On 08/09/2020 15:42, Stephen Smalley wrote: > On Tue, Sep 8, 2020 at 9:29 AM Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote: >> >> On Tue, 2020-09-08 at 08:52 -0400, Stephen Smalley wrote: >>> On Tue, Sep 8, 2020 at 8:50 AM Stephen Smalley >>> <stephen.smalley.work@xxxxxxxxx> wrote: >>>> >>>> On Tue, Sep 8, 2020 at 8:43 AM Mickaël Salaün <mic@xxxxxxxxxxx> wrote: >>>>> >>>>> >>>>> On 08/09/2020 14:28, Mimi Zohar wrote: >>>>>> Hi Mickael, >>>>>> >>>>>> On Tue, 2020-09-08 at 09:59 +0200, Mickaël Salaün wrote: >>>>>>> + mode |= MAY_INTERPRETED_EXEC; >>>>>>> + /* >>>>>>> + * For compatibility reasons, if the system-wide policy >>>>>>> + * doesn't enforce file permission checks, then >>>>>>> + * replaces the execute permission request with a read >>>>>>> + * permission request. >>>>>>> + */ >>>>>>> + mode &= ~MAY_EXEC; >>>>>>> + /* To be executed *by* user space, files must be readable. */ >>>>>>> + mode |= MAY_READ; >>>>>> >>>>>> After this change, I'm wondering if it makes sense to add a call to >>>>>> security_file_permission(). IMA doesn't currently define it, but >>>>>> could. >>>>> >>>>> Yes, that's the idea. We could replace the following inode_permission() >>>>> with file_permission(). I'm not sure how this will impact other LSMs though. >> >> I wasn't suggesting replacing the existing security_inode_permission >> hook later, but adding a new security_file_permission hook here. >> >>>> >>>> They are not equivalent at least as far as SELinux is concerned. >>>> security_file_permission() was only to be used to revalidate >>>> read/write permissions previously checked at file open to support >>>> policy changes and file or process label changes. We'd have to modify >>>> the SELinux hook if we wanted to have it check execute access even if >>>> nothing has changed since open time. >>> >>> Also Smack doesn't appear to implement file_permission at all, so it >>> would skip Smack checking. >> >> My question is whether adding a new security_file_permission call here >> would break either SELinux or Apparmor? > > selinux_inode_permission() has special handling for MAY_ACCESS so we'd > need to duplicate that into selinux_file_permission() -> > selinux_revalidate_file_permission(). Also likely need to adjust > selinux_file_permission() to explicitly check whether the mask > includes any permissions not checked at open time. So some changes > would be needed here. By default, it would be a no-op unless there > was a policy reload or the file was relabeled between the open(2) and > the faccessat(2) call. > We could create a new hook path_permission(struct path *path, int mask) as a superset of inode_permission(). To be more convenient, his new hook could then just call inode_permission() for every LSMs not implementing path_permission().
