SECURITY: Avoid memory corruption in dkim handling

Credits: Qualys

    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 ea850e27714ccda2090d781ebe89b410bc38c2c6)

1 files changed, 8 insertions, 6 deletions

diff --git a/src/src/pdkim/pdkim.c b/src/src/pdkim/pdkim.c
index 01cf5dd28..da2ea7e49 100644
--- a/src/src/pdkim/pdkim.c
+++ b/src/src/pdkim/pdkim.c
@@ -722,7 +722,9 @@ 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)
 {
-blob * canon_data = orig_data;
+const blob const * canon_data = orig_data;
+size_t len;
+
 /* Defaults to simple canon (no further treatment necessary) */
 
 if (b->canon_method == PDKIM_CANON_RELAXED)
@@ -770,13 +772,13 @@ if (b->canon_method == PDKIM_CANON_RELAXED)
 if (  b->bodylength >= 0
    && b->signed_body_bytes + (unsigned long)canon_data->len > b->bodylength
    )
-  canon_data->len = b->bodylength - b->signed_body_bytes;
+  len = b->bodylength - b->signed_body_bytes;
 
-if (canon_data->len > 0)
+if (len > 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, len);
+  b->signed_body_bytes += len;
+  DEBUG(D_acl) pdkim_quoteprint(canon_data->data, len);
   }
 
 return relaxed_data;