[axtls-general] [Bug Report] heap buffer overflow when parsing private key

[Xm C. <xic...@gm...>]

Hi, we found a bug in the latest version of axtls(v2.1.5) when parsing
private key.

Here's how we trigger the bug and the error log from AddressSanitizer.

```
./axssl s_server -accept 15001 -pass abcd -key POC10 -cert ./a.pem
=================================================================
==436778==ERROR: AddressSanitizer: heap-buffer-overflow on address
0x6020000000bf at pc 0x5583a5a5a903 bp 0x7fffb8107e00 sp 0x7fffb8107df0
READ of size 1 at 0x6020000000bf thread T0
    #0 0x5583a5a5a902 in bi_import ./axtls-code/crypto/bigint.c:631
    #1 0x5583a5a61912 in RSA_priv_key_new ./axtls-code/crypto/rsa.c:66
    #2 0x5583a5a6d786 in asn1_get_private_key ./axtls-code/ssl/asn1.c:299
    #3 0x5583a5a430d4 in add_private_key ./axtls-code/ssl/tls1.c:648
    #4 0x5583a5a3d21b in do_obj ./axtls-code/ssl/loader.c:134
    #5 0x5583a5a3d0b7 in ssl_obj_load ./axtls-code/ssl/loader.c:92
    #6 0x5583a5a3a8a3 in do_server ./axtls-code/samples/c/axssl.c:232
    #7 0x5583a5a39dcd in main ./axtls-code/samples/c/axssl.c:90
    #8 0x7f51816ce082 in __libc_start_main ../csu/libc-start.c:308
    #9 0x5583a5a39b4d in _start (axssl+0x8b4d)

0x6020000000bf is located 13 bytes to the right of 2-byte region
[0x6020000000b0,0x6020000000b2)
allocated by thread T0 here:
    #0 0x7f518194c527 in __interceptor_malloc
../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:145
    #1 0x5583a5a6cbbd in asn1_get_big_int ./axtls-code/ssl/asn1.c:168
    #2 0x5583a5a6d559 in asn1_get_private_key ./axtls-code/ssl/asn1.c:292
    #3 0x5583a5a430d4 in add_private_key ./axtls-code/ssl/tls1.c:648
    #4 0x5583a5a3d21b in do_obj ./axtls-code/ssl/loader.c:134
    #5 0x5583a5a3d0b7 in ssl_obj_load ./axtls-code/ssl/loader.c:92
    #6 0x5583a5a3a8a3 in do_server ./axtls-code/samples/c/axssl.c:232
    #7 0x5583a5a39dcd in main ./axtls-code/samples/c/axssl.c:90
    #8 0x7f51816ce082 in __libc_start_main ../csu/libc-start.c:308

SUMMARY: AddressSanitizer: heap-buffer-overflow
./axtls-code/crypto/bigint.c:631 in bi_import
Shadow bytes around the buggy address:
  0x0c047fff7fc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c047fff7fd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c047fff7fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c047fff7ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c047fff8000: fa fa 00 00 fa fa 02 fa fa fa 02 fa fa fa 02 fa
=>0x0c047fff8010: fa fa 05 fa fa fa 02[fa]fa fa 02 fa fa fa 02 fa
  0x0c047fff8020: fa fa 00 fa fa fa 04 fa fa fa 00 fa fa fa fd fa
  0x0c047fff8030: fa fa 00 00 fa fa 00 04 fa fa fd fa fa fa 00 00
  0x0c047fff8040: fa fa fd fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8050: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8060: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
  Shadow gap:              cc
==436778==ABORTING
```

We triaged the crash, and found that the bug locates at ./ssl/asn1.c.
Firstly, when invoking `RSA_priv_key_new`, function `asn1_get_private_key`
uses the values from `asn1_get_big_int` as arguments without any check.
Secondly, when calling `bi_import` from `RSA_priv_key_new`, the data and
its len may not match.
Finally, there might be an out-of-bound access in function `bi_import` at
line 631 within the loop.


You can use the attached files and the following compile command to
reproduce the bug.
```
LDFLAGS="-fsanitize=address" CFLAGS="-g -O0 -fsanitize=address"
CXXFLAGS="-g -O0 -fsanitize=address" make
```

Thank you for your work. We are prepared to provide more information and
looking forward for your reply!

Xingman Chen