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Date:   Mon, 4 Nov 2019 19:33:57 +0100 (CET)
From:   Nicolas Pitre <nico@fluxnic.net>
To:     Or Cohen <orcohen@paloaltonetworks.com>
cc:     Greg KH <gregkh@linuxfoundation.org>, jslaby@suse.com,
        textshell@uchuujin.de, Daniel Vetter <daniel.vetter@ffwll.ch>,
        sam@ravnborg.org, mpatocka@redhat.com, ghalat@redhat.com,
        linux-kernel@vger.kernel.org, jwilk@jwilk.net,
        Nadav Markus <nmarkus@paloaltonetworks.com>,
        syzkaller@googlegroups.com
Subject: Re: Bug report - slab-out-of-bounds in vcs_scr_readw
In-Reply-To: <CAM6JnLdrzCPOYyfTdmriFo7cRaGM4p2OEPd_0MHa3_WemamffA@mail.gmail.com>
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References: <CAM6JnLeEnvjjQPyLeh+8dt5wGNud_vks5k_eXJZy2T1H7ao=hQ@mail.gmail.com> <20191104152428.GA2252441@kroah.com> <nycvar.YSQ.7.76.1911041648280.30289@knanqh.ubzr> <CAM6JnLdrzCPOYyfTdmriFo7cRaGM4p2OEPd_0MHa3_WemamffA@mail.gmail.com>
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On Mon, 4 Nov 2019, Or Cohen wrote:

> @gregkh@linuxfoundation.org @nico@fluxnic.net  - Thanks for the quick response.
> @gregkh@linuxfoundation.org  - Regarding your question, I don't think
> the 1 byte buffer is related to the problem. (  it's just was there in
> the initial reproducer the fuzzer created, and I forgot to remove it
> while reducing code from the reproducer ).

I think I know what the problem is. I have no time to test it though.

Please try this (untested) patch. Also please try running the same test 
code but with vcsa6 in addition to vcsu6 to be sure.

---------- >8
Subject: [PATCH] vcs: add missing validation on vcs_size() returned value

One usage instance didn't account for the fact that vcs_size() may
return a negative error code.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>

diff --git a/drivers/tty/vt/vc_screen.c b/drivers/tty/vt/vc_screen.c
index 1f042346e7..fa07d79027 100644
--- a/drivers/tty/vt/vc_screen.c
+++ b/drivers/tty/vt/vc_screen.c
@@ -474,6 +474,10 @@ vcs_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos)
 		goto unlock_out;
 
 	size = vcs_size(inode);
+	if (size < 0) {
+		ret = size;
+		goto unlock_out;
+	}
 	ret = -EINVAL;
 	if (pos < 0 || pos > size)
 		goto unlock_out;