Messages in this thread | | | Date | Thu, 02 Oct 2014 20:56:37 +0200 | From | Heinrich Schuchardt <> | Subject | Re: [PATCH] fanotify: prove that structure size is correct |
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On 29.09.2014 01:35, Kees Cook wrote: > When building with CONFIG_DEBUG_STRICT_USER_COPY_CHECKS, this copy_to_user > wasn't provably correct (the length was in a variable but the structure > wasn't). The compiler wasn't able to check that the length was being > statically assigned, so include a BUG_ON to notice if this ever changes. > > Signed-off-by: Kees Cook <keescook@chromium.org> > --- > fs/notify/fanotify/fanotify_user.c | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c > index b13992a41bd9..7bdb43222875 100644 > --- a/fs/notify/fanotify/fanotify_user.c > +++ b/fs/notify/fanotify/fanotify_user.c > @@ -216,6 +216,7 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group, > > fd = fanotify_event_metadata.fd; > ret = -EFAULT; > + BUG_ON(fanotify_event_metadata.event_len != FAN_EVENT_METADATA_LEN);
Thank you for indicating an inconsistency in fanotify_user.c
In fanotify_user.c a mismatch exists between the assumptions made in get_one_event() and copy_event_to_user().
According to the fanotify.7 man page: "FAN_EVENT_METADATA_LEN - This is the minimum size (and currently the only size) of any event metadata."
get_one_event() assumes that the event metadata has a length of FAN_EVENT_METADATA_LEN and does not care about the value of event_len.
copy_event_to_user() assumes that event_len specifies the length of the event metadata and does not care about FAN_EVENT_METADATA_LEN.
Due to these different assumptions it cannot be proven inside fanotify_user.c that copy_to_user() is called correctly.
If we want such a prove inside fanotify_user.c, we should change get_one_event() to use the following:
// Get the event without removing it from the group. fsnotify_event *evt = fsnotify_peek_notify_event(group); // Check we have enough user memory to copy the event. if (evt->event_len > count) return ERR_PTR(-EINVAL); // Remove the event from the group. fsnotify_remove_notify_event(group); // Return the pointer we tested. // fanotify_remove_notify_event() should return the same pointer. return evt;
If after this change you still want a BUG_ON, pass count to copy_event_to_user() and write BUG_ON(fanotify_event_metadata.event_len > count);
Best regards
Heinrich Schuchardt
> if (copy_to_user(buf, &fanotify_event_metadata, > fanotify_event_metadata.event_len)) > goto out_close_fd; >
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