proc: use human-readable values for hidepid

The hidepid parameter values are becoming more and more and it becomes
difficult to remember what each new magic number means.

Backward compatibility is preserved since it is possible to specify
numerical value for the hidepid parameter. This does not break the
fsconfig since it is not possible to specify a numerical value through
it. All numeric values are converted to a string. The type
FSCONFIG_SET_BINARY cannot be used to indicate a numerical value.

Selftest has been added to verify this behavior.

Suggested-by: Andy Lutomirski <[e-mail]>
Signed-off-by: Alexey Gladkov <[e-mail]>
Reviewed-by: Alexey Dobriyan <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Signed-off-by: Eric W. Biederman <[e-mail]>

Documentation/filesystems/proc.rst

···
 	subset=		Show only the specified subset of procfs.
 	=========	========================================================
 
-hidepid=0 means classic mode - everybody may access all /proc/<pid>/ directories
-(default).
+hidepid=off or hidepid=0 means classic mode - everybody may access all
+/proc/<pid>/ directories (default).
 
-hidepid=1 means users may not access any /proc/<pid>/ directories but their
-own.  Sensitive files like cmdline, sched*, status are now protected against
-other users.  This makes it impossible to learn whether any user runs
-specific program (given the program doesn't reveal itself by its behaviour).
-As an additional bonus, as /proc/<pid>/cmdline is unaccessible for other users,
-poorly written programs passing sensitive information via program arguments are
-now protected against local eavesdroppers.
+hidepid=noaccess or hidepid=1 means users may not access any /proc/<pid>/
+directories but their own.  Sensitive files like cmdline, sched*, status are now
+protected against other users.  This makes it impossible to learn whether any
+user runs specific program (given the program doesn't reveal itself by its
+behaviour).  As an additional bonus, as /proc/<pid>/cmdline is unaccessible for
+other users, poorly written programs passing sensitive information via program
+arguments are now protected against local eavesdroppers.
 
-hidepid=2 means hidepid=1 plus all /proc/<pid>/ will be fully invisible to other
-users.  It doesn't mean that it hides a fact whether a process with a specific
-pid value exists (it can be learned by other means, e.g. by "kill -0 $PID"),
-but it hides process' uid and gid, which may be learned by stat()'ing
-/proc/<pid>/ otherwise.  It greatly complicates an intruder's task of gathering
-information about running processes, whether some daemon runs with elevated
-privileges, whether other user runs some sensitive program, whether other users
-run any program at all, etc.
+hidepid=invisible or hidepid=2 means hidepid=1 plus all /proc/<pid>/ will be
+fully invisible to other users.  It doesn't mean that it hides a fact whether a
+process with a specific pid value exists (it can be learned by other means, e.g.
+by "kill -0 $PID"), but it hides process' uid and gid, which may be learned by
+stat()'ing /proc/<pid>/ otherwise.  It greatly complicates an intruder's task of
+gathering information about running processes, whether some daemon runs with
+elevated privileges, whether other user runs some sensitive program, whether
+other users run any program at all, etc.
 
-hidepid=4 means that procfs should only contain /proc/<pid>/ directories
-that the caller can ptrace.
+hidepid=ptraceable or hidepid=4 means that procfs should only contain
+/proc/<pid>/ directories that the caller can ptrace.
 
 gid= defines a group authorized to learn processes information otherwise
 prohibited by hidepid=.  If you use some daemon like identd which needs to learn
···
 It means that it became possible to have several procfs instances
 displaying tasks with different filtering options in one pid namespace.
 
-# mount -o hidepid=2 -t proc proc /proc
-# mount -o hidepid=1 -t proc proc /tmp/proc
+# mount -o hidepid=invisible -t proc proc /proc
+# mount -o hidepid=noaccess -t proc proc /tmp/proc
 # grep ^proc /proc/mounts
-proc /proc proc rw,relatime,hidepid=2 0 0
-proc /tmp/proc proc rw,relatime,hidepid=1 0 0
+proc /proc proc rw,relatime,hidepid=invisible 0 0
+proc /tmp/proc proc rw,relatime,hidepid=noaccess 0 0

fs/proc/inode.c

···
 #include <linux/seq_file.h>
 #include <linux/slab.h>
 #include <linux/mount.h>
+#include <linux/bug.h>
 
 #include <linux/uaccess.h>
 
···
 		deactivate_super(old_sb);
 }
 
+static inline const char *hidepid2str(int v)
+{
+	switch (v) {
+		case HIDEPID_OFF: return "off";
+		case HIDEPID_NO_ACCESS: return "noaccess";
+		case HIDEPID_INVISIBLE: return "invisible";
+		case HIDEPID_NOT_PTRACEABLE: return "ptraceable";
+	}
+	WARN_ONCE(1, "bad hide_pid value: %d\n", v);
+	return "unknown";
+}
+
 static int proc_show_options(struct seq_file *seq, struct dentry *root)
 {
 	struct proc_fs_info *fs_info = proc_sb_info(root->d_sb);
···
 	if (!gid_eq(fs_info->pid_gid, GLOBAL_ROOT_GID))
 		seq_printf(seq, ",gid=%u", from_kgid_munged(&init_user_ns, fs_info->pid_gid));
 	if (fs_info->hide_pid != HIDEPID_OFF)
-		seq_printf(seq, ",hidepid=%u", fs_info->hide_pid);
+		seq_printf(seq, ",hidepid=%s", hidepid2str(fs_info->hide_pid));
 	if (fs_info->pidonly != PROC_PIDONLY_OFF)
 		seq_printf(seq, ",subset=pid");
 

fs/proc/root.c

···
 
 static const struct fs_parameter_spec proc_fs_parameters[] = {
 	fsparam_u32("gid",	Opt_gid),
-	fsparam_u32("hidepid",	Opt_hidepid),
+	fsparam_string("hidepid",	Opt_hidepid),
 	fsparam_string("subset",	Opt_subset),
 	{}
 };
···
 		value == HIDEPID_NO_ACCESS ||
 		value == HIDEPID_INVISIBLE ||
 		value == HIDEPID_NOT_PTRACEABLE);
+}
+
+static int proc_parse_hidepid_param(struct fs_context *fc, struct fs_parameter *param)
+{
+	struct proc_fs_context *ctx = fc->fs_private;
+	struct fs_parameter_spec hidepid_u32_spec = fsparam_u32("hidepid", Opt_hidepid);
+	struct fs_parse_result result;
+	int base = (unsigned long)hidepid_u32_spec.data;
+
+	if (param->type != fs_value_is_string)
+		return invalf(fc, "proc: unexpected type of hidepid value\n");
+
+	if (!kstrtouint(param->string, base, &result.uint_32)) {
+		if (!valid_hidepid(result.uint_32))
+			return invalf(fc, "proc: unknown value of hidepid - %s\n", param->string);
+		ctx->hidepid = result.uint_32;
+		return 0;
+	}
+
+	if (!strcmp(param->string, "off"))
+		ctx->hidepid = HIDEPID_OFF;
+	else if (!strcmp(param->string, "noaccess"))
+		ctx->hidepid = HIDEPID_NO_ACCESS;
+	else if (!strcmp(param->string, "invisible"))
+		ctx->hidepid = HIDEPID_INVISIBLE;
+	else if (!strcmp(param->string, "ptraceable"))
+		ctx->hidepid = HIDEPID_NOT_PTRACEABLE;
+	else
+		return invalf(fc, "proc: unknown value of hidepid - %s\n", param->string);
+
+	return 0;
 }
 
 static int proc_parse_subset_param(struct fs_context *fc, char *value)
···
 		break;
 
 	case Opt_hidepid:
-		if (!valid_hidepid(result.uint_32))
-			return invalf(fc, "proc: unknown value of hidepid.\n");
-		ctx->hidepid = result.uint_32;
+		if (proc_parse_hidepid_param(fc, param))
+			return -EINVAL;
 		break;
 
 	case Opt_subset:

tools/testing/selftests/proc/.gitignore

···
 /fd-001-lookup
 /fd-002-posix-eq
 /fd-003-kthread
+/proc-fsconfig-hidepid
 /proc-loadavg-001
 /proc-multiple-procfs
 /proc-pid-vm

tools/testing/selftests/proc/Makefile

···
 TEST_GEN_PROGS += setns-sysvipc
 TEST_GEN_PROGS += thread-self
 TEST_GEN_PROGS += proc-multiple-procfs
+TEST_GEN_PROGS += proc-fsconfig-hidepid
 
 include ../lib.mk

tools/testing/selftests/proc/proc-fsconfig-hidepid.c

···
+/*
+ * Copyright © 2020 Alexey Gladkov <[e-mail]>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+#include <assert.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <errno.h>
+#include <linux/mount.h>
+#include <linux/unistd.h>
+
+static inline int fsopen(const char *fsname, unsigned int flags)
+{
+	return syscall(__NR_fsopen, fsname, flags);
+}
+
+static inline int fsconfig(int fd, unsigned int cmd, const char *key, const void *val, int aux)
+{
+	return syscall(__NR_fsconfig, fd, cmd, key, val, aux);
+}
+
+int main(void)
+{
+	int fsfd, ret;
+	int hidepid = 2;
+
+	assert((fsfd = fsopen("proc", 0)) != -1);
+
+	ret = fsconfig(fsfd, FSCONFIG_SET_BINARY, "hidepid", &hidepid, 0);
+	assert(ret == -1);
+	assert(errno == EINVAL);
+
+	assert(!fsconfig(fsfd, FSCONFIG_SET_STRING, "hidepid", "2", 0));
+	assert(!fsconfig(fsfd, FSCONFIG_SET_STRING, "hidepid", "invisible", 0));
+
+	assert(!close(fsfd));
+
+	return 0;
+}