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psa_raw_key_agreement: return BUFFER_TOO_SMALL when warranted

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psa_raw_key_agreement() returned PSA_ERROR_INVALID_ARGUMENT instead of
PSA_ERROR_BUFFER_TOO_SMALL when the output buffer was too small for ECDH,
the only algorithm that is currently implemented. Make it return the correct
error code.

The reason for the wrong error code is that ecdh.c returns
MBEDTLS_ERR_ECP_BAD_INPUT_DATA, presumably for similarith with dhm.c. It
might make sense to change ecdh.c to use MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL,
but dhm.c doesn't have an existing BUFFER_TOO_SMALL error. To minimize the
impact of the fix, handle this in the PSA layer.

Fixes #5735.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2022-04-14 00:17:15 +02:00
parent 7be11a790d
commit 3e56130fb9
2 changed files with 19 additions and 0 deletions
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3
ChangeLog.d/psa_raw_key_agreement-buffer_too_small.txt Normal file
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@ -0,0 +1,3 @@
Bugfix
* psa_raw_key_agreement() now returns PSA_ERROR_BUFFER_TOO_SMALL when
applicable. Fixes #5735.

16
library/psa_crypto.c
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@ -5766,6 +5766,22 @@ psa_status_t psa_raw_key_agreement( psa_algorithm_t alg,
if( status != PSA_SUCCESS )
goto exit;
/* PSA_RAW_KEY_AGREEMENT_OUTPUT_SIZE() is in general an upper bound
* for the output size. The PSA specification only guarantees that this
* function works if output_size >= PSA_RAW_KEY_AGREEMENT_OUTPUT_SIZE(...),
* but it might be nice to allow smaller buffers if the output fits.
* At the time of writing this comment, with only ECDH implemented,
* PSA_RAW_KEY_AGREEMENT_OUTPUT_SIZE() is exact so the point is moot.
* If FFDH is implemented, PSA_RAW_KEY_AGREEMENT_OUTPUT_SIZE() can easily
* be exact for it as well. */
size_t expected_length =
PSA_RAW_KEY_AGREEMENT_OUTPUT_SIZE( slot->attr.type, slot->attr.bits );
if( output_size < expected_length )
{
status = PSA_ERROR_BUFFER_TOO_SMALL;
goto exit;
}
status = psa_key_agreement_raw_internal( alg, slot,
peer_key, peer_key_length,
output, output_size,