		MAP_ANON | MAP_PRIVATE, -1, 0);
> +	if (rseq == MAP_FAILED)
> +		err(1, "mmap failed");
> +	if (pkey_mprotect(rseq, page_size, PROT_READ | PROT_WRITE, pkey))
> +		err(1, "pkey_mprotect failed");
> +	if (syscall(__NR_rseq, rseq, 32, 0, 0))
> +		err(1, "rseq failed");
> +	if (pkey_set(pkey, PKEY_DISABLE_ACCESS))
> +		err(1, "pkey_set failed");
> +
> +	/*
> +	 * If the kernel misbehaves, context switches in the following loop
> +	 * will kill the process with SIGSEGV.
> +	 */
> +	for (i = 0; i < 10; i++)
> +		usleep(100);
> +	return 0;
> +}


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com