From 23a8e1b78063c0276d987e835434700267b9c8f8 Mon Sep 17 00:00:00 2001
From: Ian Moffett <ian@osmora.org>
Date: Thu, 3 Jul 2025 19:59:45 -0400
Subject: kernel: Ensure mutually exclusive console access

If multiple processes try to write to the console, a race condition
of sorts may occur. Similarly, if multiple processes try to read from the
console and contend with the console input buffer. One process may steal
keys from the other. Prevent this by implementing a mutex within the
console descriptor. Each time a process reads or writes the console, it
attaches itself. Any other processes attempting to read or write the
console while another is attached will be yielded to the scheduler
until the resource is free.

Signed-off-by: Ian Moffett <ian@osmora.org>
---
 sys/dev/cons/cons.c | 70 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)

(limited to 'sys/dev/cons/cons.c')

diff --git a/sys/dev/cons/cons.c b/sys/dev/cons/cons.c
index 2cef071..9921ff8 100644
--- a/sys/dev/cons/cons.c
+++ b/sys/dev/cons/cons.c
@@ -343,6 +343,7 @@ cons_fast_putch(struct cons_screen *scr, char c)
 static int
 dev_write(dev_t dev, struct sio_txn *sio, int flags)
 {
+    cons_attach();
     cons_putstr(&g_root_scr, sio->buf, sio->len);
     cons_flush(&g_root_scr);
     return sio->len;
@@ -372,6 +373,7 @@ dev_read(dev_t dev, struct sio_txn *sio, int flags)
         return -EAGAIN;
     }
 
+    cons_attach();
     spinlock_acquire(&g_root_scr.lock);
     for (;;) {
         /* Buffer too small */
@@ -467,6 +469,69 @@ ctl_feat_write(struct ctlfs_dev *cdp, struct sio_txn *sio)
     return sio->len;
 }
 
+/*
+ * Detach the currently running process from the
+ * console.
+ */
+int
+cons_detach(void)
+{
+    struct cons_screen *scr;
+
+    scr = &g_root_scr;
+    if (scr->atproc == NULL) {
+        return 0;
+    }
+    if (scr->atproc_lock == NULL) {
+        return 0;
+    }
+
+    scr = &g_root_scr;
+    scr->atproc = NULL;
+    mutex_release(scr->atproc_lock);
+    return 0;
+}
+
+/*
+ * Attach the current process to the
+ * console.
+ */
+int
+cons_attach(void)
+{
+    struct cons_screen *scr;
+    struct proc *td, *atproc;
+
+    td = this_td();
+    if (td == NULL) {
+        return -1;
+    }
+
+    scr = &g_root_scr;
+    if (scr->atproc_lock == NULL) {
+        return 0;
+    }
+
+    scr = &g_root_scr;
+    atproc = scr->atproc;
+
+    if (atproc != NULL) {
+        if (atproc->pid == td->pid) {
+            return 0;
+        }
+
+        /*
+         * Do not release this here as we want
+         * any other process that tries to attach
+         * to wait.
+         */
+        mutex_acquire(scr->atproc_lock, 0);
+    }
+
+    scr->atproc = td;
+    return 0;
+}
+
 /*
  * Reset console color.
  */
@@ -556,6 +621,8 @@ cons_init(void)
     g_root_scr.nrows = fbdev.height / FONT_HEIGHT;
     g_root_scr.ncols = fbdev.width / FONT_WIDTH;
     g_root_scr.fbdev = fbdev;
+    g_root_scr.atproc = NULL;
+    g_root_scr.atproc_lock = NULL;
     memset(&g_root_scr.lock, 0, sizeof(g_root_scr.lock));
     cons_init_bufs(&g_root_scr);
     SHOW_CURSOR(&g_root_scr);
@@ -578,6 +645,9 @@ cons_expose(void)
         return;
     }
 
+    /* Init the attached proc mutex lock */
+    g_root_scr.atproc_lock = mutex_new("console0");
+
     /* Register the device here */
     major = dev_alloc_major();
     dev = dev_alloc(major);
-- 
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