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}

(cherry picked from commit 92359a62a0e31734ad8069c66f64b37f9eaaccbe)

1 files changed, 5 insertions, 5 deletions

diff --git a/src/src/pdkim/pdkim.c b/src/src/pdkim/pdkim.c
index 4320ecd49..36e666fd9 100644
--- a/src/src/pdkim/pdkim.c
+++ b/src/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 *
@@ -720,9 +720,9 @@ 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, const blob const * orig_data, blob * relaxed_data)
+pdkim_update_ctx_bodyhash(pdkim_bodyhash * b, const blob * orig_data, blob * relaxed_data)
 {
-const blob const * canon_data = orig_data;
+const blob * canon_data = orig_data;
 size_t left;
 
 /* Defaults to simple canon (no further treatment necessary) */
@@ -771,9 +771,9 @@ 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 + left > b->bodylength
+   && left > (unsigned long)b->bodylength - b->signed_body_bytes
    )
-  left = b->bodylength - b->signed_body_bytes;
+  left = (unsigned long)b->bodylength - b->signed_body_bytes;
 
 if (left > 0)
   {