Re: [Qemu-devel] [PATCH for-1.4 01/12] qmp: Fix design bug and read beyond buffer in memchar-write

[Markus Armbruster]

mdroth <address@hidden> writes:

> On Wed, Feb 06, 2013 at 10:06:03AM +0100, Markus Armbruster wrote:
>> Markus Armbruster <address@hidden> writes:
>> 
>> > Eric Blake <address@hidden> writes:
>> >
>> >> On 02/05/2013 09:22 AM, Markus Armbruster wrote:
>> >>> Command memchar-write takes data and size parameter.  Begs the
>> >>> question what happens when data doesn't match size.
>> >>> 
>> >>> With format base64, qmp_memchar_write() copies the full data argument,
>> >>> regardless of size argument.
>> >>> 
>> >>> With format utf8, qmp_memchar_write() copies size bytes from data,
>> >>> happily reading beyond data.  Copies crap from the heap or even
>> >>> crashes.
>> >>> 
>> >>> Drop the size parameter, and always copy the full data argument.
>> >>> 
>> >>> Signed-off-by: Markus Armbruster <address@hidden>
>> >>> ---
>> >>>  hmp.c            | 4 +---
>> >>>  qapi-schema.json | 4 +---
>> >>>  qemu-char.c      | 8 +++-----
>> >>>  qmp-commands.hx  | 4 +---
>> >>>  4 files changed, 6 insertions(+), 14 deletions(-)
>> >>
>> >>>      if (has_format && (format == DATA_FORMAT_BASE64)) {
>> >>>          write_data = g_base64_decode(data, &write_count);
>> >>>      } else {
>> >>>          write_data = (uint8_t *)data;
>> >>> +        write_count = strlen(data);
>> >>>      }
>> >>
>> >> Obviously, base64 is the only way to write an embedded NUL.  But what
>> >
>> > Consider the JSON string "this \\u0000 is fun".  But our JSON parser
>> > chokes on it, so we can ignore it until that's fixed.  See my "[PATCH]
>> > check-qjson: More thorough testing of UTF-8 in strings", specifically
>> > the comment right at the beginning of utf8_string().
>> >
>> >> happens if the user requests base64 encoding, but the utf8 string that
>> >> got passed in through JSON is not a valid base64-encoded string?  Does
>> >> g_base64_decode report an error in that case, and should you be handling
>> >> the error here?
>> >
>> > Good question.  I wish it had occured to GLib developers:
>> > http://developer.gnome.org/glib/stable/glib-Base64-Encoding.html#g-base64-decode
>> >
>> > Seriously, I need to find out what the heck g_base64_decode() does when
>> > it's fed crap.  If it fails in a reliable and detectable manner, I need
>> > to handle the failure.  If not, I need to replace the function.
>> >
>> > Moreover, I should document which characters outside the base64 alphabet
>> > are silently ignored, if any.  All whitespace?  Just newlines?
>> 
>> As far as I can tell, it never fails, but silently ignores characters
>> outside the alphabet [A-Za-z0-9+/], as well as unpadded suffixes.  Oh,
>> and it does something weird when padding appears in the middle.
>> Craptastic.
>> 
>> We can either document this behavior as a feature, or we replace the
>> function by one that accepts only valid base64.  If we do the latter, we
>> better specify the language we want to accept now, but I guess we could
>> choose to delay the actual checking post 1.4.
>> 
>> There's another use of g_base64_decode() in qmp_guest_file_write().
>> Which means guest agent command guest-file-write is similarly
>> ill-defined.  Mike, anything to be done there?
>
> For qemu-ga I think the documentation makes it clear enough that we're
> expecting b64 inputs rather than just interpreting random input as b64,
> so I don't see a problem with making the checks stricter in the future.
>
> One other concern though:
>
>> 
>> 
>> ---<test program>---
>> #include <glib.h>
>> #include <stdio.h>
>> #include <stdlib.h>
>> 
>> int
>> main(int argc, char *argv[])
>> {
>>     char **ap, *b64;
>>     unsigned char *buf;
>>     size_t sz, i;
>> 
>>     for (ap = argv + 1; *ap; ap++) {
>>      printf("in : %s\n", *ap);
>>      buf = g_base64_decode(*ap, &sz);
>>      printf("out:");
>>      for (i = 0; i < sz; i++) {
>>          printf(" %02x", buf[i]);
>>      }
>>      putchar('\n');
>>      b64 = g_base64_encode(buf, sz);
>>      printf("b64: %s\n", b64);
>>      free(buf);
>>     }
>> }
>> ---<test run>---
>> in : 1
>> out:
>> b64: 
>> in : 1=
>> out:
>> b64: 
>> in : 1==
>> out:
>> b64: 
>> in : 1===
>> out: d4
>> b64: 1A==
>> in : 12
>> out:
>> b64: 
>> in : 123
>> out:
>> b64: 
>> in : 1234
>> out: d7 6d f8
>> b64: 1234
>> in : =1234
>> out: 03 5d b7
>> b64: A123
>> in : 1=234
>> out: d4 0d b7
>> b64: 1A23
>> in : <>?,./watch address@hidden&*()_+
>> out: ff 06 ad 72 1b 61
>> b64: /watchth
>> in : /watchthis+
>> out: ff 06 ad 72 1b 61
>> b64: /watchth
>
> Am I misinterpreting the output or is base64_encode() actually spitting
> *out* invalid base64 strings for certain inputs? If so that seems pretty
> bad for something like guest-file-read, where inputs to base64_encode()
> are for all intents completely random. Addressing it in hard freeze may
> not be reasonable, since qemu-ga users must already be prepared to receive
> garbage from malicious/buggy agents, but I'd certainly pick up a fix for a
> subsequent stable release.

Which base64_encode() outputs in my test run do you suspect of being
bad?