-fanalyzer warning for mlkem public_key_size_bytes

[falko-strenzke]

cipher/mlkem.c|528 col 18| warning: use of uninitialized value ‘public_key_size_bytes’ [CWE-457] [-Wanalyzer-use-of-uninitialized-value]
||   528 |   bit_strength = bitstrength_from_public_size_bytes (public_key_size_bytes);
||       |                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
||   ‘mlkem_encap’: events 1-4
||     |
||     |  509 | mlkem_encap (gcry_sexp_t *r_ciph,
||     |      | ^~~~~~~~~~~
||     |      | |
||     |      | (1) entry to ‘mlkem_encap’
||     |......
||     |  516 |   size_t public_key_size_bytes;
||     |      |          ~~~~~~~~~~~~~~~~~~~~~
||     |      |          |
||     |      |          (2) region created on stack here
||     |      |          (3) capacity: 8 bytes
||     |......
||     |  522 |   ec = public_key_from_sexp (
||     |      |        ~~~~~~~~~~~~~~~~~~~~~~
||     |      |        |
||     |      |        (4) calling ‘public_key_from_sexp’ from ‘mlkem_encap’
||     |  523 |       keyparms, &public_key, &public_key_size_bytes, NULL);
||     |      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
||     |
||     +--> ‘public_key_from_sexp’: events 5-8
||            |
||            |  342 | public_key_from_sexp (const gcry_sexp_t keyparms,
||            |      | ^~~~~~~~~~~~~~~~~~~~
||            |      | |
||            |      | (5) entry to ‘public_key_from_sexp’
||            |......
||            |  348 |   return extract_opaque_mpi_from_sexp (keyparms,
||            |      |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
||            |      |          |
||            |      |          (6) following ‘false’ branch (when ‘param’ is NULL)...
||            |      |          (7) ...to here
||            |      |          (8) calling ‘extract_opaque_mpi_from_sexp’ from ‘public_key_from_sexp’
||            |  349 |                                        "/y",
||            |      |                                        ~~~~~
||            |  350 |                                        pk_p,
||            |      |                                        ~~~~~
||            |  351 |                                        pk_len_p,
||            |      |                                        ~~~~~~~~~
||            |  352 |                                        param != NULL ? &param->public_key_bytes
||            |      |                                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
||            |  353 |                                                      : (u16 *)NULL,
||            |      |                                                      ~~~~~~~~~~~~~~
||            |  354 |                                        _gcry_malloc);
||            |      |                                        ~~~~~~~~~~~~~
||            |
||            +--> ‘extract_opaque_mpi_from_sexp’: events 9-10
||                   |
||                   |  233 | extract_opaque_mpi_from_sexp (const gcry_sexp_t keyparms,
||                   |      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
||                   |      | |
||                   |      | (9) entry to ‘extract_opaque_mpi_from_sexp’
||                   |......
||                   |  249 |   if (ec)
||                   |      |      ~
||                   |      |      |
||                   |      |      (10) following ‘false’ branch (when ‘ec == 0’)...
||                   |
||                 ‘extract_opaque_mpi_from_sexp’: event 11
||                   |
|../src/gcrypt-int.h|554| 32:
||                   |  554 | #define mpi_get_nbits(a)       _gcry_mpi_get_nbits ((a))
||                   |      |                                ^~~~~~~~~~~~~~~~~~~~~~~~~
||                   |      |                                |
||                   |      |                                (11) ...to here
cipher/mlkem.c|253 col 14| note: in expansion of macro ‘mpi_get_nbits’
||                   |  253 |   data_len = mpi_get_nbits (sk);
||                   |      |              ^~~~~~~~~~~~~
||                   |
||                 ‘extract_opaque_mpi_from_sexp’: events 12-19
||                   |
||                   |  254 |   if (data_len % 8)
||                   |      |      ^
||                   |      |      |
||                   |      |      (12) following ‘false’ branch...
||                   |......
||                   |  258 |   data_len /= 8;
||                   |      |   ~~~~~~~~~~~~~
||                   |      |            |
||                   |      |            (13) ...to here
||                   |  259 |   if (expected_size_bytes_opt
||                   |      |      ~
||                   |      |      |
||                   |      |      (14) following ‘false’ branch (when ‘expected_size_bytes_opt’ is NULL)...
||                   |......
||                   |  265 |   *data_p = alloc_func (data_len);
||                   |      |             ~~~~~~~~~~~~~~~~~~~~~
||                   |      |             |
||                   |      |             (15) ...to here
||                   |  266 |   if (*data_p == NULL)
||                   |      |      ~
||                   |      |      |
||                   |      |      (16) following ‘true’ branch...
||                   |  267 |     {
||                   |  268 |       ec = gpg_err_code_from_syserror ();
||                   |      |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
||                   |      |            |
||                   |      |            (17) ...to here
||                   |......
||                   |  288 |   if (ec)
||                   |      |      ~
||                   |      |      |
||                   |      |      (18) following ‘false’ branch (when ‘ec == 0’)...
||                   |......
||                   |  293 |   return ec;
||                   |      |          ~~
||                   |      |          |
||                   |      |          (19) ...to here
||                   |
||            <------+
||            |
||          ‘public_key_from_sexp’: event 20
||            |
||            |  348 |   return extract_opaque_mpi_from_sexp (keyparms,
||            |      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
||            |      |          |
||            |      |          (20) returning to ‘public_key_from_sexp’ from ‘extract_opaque_mpi_from_sexp’
||            |  349 |                                        "/y",
||            |      |                                        ~~~~~
||            |  350 |                                        pk_p,
||            |      |                                        ~~~~~
||            |  351 |                                        pk_len_p,
||            |      |                                        ~~~~~~~~~
||            |  352 |                                        param != NULL ? &param->public_key_bytes
||            |      |                                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
||            |  353 |                                                      : (u16 *)NULL,
||            |      |                                                      ~~~~~~~~~~~~~~
||            |  354 |                                        _gcry_malloc);
||            |      |                                        ~~~~~~~~~~~~~
||            |
||     <------+
||     |
||   ‘mlkem_encap’: events 21-24
||     |
||     |  522 |   ec = public_key_from_sexp (
||     |      |        ^~~~~~~~~~~~~~~~~~~~~~
||     |      |        |
||     |      |        (21) returning to ‘mlkem_encap’ from ‘public_key_from_sexp’
||     |  523 |       keyparms, &public_key, &public_key_size_bytes, NULL);
||     |      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
||     |  524 |   if (ec)
||     |      |      ~  
||     |      |      |
||     |      |      (22) following ‘false’ branch (when ‘ec == 0’)...
||     |......
||     |  528 |   bit_strength = bitstrength_from_public_size_bytes (public_key_size_bytes);
||     |      |                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
||     |      |                  |
||     |      |                  (23) ...to here
||     |      |                  (24) use of uninitialized value ‘public_key_size_bytes’ here
||     |
|| mlkem.c: In function ‘mlkem_decrypt’:

[falko-strenzke]

This is false positive: the analyzer doesn't know that the line

      ec = gpg_err_code_from_syserror ();

will assign a non-zero value to ec in case of memory exhaustion. In its further anaylsis, it thus thinks that the failing memory allocation will lead to null pointer deref since it misses that the leave is being jumped to.