On Wed, Dec 9, 2015 at 4:49 AM, Jan Kara <jack@xxxxxxx> wrote: > On Tue 08-12-15 15:28:18, Kees Cook wrote: >> Normally, when a user can modify a file that has setuid or setgid bits, >> those bits are cleared when they are not the file owner or a member >> of the group. This is enforced when using write and truncate but not >> when writing to a shared mmap on the file. This could allow the file >> writer to gain privileges by changing a binary without losing the >> setuid/setgid/caps bits. >> >> Changing the bits requires holding inode->i_mutex, so it cannot be done >> during the page fault (due to mmap_sem being held during the fault). We >> could do this during vm_mmap_pgoff, but that would need coverage in >> mprotect as well, but to check for MAP_SHARED, we'd need to hold mmap_sem >> again. >> >> Instead, detect the need to clear the bits during the page fault, and >> actually remove the bits during final fput. Since the file was open for >> writing, it wouldn't have been possible to execute it yet. >> >> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> >> --- >> Here's another way? I wonder which of these will actually work. I >> wish we could reject writes if file_remove_privs() fails. > > Yeah, the fact that we cannot do anything with file_remove_privs() failure > is rather unfortunate. So open for writing may be the best choice for > file_remove_privs() in the end? It's not perfect but it looks like the > least problematic solution. Yeah, back to just the open itself. I can't even delay this to the mmap. :( I will do a v5. :) -Kees > > Frankly writeable files that have SUID / SGID bits set are IMHO problems on > its own, with IMA attrs which are handled by file_remove_privs() as well > things may be somewhat different. > >> diff --git a/fs/file_table.c b/fs/file_table.c >> index ad17e05ebf95..abb537ef4344 100644 >> --- a/fs/file_table.c >> +++ b/fs/file_table.c >> @@ -191,6 +191,14 @@ static void __fput(struct file *file) >> >> might_sleep(); >> >> + /* >> + * XXX: While avoiding mmap_sem, we've already been written to. >> + * We must ignore the return value, since we can't reject the >> + * write. >> + */ >> + if (unlikely(file->f_remove_privs)) >> + file_remove_privs(file); >> + > > You're missing i_mutex locking again ;). > > Honza > -- > Jan Kara <jack@xxxxxxxx> > SUSE Labs, CR -- Kees Cook Chrome OS & Brillo Security -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>