Re: [PATCH v2 2/3] exec: kill "int depth" in search_binary_handler()

[Kees Cook]

On Sat, Aug 3, 2013 at 11:55 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> On 08/03, Kees Cook wrote:
>>
>> On Thu, Aug 1, 2013 at 12:05 PM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>> > Nobody except search_binary_handler() should touch ->recursion_depth,
>> > "int depth" buys nothing but complicates the code, kill it.
>>
>> I'd like to see a comment added to binfmts.h's recursion_depth field
>> that reminds people that recursion_depth is for
>> search_binary_handler()'s use only, and a binfmt loader shouldn't
>> touch it.
>
> And this comment probably makes sense even without this change

Yeah totally agreed -- I should have added this when I reorganized the
depth handling earlier. :)

>> Besides that, yeah, sensible clean up.
>
> OK, thanks, please see v2. The only change is the comment in .h
>
> ------------------------------------------------------------------------------
> Subject: [PATCH 1/1] exec: kill "int depth" in search_binary_handler()
>
> Nobody except search_binary_handler() should touch ->recursion_depth,
> "int depth" buys nothing but complicates the code, kill it.
>
> Probably we should also kill "fn" and the !NULL check, ->load_binary
> should be always defined. And it can not go away after read_unlock()
> or this code is buggy anyway.
>
> v2: add the comment about linux_binprm->recursion_depth
>
> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

Thanks,

-Kees

> ---
>  fs/exec.c               |    9 ++++-----
>  include/linux/binfmts.h |    2 +-
>  2 files changed, 5 insertions(+), 6 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index a9ae4f2..f32079c 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1370,12 +1370,11 @@ EXPORT_SYMBOL(remove_arg_zero);
>   */
>  int search_binary_handler(struct linux_binprm *bprm)
>  {
> -       unsigned int depth = bprm->recursion_depth;
> -       int try,retval;
> +       int try, retval;
>         struct linux_binfmt *fmt;
>
>         /* This allows 4 levels of binfmt rewrites before failing hard. */
> -       if (depth > 5)
> +       if (bprm->recursion_depth > 5)
>                 return -ELOOP;
>
>         retval = security_bprm_check(bprm);
> @@ -1396,9 +1395,9 @@ int search_binary_handler(struct linux_binprm *bprm)
>                         if (!try_module_get(fmt->module))
>                                 continue;
>                         read_unlock(&binfmt_lock);
> -                       bprm->recursion_depth = depth + 1;
> +                       bprm->recursion_depth++;
>                         retval = fn(bprm);
> -                       bprm->recursion_depth = depth;
> +                       bprm->recursion_depth--;
>                         if (retval >= 0) {
>                                 put_binfmt(fmt);
>                                 allow_write_access(bprm->file);
> diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
> index 70cf138..e8112ae 100644
> --- a/include/linux/binfmts.h
> +++ b/include/linux/binfmts.h
> @@ -31,7 +31,7 @@ struct linux_binprm {
>  #ifdef __alpha__
>         unsigned int taso:1;
>  #endif
> -       unsigned int recursion_depth;
> +       unsigned int recursion_depth; /* only for search_binary_handler() */
>         struct file * file;
>         struct cred *cred;      /* new credentials */
>         int unsafe;             /* how unsafe this exec is (mask of LSM_UNSAFE_*) */
> --
> 1.5.5.1
>
>



-- 
Kees Cook
Chrome OS Security
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