Hi Rik,
On Wed, Oct 11, 2023 at 09:21:28AM -0400, Rik van Riel 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.
Ahh, so the limit for each string is different than the limit for the
total. That makes sense. Sorry for the noise.
Cheers,
Alex
>
> --
> All Rights Reversed.
--
<https://www.alejandro-colomar.es/>
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