On Wed, Jan 25, 2017 at 1:43 PM, Ben Hutchings <ben@xxxxxxxxxxxxxxx> wrote: > On Wed, 2017-01-25 at 13:06 -0800, Andy Lutomirski wrote: >> If an unprivileged program opens a setgid file for write and passes >> the fd to a privileged program and the privileged program writes to >> it, we currently fail to clear the setgid bit. Fix it by checking >> f_cred instead of current's creds whenever a struct file is >> involved. > [...] > > What if, instead, a privileged program passes the fd to an un > unprivileged program? It sounds like a bad idea to start with, but at > least currently the unprivileged program is going to clear the setgid > bit when it writes. This change would make that behaviour more > dangerous. > > Perhaps there should be a capability check on both the current > credentials and file credentials? (I realise that we've considered > file credential checks to be sufficient elsewhere, but those cases > involved virtual files with special semantics, where it's clearer that > a privileged process should not pass them to an unprivileged process.) We need a set of self-tests for this whole area. :( There are so many corner cases. We still have an unfixed corner case with mmap writes not clearing set*id bits that I tried to solve last year... -Kees -- Kees Cook Nexus Security -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
