linux-kernel - Re: [PATCH v3] vt: Fix a missing-check bug in drivers/tty/vt/vt.c

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Message-ID: <20190521032118.GC5263@zhanggen-UX430UQ>
Date:   Tue, 21 May 2019 11:21:18 +0800
From:   Gen Zhang <blackgod016574@...il.com>
To:     Nicolas Pitre <nico@...xnic.net>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] vt: Fix a missing-check bug in drivers/tty/vt/vt.c

On Mon, May 20, 2019 at 10:55:40PM -0400, Nicolas Pitre wrote:
> As soon as you release the lock, another thread could come along and 
> start using the memory pointed by vc_cons[currcons].d you're about to 
> free here. This is unlikely for an initcall, but still.
> 
> You should consider this ordering instead:
> 
> err_vc_screenbuf:
> 	kfree(vc);
> 	vc_cons[currcons].d = NULL;
> err_vc:
> 	console_unlock();
> 	return -ENOMEM;
In function con_init(), the pointer variable vc_cons[currcons].d, vc and
vc->vc_screenbuf is allocated a memory space via kzalloc(). And they are
used in the following codes.
However, when there is a memory allocation error, kzalloc() can fail.
Thus null pointer (vc_cons[currcons].d, vc and vc->vc_screenbuf)
dereference may happen. And it will cause the kernel to crash. Therefore,
we should check return value and handle the error.
Further,the loop condition MIN_NR_CONSOLES is defined as 1 in
include/uapi/linux/vt.h and it is not changed. So there is no need to
unwind the loop.

Signed-off-by: Gen Zhang <blackgod016574@...il.com>

---
diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index fdd12f8..ea47eb3 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -3350,10 +3350,14 @@ static int __init con_init(void)
 
 	for (currcons = 0; currcons < MIN_NR_CONSOLES; currcons++) {
 		vc_cons[currcons].d = vc = kzalloc(sizeof(struct vc_data), GFP_NOWAIT);
+		if (!vc)
+			goto err_vc;
 		INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK);
 		tty_port_init(&vc->port);
 		visual_init(vc, currcons, 1);
 		vc->vc_screenbuf = kzalloc(vc->vc_screenbuf_size, GFP_NOWAIT);
+		if (!vc->vc_screenbuf)
+			goto err_vc_screenbuf;
 		vc_init(vc, vc->vc_rows, vc->vc_cols,
 			currcons || !vc->vc_sw->con_save_screen);
 	}
@@ -3375,6 +3379,13 @@ static int __init con_init(void)
 	register_console(&vt_console_driver);
 #endif
 	return 0;
+err_vc_screenbuf:
+	kfree(vc);
+	vc_cons[currcons].d = NULL;
+err_vc:
+	console_unlock();
+	return -ENOMEM;
+
 }
 console_initcall(con_init);
---