linux-hardening - Re: [PATCH] selftests: proc: Make sure wchan works when it exists

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Message-ID: <YWGQckkJuKB4yAfg@localhost.localdomain>
Date:   Sat, 9 Oct 2021 15:52:02 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Shuah Khan <shuah@...nel.org>, linux-kselftest@...r.kernel.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Gladkov <gladkov.alexey@...il.com>, jannh@...gle.com,
        vcaputo@...garu.com, mingo@...hat.com, juri.lelli@...hat.com,
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Subject: Re: [PATCH] selftests: proc: Make sure wchan works when it exists

On Fri, Oct 08, 2021 at 04:55:04PM -0700, Kees Cook wrote:
> This makes sure that wchan contains a sensible symbol when a process is
> blocked.

> Specifically this calls the sleep() syscall, and expects the
> architecture to have called schedule() from a function that has "sleep"
> somewhere in its name.

This exposes internal kernel symbol to userspace.
Why would want to test that?

Doing s/sleep/SLEEP/g doesn't change kernel but now the test is broken.

> For example, on the architectures I tested
> (x86_64, arm64, arm, mips, and powerpc) this is "hrtimer_nanosleep":

> +/*
> + * Make sure that wchan returns a reasonable symbol when blocked.
> + */

Test should be "contains C identifier" then?

> +int main(void)
> +{
> +	char buf[64];
> +	pid_t child;
> +	int sync[2], fd;
> +
> +	if (pipe(sync) < 0)
> +		perror_exit("pipe");
> +
> +	child = fork();
> +	if (child < 0)
> +		perror_exit("fork");
> +	if (child == 0) {
> +		/* Child */
> +		if (close(sync[0]) < 0)
> +			perror_exit("child close sync[0]");
> +		if (close(sync[1]) < 0)
> +			perror_exit("child close sync[1]");

Redundant close().

> +		sleep(10);
> +		_exit(0);
> +	}
> +	/* Parent */
> +	if (close(sync[1]) < 0)
> +		perror_exit("parent close sync[1]");

Redundant close().

> +	if (read(sync[0], buf, 1) != 0)
> +		perror_exit("parent read sync[0]");

Racy if child is scheduled out after first close in the child.

> +	snprintf(buf, sizeof(buf), "/proc/%d/wchan", child);
> +	fd = open(buf, O_RDONLY);
> +	if (fd < 0) {
> +		if (errno == ENOENT)
> +			return 4;
> +		perror_exit(buf);
> +	}
> +
> +	memset(buf, 0, sizeof(buf));
> +	if (read(fd, buf, sizeof(buf) - 1) < 1)
> +		perror_exit(buf);
> +	if (strstr(buf, "sleep") == NULL) {
> +		fprintf(stderr, "FAIL: did not find 'sleep' in wchan '%s'\n", buf);
> +		return 1;
> +	}
> +	printf("ok: found 'sleep' in wchan '%s'\n", buf);
> +
> +	if (kill(child, SIGKILL) < 0)
> +		perror_exit("kill");
> +	if (waitpid(child, NULL, 0) != child) {
> +		fprintf(stderr, "waitpid: got the wrong child!?\n");
> +		return 1;
> +	}
> +
> +	return 0;
> +}