When setting the secret with the software Diffie-Hellman implementation,
if allocating 'g' failed (e.g. if it was longer than
MAX_EXTERN_MPI_BITS), then 'p' was freed twice: once immediately, and
once later when the crypto_kpp tfm was destroyed.

Fix it by using dh_free_ctx() (renamed to dh_clear_ctx()) in the error
paths, as that correctly sets the pointers to NULL.

KASAN report:

    MPI: mpi too large (32760 bits)
    ==================================================================
    BUG: KASAN: use-after-free in mpi_free+0x131/0x170
    Read of size 4 at addr ffff88006c7cdf90 by task reproduce_doubl/367

    CPU: 1 PID: 367 Comm: reproduce_doubl Not tainted 4.14.0-rc7-00040-g05298abde6fe #7
    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
    Call Trace:
     dump_stack+0xb3/0x10b
     ? mpi_free+0x131/0x170
     print_address_description+0x79/0x2a0
     ? mpi_free+0x131/0x170
     kasan_report+0x236/0x340
     ? akcipher_register_instance+0x90/0x90
     __asan_report_load4_noabort+0x14/0x20
     mpi_free+0x131/0x170
     ? akcipher_register_instance+0x90/0x90
     dh_exit_tfm+0x3d/0x140
     crypto_kpp_exit_tfm+0x52/0x70
     crypto_destroy_tfm+0xb3/0x250
     __keyctl_dh_compute+0x640/0xe90
     ? kasan_slab_free+0x12f/0x180
     ? dh_data_from_key+0x240/0x240
     ? key_create_or_update+0x1ee/0xb20
     ? key_instantiate_and_link+0x440/0x440
     ? lock_contended+0xee0/0xee0
     ? kfree+0xcf/0x210
     ? SyS_add_key+0x268/0x340
     keyctl_dh_compute+0xb3/0xf1
     ? __keyctl_dh_compute+0xe90/0xe90
     ? SyS_add_key+0x26d/0x340
     ? entry_SYSCALL_64_fastpath+0x5/0xbe
     ? trace_hardirqs_on_caller+0x3f4/0x560
     SyS_keyctl+0x72/0x2c0
     entry_SYSCALL_64_fastpath+0x1f/0xbe
    RIP: 0033:0x43ccf9
    RSP: 002b:00007ffeeec96158 EFLAGS: 00000246 ORIG_RAX: 00000000000000fa
    RAX: ffffffffffffffda RBX: 000000000248b9b9 RCX: 000000000043ccf9
    RDX: 00007ffeeec96170 RSI: 00007ffeeec96160 RDI: 0000000000000017
    RBP: 0000000000000046 R08: 0000000000000000 R09: 0248b9b9143dc936
    R10: 0000000000001000 R11: 0000000000000246 R12: 0000000000000000
    R13: 0000000000409670 R14: 0000000000409700 R15: 0000000000000000

    Allocated by task 367:
     save_stack_trace+0x16/0x20
     kasan_kmalloc+0xeb/0x180
     kmem_cache_alloc_trace+0x114/0x300
     mpi_alloc+0x4b/0x230
     mpi_read_raw_data+0xbe/0x360
     dh_set_secret+0x1dc/0x460
     __keyctl_dh_compute+0x623/0xe90
     keyctl_dh_compute+0xb3/0xf1
     SyS_keyctl+0x72/0x2c0
     entry_SYSCALL_64_fastpath+0x1f/0xbe

    Freed by task 367:
     save_stack_trace+0x16/0x20
     kasan_slab_free+0xab/0x180
     kfree+0xb5/0x210
     mpi_free+0xcb/0x170
     dh_set_secret+0x2d7/0x460
     __keyctl_dh_compute+0x623/0xe90
     keyctl_dh_compute+0xb3/0xf1
     SyS_keyctl+0x72/0x2c0
     entry_SYSCALL_64_fastpath+0x1f/0xbe

Fixes: 802c7f1c ("crypto: dh - Add DH software implementation")
Cc: <stable@vger.kernel.org> # v4.8+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>