On Tue 12-03-19 23:26:22, Kees Cook wrote: > On Mon, Mar 11, 2019 at 1:42 PM syzbot > <syzbot+2c49971e251e36216d1f@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote: > > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=17ee410b200000 > > [...] > > ------------[ cut here ]------------ > > Bad or missing usercopy whitelist? Kernel memory exposure attempt detected > > from SLAB object 'fanotify_event' (offset 40, size 8)! > > [...] > > copy_to_user include/linux/uaccess.h:151 [inline] > > copy_fid_to_user fs/notify/fanotify/fanotify_user.c:236 [inline] > > copy_event_to_user fs/notify/fanotify/fanotify_user.c:294 [inline] > > Looks like this is the fh/ext_fh union in struct fanotify_fid, field > "fid" in struct fanotify_event. Given that "fid" is itself in a union > against a struct path, I think instead of a whitelist using > KMEM_CACHE_USERCOPY(), this should just use a bounce buffer to avoid > leaving a whitelist open for path or ext_fh exposure. Do you mean to protect it from a situation when some other code (i.e. not copy_fid_to_user()) would be tricked into copying ext_fh containing slab pointer to userspace? Honza > > Maybe something like this (untested): > > diff --git a/fs/notify/fanotify/fanotify_user.c > b/fs/notify/fanotify/fanotify_user.c > index 56992b32c6bb..b87da9580b3c 100644 > --- a/fs/notify/fanotify/fanotify_user.c > +++ b/fs/notify/fanotify/fanotify_user.c > @@ -207,6 +207,7 @@ static int process_access_response(struct > fsnotify_group *group, > static int copy_fid_to_user(struct fanotify_event *event, char __user *buf) > { > struct fanotify_event_info_fid info = { }; > + unsigned char bounce[FANOTIFY_INLINE_FH_LEN], *fh; > struct file_handle handle = { }; > size_t fh_len = event->fh_len; > size_t len = fanotify_event_info_len(event); > @@ -233,7 +234,18 @@ static int copy_fid_to_user(struct fanotify_event > *event, char __user *buf) > > buf += sizeof(handle); > len -= sizeof(handle); > - if (copy_to_user(buf, fanotify_event_fh(event), fh_len)) > + > + /* > + * For an inline fh, copy through stack to exclude the copy from > + * usercopy hardening protections. > + */ > + fh = fanotify_event_fh(event); > + if (fh_len <= sizeof(bounce)) { > + memcpy(bounce, fh, fh_len); > + fh = bounce; > + } > + > + if (copy_to_user(buf, fh, fh_len)) > return -EFAULT; > > /* Pad with 0's */ > > > -- > Kees Cook -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
