Restore the previous behaviour with maximum verification depth.

The maximum depth is not expected to include the leaf certificate - restore
the decrement prior to checking, which means the previous behaviour is
retained for the callback depth and the maximum depth. Reduce the maximum
depth by one in order to avoid the overwrite that could previously occur.

Thanks to anton@ for flagging the rust-openssl failure in regress.

ok tb@

1 files changed, 7 insertions, 7 deletions

diff --git a/src/lib/libcrypto/x509/x509_verify.c b/src/lib/libcrypto/x509/x509_verify.c
index a530275ac9..fc3fbc14da 100644
--- a/src/lib/libcrypto/x509/x509_verify.c
+++ b/src/lib/libcrypto/x509/x509_verify.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: x509_verify.c,v 1.74 2026/03/31 13:58:05 jsing Exp $ */
+/* $OpenBSD: x509_verify.c,v 1.75 2026/04/01 14:38:26 jsing Exp $ */
 /*
  * Copyright (c) 2020-2021 Bob Beck <beck@openbsd.org>
  *
@@ -666,13 +666,13 @@ x509_verify_build_chains(struct x509_verify_ctx *ctx, X509 *cert,
                 return;
 
         depth = sk_X509_num(current_chain->certs);
+        if (depth > 0)
+                depth--;
         if (depth >= ctx->max_depth) {
                 (void)x509_verify_cert_error(ctx, cert, depth,
                     X509_V_ERR_CERT_CHAIN_TOO_LONG, 0);
                 return;
         }
-        if (depth > 0)
-                depth--;
 
         count = ctx->chains_count;
 
@@ -978,8 +978,8 @@ x509_verify_ctx_new_from_xsc(X509_STORE_CTX *xsc)
             (ctx->intermediates = X509_chain_up_ref(xsc->untrusted)) == NULL)
                 goto err;
 
-        max_depth = X509_VERIFY_MAX_CHAIN_CERTS;
-        if (xsc->param->depth > 0 && xsc->param->depth < X509_VERIFY_MAX_CHAIN_CERTS)
+        max_depth = X509_VERIFY_MAX_CHAIN_CERTS - 1;
+        if (xsc->param->depth > 0 && xsc->param->depth < max_depth)
                 max_depth = xsc->param->depth;
         if (!x509_verify_ctx_set_max_depth(ctx, max_depth))
                 goto err;
@@ -1008,7 +1008,7 @@ x509_verify_ctx_new(STACK_OF(X509) *roots)
                         goto err;
         }
 
-        ctx->max_depth = X509_VERIFY_MAX_CHAIN_CERTS;
+        ctx->max_depth = X509_VERIFY_MAX_CHAIN_CERTS - 1;
         ctx->max_chains = X509_VERIFY_MAX_CHAINS;
         ctx->max_sigs = X509_VERIFY_MAX_SIGCHECKS;
 
@@ -1035,7 +1035,7 @@ x509_verify_ctx_free(struct x509_verify_ctx *ctx)
 int
 x509_verify_ctx_set_max_depth(struct x509_verify_ctx *ctx, size_t max)
 {
-        if (max < 1 || max > X509_VERIFY_MAX_CHAIN_CERTS)
+        if (max < 1 || max >= X509_VERIFY_MAX_CHAIN_CERTS)
                 return 0;
         ctx->max_depth = max;
         return 1;