Re: [PATCH] execve.2: execve also returns E2BIG if a string is too long

[Alejandro Colomar <alx@kernel.org>]

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On Wed, Oct 11, 2023 at 09:44:29AM -0400, Matthew House wrote:
> On Wed, Oct 11, 2023 at 9:21 AM Rik van Riel <riel@surriel.com> wrote:
> > On Wed, 2023-10-11 at 12:41 +0200, Alejandro Colomar wrote:
> > > Hi Rik,
> > >
> > > On Tue, Oct 10, 2023 at 11:41:53PM -0400, Rik van Riel wrote:
> > > > Document that if a command line or environment string is too long
> > > > (> MAX_ARG_STRLEN), execve will also return E2BIG.
> > >
> > > That's already implied by the current text:
> > >
> > >        E2BIG  The total number of bytes in the environment (envp) and
> > > argument
> > >               list (argv) is too large.
> > >
> > > That means that
> > >
> > > size_t  bytes;
> > >
> > > bytes = 0;
> > > for (char *e = envp; e != NULL; e++)
> > >         bytes += strlen(e) + 1;  // I have doubts about the +1
> > > for (char *a = argv; a != NULL; a++)
> > >         bytes += strlen(a) + 1;  // Same doubts
> > >
> > > if (bytes > MAX_ARG_STRLEN)  // Maybe >= ?
> > >         return -E2BIG;
> >
> > The code in fs/exec.c enforces MAX_ARG_STRLEN against
> > each individual string, not against the total.
> >
> > If any string, either argument or environment, is larger
> > than 32 * PAGE_SIZE, the kernel will return -E2BIG.
> >
> > do_execveat_common() has this code, which uses copy_strings
> > to copy both the strings from the environment, and from
> > the command line arguments:
> >
> >         retval = copy_strings(bprm->envc, envp, bprm);
> >         if (retval < 0)
> >                 goto out_free;
> >
> >         retval = copy_strings(bprm->argc, argv, bprm);
> >         if (retval < 0)
> >                 goto out_free;
> >
> > Inside copy_strings() we have this code:
> >
> >
> >         while (argc-- > 0) {
> > ...
> >                 len = strnlen_user(str, MAX_ARG_STRLEN);
> >                 if (!len)
> >                         goto out;
> >
> >                 ret = -E2BIG;
> >                 if (!valid_arg_len(bprm, len))
> >                         goto out;
> >
> > The valid_arg_len() function does not need explanation:
> >
> > static bool valid_arg_len(struct linux_binprm *bprm, long len)
> > {
> >         return len <= MAX_ARG_STRLEN;
> > }
> >
> >
> > The current man page wording is very clear about the total
> > length being enforced, but IMHO not as clear about the limit
> > that gets enforced on each individual string.
> >
> > The total length limit of environment & commandline arguments
> > is enforced by bprm_stack_limits(), and is checked against
> > either 1/4 of the maximum stack size, or 3/4 of _STK_LIM, whichever
> > is smaller. The MAX_ARG_STRLEN value does not come into play when
> > enforcing the total.
> 
> To expand on this, there are basically two separate byte limits in
> fs/exec.c, one for each individual argv/envp string, and another for all
> strings and all pointers to them as a whole. To put the whole thing in
> pseudocode, the checks work effectively like this, assuming I haven't made
> any errors:
> 
> int argc, envc;
> unsigned long bytes, limit;
> 
> /* assume that argv has already been adjusted to add an empty argv[0] */
> argc = 0, envc = 0, bytes = 0;
> for (char **a = argv; *a != NULL; a++, argc++) {
>     if (strlen(*a) >= MAX_ARG_STRLEN)

Are you sure this is >= and not > ?

>         return -E2BIG;
>     bytes += strlen(*a) + 1;
> }
> for (char **e = envp; *e != NULL; e++, envc++) {
>     if (strlen(*e) >= MAX_ARG_STRLEN)
>         return -E2BIG;
>     bytes += strlen(*e) + 1;
> }
> 
> if (argc > MAX_ARG_STRINGS || envc > MAX_ARG_STRINGS)
>     return -E2BIG;
> bytes += (argc + envc) * sizeof(void *);
> 
> limit = max(min(_STK_LIM / 4 * 3, rlim_stack.rlim_cur / 4), ARG_MAX);
> if (bytes > limit)
>     return -E2BIG;
> 
> Thank you,
> Matthew House

-- 
<https://www.alejandro-colomar.es/>

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