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Merge pull request #705 from mpg/l13-hw-starts-finish-2.7-restricted

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[backport 2.7] Use starts/finish around Lucky 13 dummy compressions
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Manuel Pégourié-Gonnard 2020-06-23 10:43:22 +02:00 committed by GitHub
commit f2027b5c46
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2 changed files with 19 additions and 2 deletions
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7
ChangeLog.d/l13-hw-accel.txt Normal file
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@ -0,0 +1,7 @@
Security
* Fix issue in Lucky 13 counter-measure that could make it ineffective when
hardware accelerators were used (using one of the MBEDTLS_SHAxxx_ALT
macros). This would cause the original Lucky 13 attack to be possible in
those configurations, allowing an active network attacker to recover
plaintext after repeated timing measurements under some conditions.
Reported and fix suggested by Luc Perneel in #3246.

14
library/ssl_tls.c
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@ -2092,10 +2092,20 @@ static int ssl_decrypt_buf( mbedtls_ssl_context *ssl )
ssl_read_memory( ssl->in_msg + ssl->in_msglen, padlen );
mbedtls_md_hmac_finish( &ssl->transform_in->md_ctx_dec, mac_expect );
/* Call mbedtls_md_process at least once due to cache attacks
* that observe whether md_process() was called of not */
/* Dummy calls to compression function.
* Call mbedtls_md_process at least once due to cache attacks
* that observe whether md_process() was called of not.
* Respect the usual start-(process|update)-finish sequence for
* the sake of hardware accelerators that might require it. */
mbedtls_md_starts( &ssl->transform_in->md_ctx_dec );
for( j = 0; j < extra_run + 1; j++ )
mbedtls_md_process( &ssl->transform_in->md_ctx_dec, ssl->in_msg );
{
/* The switch statement above already checks that we're using
* one of MD-5, SHA-1, SHA-256 or SHA-384. */
unsigned char tmp[384 / 8];
mbedtls_md_finish( &ssl->transform_in->md_ctx_dec, tmp );
}
mbedtls_md_hmac_reset( &ssl->transform_in->md_ctx_dec );