[openssl-dev] [openssl.org #4611] PKCS12_create() not thread-safe for ECDSA

[Matt Slot via RT]

OpenSSL 1.0.2h

The function eckey_priv_encode() may crash if the same pkey is serialized from multiple threads. Here is a sample backtrace:

#0 0x00007fff8f321f92 in _platform_memmove$VARIANT$Haswell ()
#1 0x0000000100196132 in i2c_ASN1_BIT_STRING
#2 0x00000001001a1fb2 in asn1_ex_i2c
#3 0x00000001001a1e68 in asn1_i2d_ex_primitive
#4 0x00000001001a19fd in ASN1_item_ex_i2d
#5 0x00000001001a1d8b in asn1_template_ex_i2d
#6 0x00000001001a1a8b in ASN1_item_ex_i2d
#7 0x00000001001a16d0 in asn1_item_flags_i2d
#8 0x0000000100149d3e in i2d_ECPrivateKey
#9 0x000000010014e576 in eckey_priv_encode
#10 0x000000010018e9f9 in EVP_PKEY2PKCS8_broken
#11 0x00000001001d38d9 in PKCS12_add_key
#12 0x00000001001d35a3 in PKCS12_create

EC_KEY_set_enc_flags() is called to modify/restore the encoding flags within the EC_KEY. Two calls are made to i2d_ECPrivateKey(). The first calculates the necessary buffer length, the second serializes the data into an allocated buffer. If the flags change during this period, the second call overwrites the buffer.

These APIs are documented as thread safe, and should not change the internal flags of the pkey without proper locking.

Matt Slot
Principal Software Engineer

Barracuda Networks, Inc.
317 Maynard St. Ann Arbor, MI 48104
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