On Mon, May 24, 2021 at 04:53:21PM +0300, Amir Goldstein wrote: > Reporting event->pid should depend on the privileges of the user that > initialized the group, not the privileges of the user reading the > events. I think it's in general a good permission model to not have the result depend as little on the reader as possible post-open/init. So this makes a lot of sense to me and I'm a bit surprised it wasn't like that right away. :) > > Use an internal group flag FANOTIFY_UNPRIV to record the fact that the > group was initialized by an unprivileged user. > > To be on the safe side, the premissions to setup filesystem and mount > marks now require that both the user that initialized the group and > the user setting up the mark have CAP_SYS_ADMIN. > > Link: https://lore.kernel.org/linux-fsdevel/CAOQ4uxiA77_P5vtv7e83g0+9d7B5W9ZTE4GfQEYbWmfT1rA=VA@xxxxxxxxxxxxxx/ > Fixes: 7cea2a3c505e ("fanotify: support limited functionality for unprivileged users") > Cc: <Stable@xxxxxxxxxxxxxxx> # v5.12+ > Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx> > --- Looks good, Acked-by: Christian Brauner <christian.brauner@xxxxxxxxxx> > > Changes since v1: > - Address Matthew's editorial review comments > - Rename macro FANOTIFY_INTERNAL_GROUP_FLAGS > > fs/notify/fanotify/fanotify_user.c | 30 ++++++++++++++++++++++++------ > fs/notify/fdinfo.c | 2 +- > include/linux/fanotify.h | 4 ++++ > 3 files changed, 29 insertions(+), 7 deletions(-) > > diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c > index 71fefb30e015..be5b6d2c01e7 100644 > --- a/fs/notify/fanotify/fanotify_user.c > +++ b/fs/notify/fanotify/fanotify_user.c > @@ -424,11 +424,18 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group, > * events generated by the listener process itself, without disclosing > * the pids of other processes. > */ > - if (!capable(CAP_SYS_ADMIN) && > + if (FAN_GROUP_FLAG(group, FANOTIFY_UNPRIV) && > task_tgid(current) != event->pid) > metadata.pid = 0; > > - if (path && path->mnt && path->dentry) { > + /* > + * For now, fid mode is required for an unprivileged listener and > + * fid mode does not report fd in events. Keep this check anyway > + * for safety in case fid mode requirement is relaxed in the future > + * to allow unprivileged listener to get events with no fd and no fid. > + */ > + if (!FAN_GROUP_FLAG(group, FANOTIFY_UNPRIV) && > + path && path->mnt && path->dentry) { > fd = create_fd(group, path, &f); > if (fd < 0) > return fd; > @@ -1040,6 +1047,7 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags) > int f_flags, fd; > unsigned int fid_mode = flags & FANOTIFY_FID_BITS; > unsigned int class = flags & FANOTIFY_CLASS_BITS; > + unsigned int internal_flags = 0; > > pr_debug("%s: flags=%x event_f_flags=%x\n", > __func__, flags, event_f_flags); > @@ -1053,6 +1061,13 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags) > */ > if ((flags & FANOTIFY_ADMIN_INIT_FLAGS) || !fid_mode) > return -EPERM; > + > + /* > + * Setting the internal flag FANOTIFY_UNPRIV on the group > + * prevents setting mount/filesystem marks on this group and > + * prevents reporting pid and open fd in events. > + */ > + internal_flags |= FANOTIFY_UNPRIV; > } > > #ifdef CONFIG_AUDITSYSCALL > @@ -1105,7 +1120,7 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags) > goto out_destroy_group; > } > > - group->fanotify_data.flags = flags; > + group->fanotify_data.flags = flags | internal_flags; > group->memcg = get_mem_cgroup_from_mm(current->mm); > > group->fanotify_data.merge_hash = fanotify_alloc_merge_hash(); > @@ -1305,11 +1320,13 @@ static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask, > group = f.file->private_data; > > /* > - * An unprivileged user is not allowed to watch a mount point nor > - * a filesystem. > + * An unprivileged user is not allowed to setup mount nor filesystem > + * marks. This also includes setting up such marks by a group that > + * was initialized by an unprivileged user. > */ > ret = -EPERM; > - if (!capable(CAP_SYS_ADMIN) && > + if ((!capable(CAP_SYS_ADMIN) || > + FAN_GROUP_FLAG(group, FANOTIFY_UNPRIV)) && > mark_type != FAN_MARK_INODE) > goto fput_and_out; > > @@ -1460,6 +1477,7 @@ static int __init fanotify_user_setup(void) > max_marks = clamp(max_marks, FANOTIFY_OLD_DEFAULT_MAX_MARKS, > FANOTIFY_DEFAULT_MAX_USER_MARKS); > > + BUILD_BUG_ON(FANOTIFY_INIT_FLAGS & FANOTIFY_INTERNAL_GROUP_FLAGS); > BUILD_BUG_ON(HWEIGHT32(FANOTIFY_INIT_FLAGS) != 10); > BUILD_BUG_ON(HWEIGHT32(FANOTIFY_MARK_FLAGS) != 9); > > diff --git a/fs/notify/fdinfo.c b/fs/notify/fdinfo.c > index a712b2aaa9ac..57f0d5d9f934 100644 > --- a/fs/notify/fdinfo.c > +++ b/fs/notify/fdinfo.c > @@ -144,7 +144,7 @@ void fanotify_show_fdinfo(struct seq_file *m, struct file *f) > struct fsnotify_group *group = f->private_data; > > seq_printf(m, "fanotify flags:%x event-flags:%x\n", > - group->fanotify_data.flags, > + group->fanotify_data.flags & FANOTIFY_INIT_FLAGS, > group->fanotify_data.f_flags); > > show_fdinfo(m, f, fanotify_fdinfo); > diff --git a/include/linux/fanotify.h b/include/linux/fanotify.h > index bad41bcb25df..a16dbeced152 100644 > --- a/include/linux/fanotify.h > +++ b/include/linux/fanotify.h > @@ -51,6 +51,10 @@ extern struct ctl_table fanotify_table[]; /* for sysctl */ > #define FANOTIFY_INIT_FLAGS (FANOTIFY_ADMIN_INIT_FLAGS | \ > FANOTIFY_USER_INIT_FLAGS) > > +/* Internal group flags */ > +#define FANOTIFY_UNPRIV 0x80000000 > +#define FANOTIFY_INTERNAL_GROUP_FLAGS (FANOTIFY_UNPRIV) > + > #define FANOTIFY_MARK_TYPE_BITS (FAN_MARK_INODE | FAN_MARK_MOUNT | \ > FAN_MARK_FILESYSTEM) > > -- > 2.25.1 >
