// SPDX-License-Identifier: GPL-2.0/* * A pre-opened interpreter - what 'F' gives a static entry and what a 'B' * entry binds - keeps a file open for as long as the entry lives, so it pins * the mount it came from. It costs no file descriptor, and binfmt_misc is * FS_USERNS_MOUNT, so an unprivileged user namespace can create them without * bound. Check that UCOUNT_BINFMT_MISC_INTERPRETERS bounds it, that an entry * that pre-opens nothing is not charged, that removing an entry gives the * charge back, and that nesting a user namespace does not evade it. * * Runs unprivileged in a user namespace. */#define _GNU_SOURCE#include <errno.h>#include <fcntl.h>#include <limits.h>#include <stdio.h>#include <string.h>#include <sys/mount.h>#include <sys/stat.h>#include <unistd.h> #include "../filesystems/utils.h"#include "kselftest_harness.h" #define MNT		"/tmp/binfmt_interplimit"#define NESTED_MNT	"/tmp/binfmt_interplimit_nested"#define LIMIT_SYSCTL	"/proc/sys/user/max_binfmt_misc_interpreters" #define MAGIC		"\\xde\\xad"/* Not on the instance, and unlike /bin/true it always exists. */#define INTERP		"/proc/self/exe" /* Small enough to fill by hand, big enough that a refund is visible. */#define LIMIT		4 /* What UCOUNT_ENTRY() lets a namespace raise its own limit to. */#define LIMIT_MAX	"2147483647" static int ensure_dir(const char *path){	if (mkdir(path, 0755) && errno != EEXIST)		return -1;	return 0;} /* Write @val to @path, preserving write(2)'s errno for the caller. */static int write_keep_errno(const char *path, const char *val){	int fd, saved;	ssize_t n; 	fd = open(path, O_WRONLY | O_CLOEXEC);	if (fd < 0)		return -1;	n = write(fd, val, strlen(val));	saved = errno;	close(fd);	errno = saved;	return n < 0 ? -1 : 0;} static int set_limit(const char *val){	return write_keep_errno(LIMIT_SYSCTL, val);} static int register_at(const char *mnt, const char *rule){	char path[PATH_MAX]; 	snprintf(path, sizeof(path), "%s/register", mnt);	return write_keep_errno(path, rule);} /* An 'F' entry: one interpreter pre-opened at registration, one charge. */static int register_fixed(const char *mnt, const char *name){	char rule[PATH_MAX]; 	snprintf(rule, sizeof(rule), ":%s:M::" MAGIC "::" INTERP ":F", name);	return register_at(mnt, rule);} /* The same entry without 'F': the interpreter is opened per exec instead. */static int register_plain(const char *mnt, const char *name){	char rule[PATH_MAX]; 	snprintf(rule, sizeof(rule), ":%s:M::" MAGIC "::" INTERP ":", name);	return register_at(mnt, rule);} static int remove_entry(const char *mnt, const char *name){	char path[PATH_MAX]; 	snprintf(path, sizeof(path), "%s/%s", mnt, name);	return write_keep_errno(path, "-1\n");} static bool entry_exists(const char *mnt, const char *name){	char path[PATH_MAX]; 	snprintf(path, sizeof(path), "%s/%s", mnt, name);	return access(path, F_OK) == 0;} /* Register @n 'F' entries, each with a name of its own. */static int fill_budget(const char *mnt, unsigned int n){	char name[32];	unsigned int i; 	for (i = 0; i < n; i++) {		snprintf(name, sizeof(name), "fixed%u", i);		if (register_fixed(mnt, name))			return -1;	}	return 0;} FIXTURE(interp_limit) {}; FIXTURE_SETUP(interp_limit){	/* setup_userns() exits rather than returns if this is not there. */	if (access("/proc/self/ns/user", F_OK))		SKIP(return, "kernel without user namespaces");	ASSERT_EQ(setup_userns(), 0); 	/* CAP_SYS_RESOURCE in this namespace is what makes it writable. */	if (set_limit(LIMIT_MAX)) {		if (errno == ENOENT)			SKIP(return, "kernel without " LIMIT_SYSCTL);		SKIP(return, "cannot set the limit: %s", strerror(errno));	} 	ASSERT_EQ(ensure_dir(MNT), 0);	if (mount("binfmt_misc", MNT, "binfmt_misc", 0, NULL)) {		int saved = errno; 		/* Teardown doesn't run when setup skips, so clean up here. */		rmdir(MNT);		SKIP(return, "no binfmt_misc: %s", strerror(saved));	}} FIXTURE_TEARDOWN(interp_limit){	/* The namespaces go with the process; just don't litter /tmp. */	umount2(NESTED_MNT, MNT_DETACH);	umount2(MNT, MNT_DETACH);	rmdir(NESTED_MNT);	rmdir(MNT);} /* Every pre-opened interpreter is charged, and the budget is a hard stop. */TEST_F(interp_limit, fixed_interpreters_are_charged){	char buf[32]; 	snprintf(buf, sizeof(buf), "%u", LIMIT);	ASSERT_EQ(set_limit(buf), 0); 	ASSERT_EQ(fill_budget(MNT, LIMIT), 0); 	EXPECT_NE(register_fixed(MNT, "over"), 0);	EXPECT_EQ(errno, ENOSPC); 	/* A refused registration leaves nothing behind. */	EXPECT_FALSE(entry_exists(MNT, "over"));} /* An entry that pre-opens nothing pins nothing, so it is not charged. */TEST_F(interp_limit, plain_entries_are_not_charged){	ASSERT_EQ(set_limit("0"), 0); 	EXPECT_EQ(register_plain(MNT, "plain"), 0);	EXPECT_TRUE(entry_exists(MNT, "plain")); 	/* ... while the same entry with 'F' has nothing to spend. */	EXPECT_NE(register_fixed(MNT, "fixed"), 0);	EXPECT_EQ(errno, ENOSPC);} /* Removing an entry closes its interpreters and gives the charge back. */TEST_F(interp_limit, removal_refunds_the_charge){	char buf[32]; 	snprintf(buf, sizeof(buf), "%u", LIMIT);	ASSERT_EQ(set_limit(buf), 0); 	ASSERT_EQ(fill_budget(MNT, LIMIT), 0);	ASSERT_NE(register_fixed(MNT, "over"), 0); 	ASSERT_EQ(remove_entry(MNT, "fixed0"), 0);	EXPECT_EQ(register_fixed(MNT, "over"), 0);} /* * The charge walks the ancestors, so a namespace cannot buy itself budget by * nesting: it may raise only its own limit, and the parent it was created * from is charged for every binding made below it. */TEST_F(interp_limit, nesting_does_not_evade_it){	char buf[32]; 	snprintf(buf, sizeof(buf), "%u", LIMIT);	ASSERT_EQ(set_limit(buf), 0);	ASSERT_EQ(fill_budget(MNT, LIMIT), 0); 	ASSERT_EQ(setup_userns(), 0);	ASSERT_EQ(set_limit(LIMIT_MAX), 0); 	ASSERT_EQ(ensure_dir(NESTED_MNT), 0);	ASSERT_EQ(mount("binfmt_misc", NESTED_MNT, "binfmt_misc", 0, NULL), 0); 	/* A fresh instance with an unlimited budget of its own, and yet: */	EXPECT_NE(register_fixed(NESTED_MNT, "nested"), 0);	EXPECT_EQ(errno, ENOSPC); 	/* The nested instance works for anything that pins no file. */	EXPECT_EQ(register_plain(NESTED_MNT, "nested_plain"), 0);} TEST_HARNESS_MAIN