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SubjectRe: [PATCH v2 6/8] exec/binfmt_script: Don't modify bprm->buf and then return -ENOEXEC
On Mon, May 18, 2020 at 07:33:21PM -0500, Eric W. Biederman wrote:
>
> The return code -ENOEXEC serves to tell search_binary_handler that it
> should continue searching for the binfmt to handle a given file. This
> makes return -ENOEXEC with a bprm->buf that is needed to continue the
> search problematic.
>
> The current binfmt_script manages to escape problems as it closes and
> clears bprm->file before return -ENOEXEC with bprm->buf modified.
> This prevents search_binary_handler from looping as it explicitly
> handles a NULL bprm->file.
>
> I plan on moving all of the bprm->file managment into fs/exec.c and out
> of the binary handlers so this will become a problem.
>
> Move closing bprm->file and the test for BINPRM_PATH_INACCESSIBLE
> down below the last return of -ENOEXEC.
>
> Introduce i_sep and i_end to track the end of the first argument and
> the end of the parameters respectively. Using those, constification
> of all char * pointers, and the helpers next_terminator and
> next_non_spacetab guarantee the parameter parsing will not modify
> bprm->buf.

I'm quite pleased this could be implemented using the existing helpers!
It seems Linus and I were on the right track with these. :)

>
> Only modify bprm->buf to terminate the strings i_arg and i_name with
> '\0' for passing to copy_strings_kernel.
>
> When replacing loops with next_non_spacetab and next_terminator care
> has been take that the logic of the parsing code (short of replacing
> characters by '\0') remains the same.

Ah, interesting. As in, bprm->buf must not be modified unless the binfmt
handler is going to succeed. I think this requirement should be
documented in the binfmt struct header file.

> [...]
> diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c
> index 8d718d8fd0fe..85e0ef86eb11 100644
> --- a/fs/binfmt_script.c
> +++ b/fs/binfmt_script.c
> @@ -71,39 +56,48 @@ static int load_script(struct linux_binprm *bprm)
> * parse them on its own.
> */
> buf_end = bprm->buf + sizeof(bprm->buf) - 1;
> - cp = strnchr(bprm->buf, sizeof(bprm->buf), '\n');
> - if (!cp) {
> - cp = next_non_spacetab(bprm->buf + 2, buf_end);
> - if (!cp)
> + i_end = strnchr(bprm->buf, sizeof(bprm->buf), '\n');
> + if (!i_end) {
> + i_end = next_non_spacetab(bprm->buf + 2, buf_end);
> + if (!i_end)
> return -ENOEXEC; /* Entire buf is spaces/tabs */
> /*
> * If there is no later space/tab/NUL we must assume the
> * interpreter path is truncated.
> */
> - if (!next_terminator(cp, buf_end))
> + if (!next_terminator(i_end, buf_end))
> return -ENOEXEC;
> - cp = buf_end;
> + i_end = buf_end;
> }
> - /* NUL-terminate the buffer and any trailing spaces/tabs. */
> - *cp = '\0';
> - while (cp > bprm->buf) {
> - cp--;
> - if ((*cp == ' ') || (*cp == '\t'))
> - *cp = '\0';
> - else
> - break;
> - }
> - for (cp = bprm->buf+2; (*cp == ' ') || (*cp == '\t'); cp++);
> - if (*cp == '\0')
> + /* Trim any trailing spaces/tabs from i_end */
> + while (spacetab(i_end[-1]))
> + i_end--;
> +
> + /* Skip over leading spaces/tabs */
> + i_name = next_non_spacetab(bprm->buf+2, i_end);
> + if (!i_name || (i_name == i_end))
> return -ENOEXEC; /* No interpreter name found */
> - i_name = cp;
> +
> + /* Is there an optional argument? */
> i_arg = NULL;
> - for ( ; *cp && (*cp != ' ') && (*cp != '\t'); cp++)
> - /* nothing */ ;
> - while ((*cp == ' ') || (*cp == '\t'))
> - *cp++ = '\0';
> - if (*cp)
> - i_arg = cp;
> + i_sep = next_terminator(i_name, i_end);
> + if (i_sep && (*i_sep != '\0'))
> + i_arg = next_non_spacetab(i_sep, i_end);
> +
> + /*
> + * If the script filename will be inaccessible after exec, typically
> + * because it is a "/dev/fd/<fd>/.." path against an O_CLOEXEC fd, give
> + * up now (on the assumption that the interpreter will want to load
> + * this file).
> + */
> + if (bprm->interp_flags & BINPRM_FLAGS_PATH_INACCESSIBLE)
> + return -ENOENT;
> +
> + /* Release since we are not mapping a binary into memory. */
> + allow_write_access(bprm->file);
> + fput(bprm->file);
> + bprm->file = NULL;
> +
> /*
> * OK, we've parsed out the interpreter name and
> * (optional) argument.
> @@ -121,7 +115,9 @@ static int load_script(struct linux_binprm *bprm)
> if (retval < 0)
> return retval;
> bprm->argc++;
> + *((char *)i_end) = '\0';
> if (i_arg) {
> + *((char *)i_sep) = '\0';
> retval = copy_strings_kernel(1, &i_arg, bprm);
> if (retval < 0)
> return retval;

I think this is all correct, though I'm always suspicious of my visual
inspection of string parsers. ;)

I had a worry the \n was not handled correctly in some case. I.e. before
any \n was converted into \0, and so next_terminator() didn't need to
consider \n separately. (next_non_spacetab() doesn't care since \n and \0
are both not ' ' nor '\t'.) For next_terminator(), though, I was worried
there was a case where *i_end == '\n', and next_terminator()
will return NULL instead of "last" due to *last being '\n' instead of
'\0', causing a problem, but you're using the adjusted i_end so I think
it's correct. And you've handled i_name == i_end.

I will see if I can find my testing scripts I used when commit
b5372fe5dc84 originally landed to double-check... until then:

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

--
Kees Cook

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