From d30ef639647ad263d09740c931a5bfb5a8b6a5f6 Mon Sep 17 00:00:00 2001 From: Matt Caswell Date: Fri, 8 May 2020 11:12:10 +0100 Subject: [PATCH] Correct alignment calculation in ssl3_setup_write The alignment calculation in ssl3_setup_write incorrectly results in an alignment allowance of (-SSL3_RT_HEADER_LENGTH) & (SSL3_ALIGN_PAYLOAD - 1) bytes. This equals 3 in almost all cases. The maximum alignment actually used in do_ssl3_write is (SSL3_ALIGN_PAYLOAD - 1). This equals 7 bytes in almost all cases. So there is a potential to overrun the buffer by up to 4 bytes. Fortunately, the encryption overhead allowed for is 80 bytes which consists of 16 bytes for the cipher block size and 64 bytes for the MAC output. However the biggest MAC that we ever produce is HMAC-384 which is 48 bytes - so we have a headroom of 16 bytes (i.e. more than the 4 bytes of potential overrun). Thanks to Nagesh Hegde for reporting this. Fixes #11766 Reviewed-by: Ben Kaduk (Merged from https://github.com/openssl/openssl/pull/11768) --- ssl/record/ssl3_buffer.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/ssl/record/ssl3_buffer.c b/ssl/record/ssl3_buffer.c index 5e2cceee27..2c25099e10 100644 --- a/ssl/record/ssl3_buffer.c +++ b/ssl/record/ssl3_buffer.c @@ -94,7 +94,7 @@ int ssl3_setup_write_buffer(SSL *s, size_t numwpipes, size_t len) headerlen = SSL3_RT_HEADER_LENGTH; #if defined(SSL3_ALIGN_PAYLOAD) && SSL3_ALIGN_PAYLOAD!=0 - align = (-SSL3_RT_HEADER_LENGTH) & (SSL3_ALIGN_PAYLOAD - 1); + align = SSL3_ALIGN_PAYLOAD - 1; #endif len = ssl_get_max_send_fragment(s) -- 2.25.1