Re: [PATCH] Prevent buffer overflows when path is too long

[Antoine Pelisse <apelisse@gmail.com>]

On Tue, Nov 26, 2013 at 8:50 PM, Junio C Hamano <gitster@pobox.com> wrote:
> Antoine Pelisse <apelisse@gmail.com> writes:
>
>> Some buffers created with PATH_MAX length are not checked when being
>> written, and can overflow if PATH_MAX is not big enough to hold the
>> path.
>
> Perhaps it is time to update all of them to use strbuf?  The callers
> of prefix_filename() aren't that many, and all of them are prepared
> to stash the returned value away when they keep it longer term, so
> they would not notice if we used "static struct strbuf path" and
> gave back "path.buf" (without strbuf_detach() on it).  The buffer
> used in clear_ce_flags() and passed to clear_ce_flags_{1,dir} are
> not seen outside the callchain, and can safely become strbuf, I
> think.

Let's do that, but shouldn't we also modify those that are currently
safe, like absolute_path() just above prefix_filename() ?

>>  abspath.c        | 10 ++++++++--
>>  diffcore-order.c | 14 +++++++++-----
>>  unpack-trees.c   |  2 ++
>>  3 files changed, 19 insertions(+), 7 deletions(-)
>>
>> diff --git a/abspath.c b/abspath.c
>> index e390994..29a5f9d 100644
>> --- a/abspath.c
>> +++ b/abspath.c
>> @@ -216,11 +216,16 @@ const char *absolute_path(const char *path)
>>  const char *prefix_filename(const char *pfx, int pfx_len, const char *arg)
>>  {
>>       static char path[PATH_MAX];
>> +
>> +     if (pfx_len >= PATH_MAX)
>> +             die("Too long prefix path: %s", pfx);
>
> I do not think this is needed, and will reject a valid input that
> used to be accepted (e.g. arg is absolute so pfx does not matter).

My mistake

>> -     strcpy(path + pfx_len, arg);
>> +     if (strlcpy(path + pfx_len, arg, PATH_MAX - pfx_len) > PATH_MAX)
>> +             die("Too long path: %s", path);
>>       for (p = path + pfx_len; *p; p++)
>>               if (*p == '\\')
>>                       *p = '/';
>
> The above is curious. Unless we are doing the short-cut for "no
> prefix so we can just return arg" codepath, we know that the
> resulting length is always pfx_len + strlen(arg), no?

If you mean that the test should be more like the following:
+     if (strlcpy(path + pfx_len, arg, PATH_MAX - pfx_len) > PATH_MAX - pfx_len)

Then of course, you are right, that's my mistake.

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