On Mon, Oct 1, 2018 at 1:58 PM Jan Kara <jack@xxxxxxx> wrote: > > On Thu 27-09-18 23:05:14, Matthew Bobrowski wrote: > > This is a reduced version of a patch that I originally submitted a while ago. > > > > In short, the fanotify API currently does not provide any means for user space > > programs to receive events specifically when a file has been opened with the > > intent to be executed. The FAN_EXEC flag will be set within the event mask when > > a object has been opened with one of the open flags being __FMODE_EXEC. > > > > Linux is used as an Operating System in some products, with an environment that > > can be certified under the Common Criteria Operating System Protection Profile > > (OSPP). This is a formal threat model for a class of technology. It requires > > specific countermeasures to mitigate threats. It requires documentation to > > explain how a product implements these countermeasures. It requires proof via a > > test suite to demonstrate that the requirements are met, observed and checked by > > an independent qualified third party. The latest set of requirements for OSPP > > v4.2 can be found here: > > > > https://www.niap-ccevs.org/Profile/Info.cfm?PPID=424&id=424 > > > > If you look on page 58, you will see the following requirement: > > > > FPT_SRP_EXT.1 Software Restriction Policies FPT_SRP_EXT.1.1 > > > > The OS shall restrict execution to only programs which match an administrator > > specified [selection: > > file path, > > file digital signature, > > version, > > hash, > > [assignment: other characteristics] > > ] > > > > This patch is to help aid in meeting this requirement. > > > > Signed-off-by: Matthew Bobrowski <mbobrowski@xxxxxxxxxxxxxx> > > I agree with Amir's points wrt API so I won't repeat those. But I have one > more API question: > > You implement FS_EXEC as a flag that can get set for certain FAN_OPEN > events. That is a new API concept for fanotify. So far you can only request > event of a certain type and then you get the same flag back when the event > happens. There is also a case of FAN_ONDIR where you can restrict set of > events only to events on a particular inode type but that's again > different. Hence my question: Is there a good reason why we don't create > FAN_OPEN_EXEC event that would trigger only on executable opens? > > If someone is interested only in executable opens, he'd have less events to > care about. OTOH additional FS_EXEC flag is probably more flexible (e.g. > you can easily implement equivalent of FAN_OPEN_NOEXEC in userspace if you > wished). Just the inconsistency of the FS_EXEC and e.g. how we handle > FAN_CLOSE_NOWRITE & FAN_CLOSE_WRITE is bothering me... > I understand why the inconsitency is bothering you, but IMO it is too late to change that. By trying to be more consistent in the *implementation* of the flags, we will end up confusing users instead of making their life easy by sticking to FAN_OPEN sematics they are used to. IMO we need to report FAN_OPEN for every open like we do now. and additionally report FAN_OPEN_EXEC for open for exec. Then user can implement FAN_OPEN_NOEXEC by setting ignore mask with FAN_OPEN_EXEC. You can make the analogy to the compound event FAN_CLOSE. If user sets a mask to the compound event FAN_CLOSE and sets ignore mask with FAN_CLOSE_WRITE, then user effectively implemented FAN_CLOSE_NOWRITE with similar semantics to implementation of FAN_OPEN_NOEXEC. Similarly, if user requests FAN_OPEN|FAN_CLOSE and then checks (event->mask & FAN_OPEN) or (event->mask & FAN_CLOSE) it has similar meaning. Testing (event->mask & FAN_OPEN_EXEC) and (event->mask & FAN_CLOSE_WRITE) in this case is similarly informative. Honestly, I can't think of an application interested only in FAN_CLOSE_NOWRITE nor only in FAN_OPEN_NOEXEC, but the functionality is available for both. The fact that user *can* implement the former without ignore mask and cannot implement to latter without ignore mask is IMO the neccesary evil we need to carry for historic API decisions. Thanks, Amir.
