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From a4e1b7755ebbdee2689d40683ba69f09e38a8d7f Mon Sep 17 00:00:00 2001
From: Qualys Security Advisory <qsa@qualys.com>
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 <lineending+8>
(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
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