On Fri, Nov 6, 2015 at 2:30 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: > On Fri, Nov 6, 2015 at 1:59 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: >> Adding Ted, who might know how this all hooks together. (The context >> is that a write() or truncate() on a setgid file clears the setgid, >> but mmap writes don't.) >> >> On Wed, Nov 4, 2015 at 9:59 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >>> On Tue, Nov 3, 2015 at 10:58 PM, Willy Tarreau <w@xxxxxx> wrote: >>>> On Tue, Nov 03, 2015 at 03:29:55PM -0800, Kees Cook wrote: >>>>> Using "write" does kill the set-gid bit. I haven't looked at >>>>> why. >>>>> Al or anyone else, is there a meaningful distinction here? >>>> >>>> I remember this one, I got caught once while trying to put a shell into >>>> a suid-writable file to get some privileges someone forgot to offer me :-) >>>> >>>> It's done by should_remove_suid() which is called upon write() and truncate(). >> >> file_remove_privs() seems to be the right entry point. >> __generic_file_write_iter in mm/filemap.c calls it, though. Are these >> callbacks not used for mmap writes? > > They're certainly not used early enough -- we need to remove suid when > the page becomes writable via mmap (wp_page_shared), not when > writeback happens, or at least not only when writeback happens. Well, I'm shy about the change there. For example, we don't strip in on open(RDWR), just on write(). > But IIRC mmaped writes go through a different path -- they go through > the address_space ops with names like writepages. Ah-ha. >>>>> Should the >>>>> mmap MAP_SHARED-write trigger the loss of the set-gid bit too? While >>>>> holding the file open with either open or mmap, I get a Text-in-use >>>>> error, so I would kind of expect the same behavior between either >>>>> close() and munmap(). I wonder if this is a bug, and if so, then your >>>>> link patch is indeed useful again. :) >>>> >>>> I don't see how this could be done with mmap(). Maybe we have a way to know >>>> when the first write is performed via this path, I have no idea. >>> >>> do_wp_page might be a decent bet. >> >> Or wp_page_shared? Can we get back to a file from the mm at that point? > > vma->vm_file, presumably (after checking whether it's null). > wp_page_shared AFAIK only happens from process context, and the vma > and its file should be valid. > > We could also get to an inode via page->address_space->mapping, but > I'm guessing that vma->vm_file would be more appropriate here. Yeah. Let me give it a try... -Kees -- Kees Cook Chrome OS 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
