On Mon, May 1, 2017 at 10:36 AM, Jann Horn <jannh@xxxxxxxxxx> wrote: > On Sun, Apr 30, 2017 at 12:04 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: >> New AT_... flag - AT_NO_JUMPS >> >> Semantics: pathname resolution must not involve >> * traversals of absolute symlinks >> * traversals of procfs-style symlinks >> * traversals of mountpoints (including bindings, referrals, etc.) >> * traversal of .. in the starting point of pathname resolution. >> >> All of those lead to failure with -ELOOP. Relative symlinks are fine, >> as long as their resolution does not end up stepping into the conditions >> above. >> >> It guarantees that result of successful pathname resolution will be on the >> same filesystem as its starting point and within the subtree rooted at >> the starting point. >> >> Right now I have it hooked only for fstatat() and friends; it could be >> easily extended to any ...at() syscalls. Objections? > > Oh, nice! > > It looks like this is somewhat similar to the old O_BENEATH proposal, > but because the intentions behind the proposals are different > (application sandboxing versus permitting an application to restrict its > own filesystem accesses), the semantics differ: AT_NO_JUMPS > doesn't prevent starting the path with "/", but does prevent mountpoint > traversal. Is that correct? > I missed that. I think that AT_HOTEL_CALIFORNIA or whatever we call it should disallow even explicit absolute paths. If I do: openat([fd to /var/www], "possibly untrusted path here", AT_HOTEL_CALIFORNIA, O_WHATEVER); I should not have to separately verify that the path doesn't start with "/" to make sure that I don't escape. There's a big added advantage of this approach, too: I could write a seccomp rule that only lets me call openat() with this new flag set, and now I can't escape. > I think that, as Andy mentioned, it might make sense to split out (or > even remove?) the prevention of mountpoint traversal. A user who > can create visible mountpoints needs to have capabilities over the > mount namespace the file descriptor refers to already. Agreed. There's a big difference between the admin bind-mounting /etc into /var/www and some web app putting a symlink to /etc into /var/www. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
