Messages in this thread Patch in this message | | | From | Kees Cook <> | Date | Tue, 11 Sep 2018 12:06:05 -0700 | Subject | Re: get_arg_page() && ptr_size accounting |
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Oh, I like this patch! This is much cleaner. Notes below...
On Tue, Sep 11, 2018 at 7:13 AM, Oleg Nesterov <oleg@redhat.com> wrote: > diff --git a/fs/exec.c b/fs/exec.c > index 1ebf6e5..7804a5c 100644 > --- a/fs/exec.c > +++ b/fs/exec.c > @@ -218,55 +218,10 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos, > if (ret <= 0) > return NULL; > > - if (write) { > - unsigned long size = bprm->vma->vm_end - bprm->vma->vm_start; > - unsigned long ptr_size, limit; > - > - /* > - * Since the stack will hold pointers to the strings, we > - * must account for them as well. > - * > - * The size calculation is the entire vma while each arg page is > - * built, so each time we get here it's calculating how far it > - * is currently (rather than each call being just the newly > - * added size from the arg page). As a result, we need to > - * always add the entire size of the pointers, so that on the > - * last call to get_arg_page() we'll actually have the entire > - * correct size. > - */ > - ptr_size = (bprm->argc + bprm->envc) * sizeof(void *); > - if (ptr_size > ULONG_MAX - size) > - goto fail; > - size += ptr_size; > - > - acct_arg_size(bprm, size / PAGE_SIZE); > - > - /* > - * We've historically supported up to 32 pages (ARG_MAX) > - * of argument strings even with small stacks > - */ > - if (size <= ARG_MAX) > - return page; > - > - /* > - * Limit to 1/4 of the max stack size or 3/4 of _STK_LIM > - * (whichever is smaller) for the argv+env strings. > - * This ensures that: > - * - the remaining binfmt code will not run out of stack space, > - * - the program will have a reasonable amount of stack left > - * to work from. > - */ > - limit = _STK_LIM / 4 * 3; > - limit = min(limit, bprm->rlim_stack.rlim_cur / 4); > - if (size > limit) > - goto fail; > - } > + if (write) > + acct_arg_size(bprm, vma_pages(bprm->vma)); > > return page; > - > -fail: > - put_page(page); > - return NULL; > } > > static void put_arg_page(struct page *page) > @@ -410,11 +365,6 @@ static int bprm_mm_init(struct linux_binprm *bprm) > if (!mm) > goto err; > > - /* Save current stack limit for all calculations made during exec. */ > - task_lock(current->group_leader); > - bprm->rlim_stack = current->signal->rlim[RLIMIT_STACK]; > - task_unlock(current->group_leader); > -
I would prefer this hunk stay here since it will be more robust against weird arch-specific things happening against rlim_stack. I had to clean up some of these tests in arch code, so I'm nervous about moving this further away. Here is before we call arch_bprm_mm_init(), and I think it's better to do this as early as possible.
> err = __bprm_mm_init(bprm); > if (err) > goto err; > @@ -492,6 +442,27 @@ static int count(struct user_arg_ptr argv, int max) > return i; > } > > +static int prepare_rlim_stack(struct linux_binprm *bprm)
How about collapsing this with:
bprm->argc = count(argv, MAX_ARG_STRINGS); if ((retval = bprm->argc) < 0) goto out;
bprm->envc = count(envp, MAX_ARG_STRINGS); if ((retval = bprm->envc) < 0) goto out;
and call it prepare_arg_count(struct linux_binprm *bprm, struct user_arg_ptr argv, struct user_arg_ptr envp)
> +{ > + unsigned long limit, ptr_size; > + > + task_lock(current->group_leader); > + bprm->rlim_stack = current->signal->rlim[RLIMIT_STACK]; > + task_unlock(current->group_leader); > +
Let's try to retain the comments here:
/* * Limit to 1/4 of the max stack size or 3/4 of _STK_LIM * (whichever is smaller) for the argv+env strings. * This ensures that: * - the remaining binfmt code will not run out of stack space, * - the program will have a reasonable amount of stack left * to work from. */ > + limit = _STK_LIM / 4 * 3; > + limit = min(limit, bprm->rlim_stack.rlim_cur / 4);
and here:
/* * We've historically supported up to 32 pages (ARG_MAX) * of argument strings even with small stacks */ > + limit = max(limit, (unsigned long)ARG_MAX); > + /* COMMENT */
This comment should likely be something like:
/* * We must account for the size of all the argv and envp pointers to * the argv and envp strings, since they will also take up space in * the stack. They aren't stored until much later when we can't * signal to the parent that the child has run out of stack space. * Instead, calculate it here so it's possible to fail gracefully. */
> + ptr_size = (bprm->argc + bprm->envc) * sizeof(void *);
BTW, in re-reading create_elf_tables() and its calculation of "items", I realize the above should actually include the trailing NULL pointers and argc, so it should be:
ptr_size = (1 + bprm->argc + 1 + bprm->envc + 1) * sizeof(void *);
> + if (limit <= ptr_size) > + return -E2BIG; > + limit -= ptr_size; > + > + bprm->p_min = bprm->p - limit; > + return 0; > +} > + > /* > * 'copy_strings()' copies argument/environment strings from the old > * processes's memory to the new process's stack. The call to get_user_pages() > @@ -527,6 +498,8 @@ static int copy_strings(int argc, struct user_arg_ptr argv, > pos = bprm->p; > str += len; > bprm->p -= len; > + if (bprm->p <= bprm->p_min) > + goto out; > > while (len > 0) { > int offset, bytes_to_copy; > @@ -1801,6 +1774,10 @@ static int __do_execve_file(int fd, struct filename *filename, > if (retval < 0) > goto out; > > + retval = prepare_rlim_stack(bprm); > + if (retval < 0) > + goto out; > + > retval = copy_strings_kernel(1, &bprm->filename, bprm); > if (retval < 0) > goto out; > diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h > index c05f24f..423e8c1 100644 > --- a/include/linux/binfmts.h > +++ b/include/linux/binfmts.h > @@ -24,7 +24,7 @@ struct linux_binprm { > struct page *page[MAX_ARG_PAGES]; > #endif > struct mm_struct *mm; > - unsigned long p; /* current top of mem */ > + unsigned long p, p_min; /* current top of mem */
Can you split this out to a separate line (with a new comment) to avoid comment-confusion? Something like:
unsigned long p; /* current top of mem */ unsigned long p_min; /* the minimum allowed mem position */
> unsigned int > /* > * True after the bprm_set_creds hook has been called once >
I've also spent some more time convincing myself again that there aren't races between count(), copy_strings(), and create_elf_tables(). A malicious parent would only be able to zero-fill the stack of the child, but not escape the counts that I can see.
-Kees
-- Kees Cook Pixel Security
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