util/readline: Fix out-of-bounds access in readline_insert_char().

Currently, the readline_insert_char() function is guarded by the cursor
position (cmd_buf_index) rather than the actual buffer fill level(cmd_buf_size).
The current check is:
	if (rs->cmd_buf_index < READLINE_CMD_BUF_SIZE)

This logic is flawed because if the command buffer is full and a user moves the
cursor backward (e.g. by sending left arrow key), cmd_buf_index can be
decreased without descreasing of buffer size.
This allow subsequent insertions to increase cmd_buf_size past its maximum
limit of rs->cmd_buf.

Because in the ReadLineState struct, cmd_buf[READLINE_CMD_BUF_SIZE + 1] is
immediately followed by the cmd_buf_index integer, once the buffer size is
sufficiently inflated, the memmove() operation inside readline_insert_char()
can write past the end of cmd_buf[] and overwrites cmd_buf_index itself.

The subsequent line:
	rs->cmd_buf[rs->cmd_buf_index] = ch;

then writes the input character to an address determined by the now-corrupted
index.

By providing a specifically crafted input sequence via HMP, this flaw can be
used to redirect the write operation to overwrite any field within the
ReadLineState structure, which can lead to unpredictable behavior or
application crashes.

Fix this by adding the guard to check for buffer fullness.

Cc: qemu-stable@nongnu.org
Signed-off-by: Nguyen Dinh Phi <phind.uet@gmail.com>
Message-id: 20260406050454.284873-2-phind.uet@gmail.com
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

diff --git a/util/readline.c b/util/readline.c
index 0f19674f52631e49f2c2adde38cd5bdaab09eb5d..e2664e48ca1033e6e2692832a95c4e436ac7b039 100644 (file)
--- a/util/readline.c
+++ b/util/readline.c
@@ -84,7 +84,9 @@ static void readline_update(ReadLineState *rs)
 
 static void readline_insert_char(ReadLineState *rs, int ch)
 {
-    if (rs->cmd_buf_index < READLINE_CMD_BUF_SIZE) {
+    assert(rs->cmd_buf_index <= rs->cmd_buf_size);
+
+    if (rs->cmd_buf_size < READLINE_CMD_BUF_SIZE) {
         memmove(rs->cmd_buf + rs->cmd_buf_index + 1,
                 rs->cmd_buf + rs->cmd_buf_index,
                 rs->cmd_buf_size - rs->cmd_buf_index);