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    Subject[PATCH 3.16 176/178] fs/exec.c: account for argv/envp pointers
    3.16.46-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Kees Cook <keescook@chromium.org>

    commit 98da7d08850fb8bdeb395d6368ed15753304aa0c upstream.

    When limiting the argv/envp strings during exec to 1/4 of the stack limit,
    the storage of the pointers to the strings was not included. This means
    that an exec with huge numbers of tiny strings could eat 1/4 of the stack
    limit in strings and then additional space would be later used by the
    pointers to the strings.

    For example, on 32-bit with a 8MB stack rlimit, an exec with 1677721
    single-byte strings would consume less than 2MB of stack, the max (8MB /
    4) amount allowed, but the pointers to the strings would consume the
    remaining additional stack space (1677721 * 4 == 6710884).

    The result (1677721 + 6710884 == 8388605) would exhaust stack space
    entirely. Controlling this stack exhaustion could result in
    pathological behavior in setuid binaries (CVE-2017-1000365).

    [akpm@linux-foundation.org: additional commenting from Kees]
    Fixes: b6a2fea39318 ("mm: variable length argument support")
    Link: http://lkml.kernel.org/r/20170622001720.GA32173@beast
    Signed-off-by: Kees Cook <keescook@chromium.org>
    Acked-by: Rik van Riel <riel@redhat.com>
    Acked-by: Michal Hocko <mhocko@suse.com>
    Cc: Alexander Viro <viro@zeniv.linux.org.uk>
    Cc: Qualys Security Advisory <qsa@qualys.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    [bwh: Backported to 3.16: use ACCESS_ONCE() instead of READ_ONCE()]
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    fs/exec.c | 28 ++++++++++++++++++++++++----
    1 file changed, 24 insertions(+), 4 deletions(-)

    --- a/fs/exec.c
    +++ b/fs/exec.c
    @@ -205,8 +205,26 @@ static struct page *get_arg_page(struct

    if (write) {
    unsigned long size = bprm->vma->vm_end - bprm->vma->vm_start;
    + unsigned long ptr_size;
    struct rlimit *rlim;

    + /*
    + * 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);

    /*
    @@ -224,13 +242,15 @@ static struct page *get_arg_page(struct
    * to work from.
    */
    rlim = current->signal->rlim;
    - if (size > ACCESS_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4) {
    - put_page(page);
    - return NULL;
    - }
    + if (size > ACCESS_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4)
    + goto fail;
    }

    return page;
    +
    +fail:
    + put_page(page);
    + return NULL;
    }

    static void put_arg_page(struct page *page)
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