From 701aa270db8ad424cece68702b9bb2e05290af9b Mon Sep 17 00:00:00 2001
From: Neil Horman <nhorman@openssl.org>
Date: Wed, 7 Jan 2026 11:52:09 -0500
Subject: [PATCH] Fix heap buffer overflow in BIO_f_linebuffer

When a FIO_f_linebuffer is part of a bio chain, and the next BIO
preforms short writes, the remainder of the unwritten buffer is copied
unconditionally to the internal buffer ctx->obuf, which may not be
sufficiently sized to handle the remaining data, resulting in a buffer
overflow.

Fix it by only copying data when ctx->obuf has space, flushing to the
next BIO to increase available storage if needed.

Fixes CVE-2025-68160
---
 crypto/bio/bf_lbuf.c | 32 ++++++++++++++++++++++++++------
 1 file changed, 26 insertions(+), 6 deletions(-)

diff --git a/crypto/bio/bf_lbuf.c b/crypto/bio/bf_lbuf.c
index 1dfcac8f2ea..e4af2a8c4ff 100644
--- a/crypto/bio/bf_lbuf.c
+++ b/crypto/bio/bf_lbuf.c
@@ -187,14 +187,34 @@ static int linebuffer_write(BIO *b, const char *in, int inl)
     while (foundnl && inl > 0);
     /*
      * We've written as much as we can.  The rest of the input buffer, if
-     * any, is text that doesn't and with a NL and therefore needs to be
-     * saved for the next trip.
+     * any, is text that doesn't end with a NL and therefore we need to try
+     * free up some space in our obuf so we can make forward progress.
      */
-    if (inl > 0) {
-        memcpy(&(ctx->obuf[ctx->obuf_len]), in, inl);
-        ctx->obuf_len += inl;
-        num += inl;
+    while (inl > 0) {
+        size_t avail = (size_t)ctx->obuf_size - (size_t)ctx->obuf_len;
+        size_t to_copy;
+
+        if (avail == 0) {
+            /* Flush buffered data to make room */
+            i = BIO_write(b->next_bio, ctx->obuf, ctx->obuf_len);
+            if (i <= 0) {
+                BIO_copy_next_retry(b);
+                return num > 0 ? num : i;
+            }
+            if (i < ctx->obuf_len)
+                memmove(ctx->obuf, ctx->obuf + i, ctx->obuf_len - i);
+            ctx->obuf_len -= i;
+            continue;
+        }
+
+        to_copy = inl > (int)avail ? avail : (size_t)inl;
+        memcpy(&(ctx->obuf[ctx->obuf_len]), in, to_copy);
+        ctx->obuf_len += (int)to_copy;
+        in += to_copy;
+        inl -= (int)to_copy;
+        num += (int)to_copy;
     }
+
     return num;
 }
 
