segfault bb_make_directory + dirname with musl

[Daniel Sabogal]

On Thu, Dec 1, 2016 at 3:13 PM, Daniel Sabogal <dsabogalcc at gmail.com> wrote:
> On Wed, Nov 30, 2016 at 5:52 PM, Denys Vlasenko
> <vda.linux at googlemail.com> wrote:
>> On Wed, Nov 30, 2016 at 3:46 AM, Daniel Sabogal <dsabogalcc at gmail.com> wrote:
>>> The following commands cause busybox to segfault on musl-based systems.
>>>
>>> $ install -D a /
>>> $ install -D a /b
>>> $ install -D a /b/
>>>
>>> This happens because the code in
>>>
>>> https://git.busybox.net/busybox/tree/coreutils/install.c?h=1_25_1#n196
>>>
>>> passes the result of dirname() to bb_make_directory() which modifies its
>>> contents. For paths of the above forms, musl's dirname returns a string
>>> literal "/" which shouldn't be modified.
>>>
>>> See http://git.musl-libc.org/cgit/musl/tree/src/misc/dirname.c
>>>
>>> There are a few other occurrences of the code shown above, but I've not
>>> checked to see if they could be made to segfault.
>>
>> Does this fix the problem?
>>
>>                         /* Bypass leading non-'/'s and then subsequent '/'s */
>>                         while (*s) {
>>                                 if (*s == '/') {
>>                                         do {
>>                                                 ++s;
>>                                         } while (*s == '/');
>>                                         c = *s; /* Save the current char */
>> ====added line==>                       if (c)
>>                                                 *s = '\0'; /* and
>> replace it with nul */
>>                                         break;
>
> This does prevent the segfault, but I'm not sure if depending on being able to
> modify the result of dirname() is reliable.

https://git.busybox.net/busybox/commit/?id=cf2600c3661c11491a838ef29733583afb6ad968

There are other occurrences of dirname + bb_make_directory that may have
this issue.

The following also segfaults.

$ cp --parents a /