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. But IIRC mmaped writes go through a different path -- they go through the address_space ops with names like writepages. > >>> >>>> 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. --Andy -- 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
