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SubjectRe: [PATCH] exec: separate MM_ANONPAGES and RLIMIT_STACK accounting
On Mon, Nov 12, 2018 at 10:09 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> get_arg_page() checks bprm->rlim_stack.rlim_cur and re-calculates the
> "extra" size for argv/envp pointers every time, this is a bit ugly and
> even not strictly correct: acct_arg_size() must not account this size.
>
> Remove all the rlimit code in get_arg_page(). Instead, add bprm->argmin
> calculated once at the start of __do_execve_file() and change copy_strings
> to check bprm->p >= bprm->argmin.
>
> The patch adds the new helper, prepare_arg_pages() which initializes
> bprm->argc/envc and bprm->argmin.
>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Acked-by: Kees Cook <keescook@chromium.org>

Thanks for nailing this all down. :)

-Kees

> ---
> fs/exec.c | 103 +++++++++++++++++++++++-------------------------
> include/linux/binfmts.h | 1 +
> 2 files changed, 51 insertions(+), 53 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index fc281b7..61a5460 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)
> @@ -492,6 +447,50 @@ static int count(struct user_arg_ptr argv, int max)
> return i;
> }
>
> +static int prepare_arg_pages(struct linux_binprm *bprm,
> + struct user_arg_ptr argv, struct user_arg_ptr envp)
> +{
> + unsigned long limit, ptr_size;
> +
> + bprm->argc = count(argv, MAX_ARG_STRINGS);
> + if (bprm->argc < 0)
> + return bprm->argc;
> +
> + bprm->envc = count(envp, MAX_ARG_STRINGS);
> + if (bprm->envc < 0)
> + return bprm->envc;
> +
> + /*
> + * 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);
> + /*
> + * We've historically supported up to 32 pages (ARG_MAX)
> + * of argument strings even with small stacks
> + */
> + limit = max(limit, (unsigned long)ARG_MAX);
> + /*
> + * 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 *);
> + if (limit <= ptr_size)
> + return -E2BIG;
> + limit -= ptr_size;
> +
> + bprm->argmin = 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 +526,8 @@ static int copy_strings(int argc, struct user_arg_ptr argv,
> pos = bprm->p;
> str += len;
> bprm->p -= len;
> + if (bprm->p < bprm->argmin)
> + goto out;
>
> while (len > 0) {
> int offset, bytes_to_copy;
> @@ -1789,12 +1790,8 @@ static int __do_execve_file(int fd, struct filename *filename,
> if (retval)
> goto out_unmark;
>
> - 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)
> + retval = prepare_arg_pages(bprm, argv, envp);
> + if (retval < 0)
> goto out;
>
> retval = prepare_binprm(bprm);
> diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
> index e9f5fe6..03200a8 100644
> --- a/include/linux/binfmts.h
> +++ b/include/linux/binfmts.h
> @@ -25,6 +25,7 @@ struct linux_binprm {
> #endif
> struct mm_struct *mm;
> unsigned long p; /* current top of mem */
> + unsigned long argmin; /* rlimit marker for copy_strings() */
> unsigned int
> /*
> * True after the bprm_set_creds hook has been called once
> --
> 2.5.0
>
>



--
Kees Cook

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