On Thu, Jan 21, 2016 at 3:22 PM, Jann Horn <jann@xxxxxxxxx> wrote: > On Mon, Jan 11, 2016 at 02:57:50PM -0800, 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. We could clear at open() time, but it's possible things are >> accidentally opening with O_RDWR and only reading. Better to clear on >> close and error failures (i.e. an improvement over now, which is not >> clearing at all). >> >> 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 (ETXTBSY). >> >> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> >> --- > [...] >> diff --git a/fs/file_table.c b/fs/file_table.c >> index ad17e05ebf95..ca11b86613cf 100644 >> --- a/fs/file_table.c >> +++ b/fs/file_table.c >> @@ -191,6 +191,21 @@ static void __fput(struct file *file) >> >> might_sleep(); >> >> + /* >> + * XXX: This is a delayed removal of privs (we've already been >> + * written to), since we must avoid mmap_sem. But a race shouldn't >> + * be possible since when open for writing, execve() will fail >> + * with ETXTBSY (via deny_write_access()). A remaining problem >> + * is that since we've already been written to, we must ignore the >> + * return value of file_remove_privs(), since we can't reject the >> + * writes of the past. >> + */ >> + if (unlikely(file->f_flags & O_REMOVEPRIV)) { >> + mutex_lock(&inode->i_mutex); >> + file_remove_privs(file); >> + mutex_unlock(&inode->i_mutex); >> + } >> + > > If there is any other setuid file I can run, can't I just do this? > > pid_t child = fork(); > if (child == 0) { > /* fd will be 3 or so */ > int fd = open("setuid-file-with-bad-privs", O_WRONLY); > char *ptr = mmap(..., fd, 0); > memcpy(ptr, my_evil_code, sizeof(my_evil_code)); > /* su --bad-option just prints usage and exits, without touching > * the fd - but since su has the last reference to the fd, __fput > * will run with its privileges */ > execlp("su", "su", "--bad-option", NULL); > } > int status; > wait(&status); > execlp("setuid-file-with-bad-privs", "setuid-file-with-bad-privs", NULL); > > I think that file_remove_privs() really needs to be changed to use f_cred > instead of current_cred(). That would also fix the known bypass where > you pass the fd to a setuid process as fd 1, causing the setuid process > to write more-or-less controlled data to a chosen offset, or similar > stuff (see > http://www.halfdog.net/Security/2015/SetgidDirectoryPrivilegeEscalation/). > > Or was there already another patch that does this that I didn't see? Andy brought it up as an issue, but I view it as a separate problem. Both things need to be fixed. :) -Kees -- Kees Cook Chrome OS & Brillo Security -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html