From a4e1b7755ebbdee2689d40683ba69f09e38a8d7f Mon Sep 17 00:00:00 2001 From: Qualys Security Advisory Date: Sun, 21 Feb 2021 22:30:03 -0800 Subject: [PATCH 21/29] Security: Avoid modification of constant data in dkim handling Based on Heiko Schlittermann's commits f880c7f3 and c118c7f4. This fixes: 6/ In src/pdkim/pdkim.c, pdkim_update_ctx_bodyhash() is sometimes called with a global orig_data and hence canon_data, and the following line can therefore modify data that should be constant: 773 canon_data->len = b->bodylength - b->signed_body_bytes; For example, the following proof of concept sets lineending.len to 0 (this should not be possible): (sleep 10; echo 'EHLO test'; sleep 3; echo 'MAIL FROM:<>'; sleep 3; echo 'RCPT TO:postmaster'; sleep 3; echo 'DATA'; date >&2; sleep 30; printf 'DKIM-Signature:a=rsa-sha1;c=simple/simple;l=0\r\n\r\n\r\nXXX\r\n.\r\n'; sleep 30) | nc -n -v 192.168.56.102 25 (gdb) print lineending $1 = {data = 0x55e18035b2ad "\r\n", len = 2} (gdb) print &lineending.len $3 = (size_t *) 0x55e180385948 (gdb) watch *(size_t *) 0x55e180385948 Hardware watchpoint 1: *(size_t *) 0x55e180385948 Old value = 2 New value = 0 (gdb) print lineending $5 = {data = 0x55e18035b2ad "\r\n", len = 0} --- src/pdkim/pdkim.c | 21 ++++++++++++--------- 1 file changed, 12 insertions(+), 9 deletions(-) diff --git a/src/pdkim/pdkim.c b/src/pdkim/pdkim.c index e3233e9f0..512a3e352 100644 --- a/src/pdkim/pdkim.c +++ b/src/pdkim/pdkim.c @@ -107,7 +107,7 @@ pdkim_combined_canon_entry pdkim_combined_canons[] = { }; -static blob lineending = {.data = US"\r\n", .len = 2}; +static const blob lineending = {.data = US"\r\n", .len = 2}; /* -------------------------------------------------------------------------- */ uschar * @@ -719,9 +719,11 @@ return NULL; If we have to relax the data for this sig, return our copy of it. */ static blob * -pdkim_update_ctx_bodyhash(pdkim_bodyhash * b, blob * orig_data, blob * relaxed_data) +pdkim_update_ctx_bodyhash(pdkim_bodyhash * b, const blob * orig_data, blob * relaxed_data) { -blob * canon_data = orig_data; +const blob * canon_data = orig_data; +size_t left; + /* Defaults to simple canon (no further treatment necessary) */ if (b->canon_method == PDKIM_CANON_RELAXED) @@ -767,16 +769,17 @@ if (b->canon_method == PDKIM_CANON_RELAXED) } /* Make sure we don't exceed the to-be-signed body length */ +left = canon_data->len; if ( b->bodylength >= 0 - && b->signed_body_bytes + (unsigned long)canon_data->len > b->bodylength + && left > (unsigned long)b->bodylength - b->signed_body_bytes ) - canon_data->len = b->bodylength - b->signed_body_bytes; + left = (unsigned long)b->bodylength - b->signed_body_bytes; -if (canon_data->len > 0) +if (left > 0) { - exim_sha_update(&b->body_hash_ctx, CUS canon_data->data, canon_data->len); - b->signed_body_bytes += canon_data->len; - DEBUG(D_acl) pdkim_quoteprint(canon_data->data, canon_data->len); + exim_sha_update(&b->body_hash_ctx, CUS canon_data->data, left); + b->signed_body_bytes += left; + DEBUG(D_acl) pdkim_quoteprint(canon_data->data, left); } return relaxed_data; -- 2.30.2