Re: [PATCH v10 8/9] proc: use human-readable values for hidehid

[Alexey Gladkov <gladkov.alexey@gmail.com>]

On Sat, Mar 28, 2020 at 01:28:28PM -0700, Kees Cook wrote:
> On Fri, Mar 27, 2020 at 06:23:30PM +0100, Alexey Gladkov wrote:
> > The hidepid parameter values are becoming more and more and it becomes
> > difficult to remember what each new magic number means.
> > 
> > Suggested-by: Andy Lutomirski <luto@kernel.org>
> > Reviewed-by: Alexey Dobriyan <adobriyan@gmail.com>
> > Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
> > ---
> >  Documentation/filesystems/proc.txt | 52 +++++++++++++++---------------
> >  fs/proc/inode.c                    | 13 +++++++-
> >  fs/proc/root.c                     | 36 +++++++++++++++++++--
> >  3 files changed, 71 insertions(+), 30 deletions(-)
> > 
> > diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
> > index bd0e0ab85048..af47672cb2cb 100644
> > --- a/Documentation/filesystems/proc.txt
> > +++ b/Documentation/filesystems/proc.txt
> > @@ -2025,28 +2025,28 @@ The following mount options are supported:
> >  	gid=		Set the group authorized to learn processes information.
> >  	subset=		Show only the specified subset of procfs.
> >  
> > -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=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=off or hidepid=0 means classic mode - everybody may access all
> > +/proc/<pid>/ directories (default).
> > +
> > +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=invisible or hidepid=2 means hidepid=noaccess 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=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
> > @@ -2093,8 +2093,8 @@ creates a new procfs instance. Mount options affect own procfs instance.
> >  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
> > diff --git a/fs/proc/inode.c b/fs/proc/inode.c
> > index e6577ce6027b..f01fb4bed75c 100644
> > --- a/fs/proc/inode.c
> > +++ b/fs/proc/inode.c
> > @@ -165,6 +165,17 @@ void proc_invalidate_siblings_dcache(struct hlist_head *inodes, spinlock_t *lock
> >  		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";
> > +	}
> > +	BUG();
> 
> Please don't use BUG()[1]. Add a default case with a warn and return
> "unknown":
> 
> 	switch (v) {
> 	case HIDEPID_OFF: return "off";
> 	case HIDEPID_NO_ACCESS: return "noaccess";
> 	case HIDEPID_INVISIBLE: return "invisible";
> 	case HIDEPID_NOT_PTRACEABLE: return "ptraceable";
> 	default:
> 		WARN_ON_ONCE("bad hide_pid value: %d\n", v);
> 		return "unknown";
> 	}
> 
> [1] https://lore.kernel.org/lkml/202003141524.59C619B51A@keescook/

Make sense. I will change it.

> > +}
> > +
> >  static int proc_show_options(struct seq_file *seq, struct dentry *root)
> >  {
> >  	struct proc_fs_info *fs_info = proc_sb_info(root->d_sb);
> > @@ -172,7 +183,7 @@ static int proc_show_options(struct seq_file *seq, struct dentry *root)
> >  	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");
> >  
> > diff --git a/fs/proc/root.c b/fs/proc/root.c
> > index dbcd96f07c7a..ba782d6e6197 100644
> > --- a/fs/proc/root.c
> > +++ b/fs/proc/root.c
> > @@ -45,7 +45,7 @@ enum proc_param {
> >  
> >  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),
> >  	{}
> >  };
> > @@ -58,6 +58,35 @@ static inline int valid_hidepid(unsigned int value)
> >  		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)) {
> > +		ctx->hidepid = result.uint_32;
> 
> This need to bounds-check the value with a call to valid_hidepid(), yes?

The kstrtouint returns 0 on success and -ERANGE on overflow [1].

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/lib/kstrtox.c#n217

> > +		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)
> >  {
> >  	struct proc_fs_context *ctx = fc->fs_private;
> > @@ -97,9 +126,10 @@ static int proc_parse_param(struct fs_context *fc, struct fs_parameter *param)
> >  		break;
> >  
> >  	case Opt_hidepid:
> > -		if (!valid_hidepid(result.uint_32))
> > +		if (proc_parse_hidepid_param(fc, param))
> > +			return -EINVAL;
> > +		if (!valid_hidepid(ctx->hidepid))
> >  			return invalf(fc, "proc: unknown value of hidepid.\n");
> > -		ctx->hidepid = result.uint_32;
> >  		break;
> >  
> >  	case Opt_subset:
> > -- 
> > 2.25.2
> > 
> 
> -- 
> Kees Cook
> 

-- 
Rgrds, legion