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SubjectRe: [PATCH v2 6/8] exec/binfmt_script: Don't modify bprm->buf and then return -ENOEXEC
Kees Cook <keescook@chromium.org> writes:

> On Mon, May 18, 2020 at 07:33:21PM -0500, Eric W. Biederman wrote:
>>
>> 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.

I think the best way to document this is to modify bprm->buf to be
"const char buf[BINPRM_BUF_SIZE]" or something like that and not
allow any modifications by anything except for the code that
initially reads in contets of the file.

That unfortunately requires copy_strings_kernel which has become
copy_string_kernel to take a length. Then I don't need to modify the
buffer at all here.

I believe binfmt_scripts is a bit unique in wanting to modify the buffer
because it is parsing strings.

The requirement is that a binfmt should not modify bprm unless it will
succeed or fail with an error that is not -ENOEXEC. The fundamental
issue is that search_binary_handler will reuse bprm if -ENOEXEC is
returned.

Until the next patch there is an escape hatch by clearing and closing
bprm->file but that goes away. Which is why I need this patch.

I guess I can see adding a comment about the general case of not
changing bprm unless you are doing something other than returning
-ENOEXEC and letting the search continue.

Eric


>> [...]
>> 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>

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