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Message-ID: <ef549144-6992-d1fe-8f07-47757cd4ad85@samsung.com>
Date: Mon, 22 Jul 2019 16:05:37 +0200
From: Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
To: Daniel Vetter <daniel@...ll.ch>, Linus Torvalds
<torvalds@...ux-foundation.org>
Cc: Tavis Ormandy <taviso@...il.com>, Daniel Vetter
<daniel.vetter@...ll.ch>, oss-security@...ts.openwall.com
Subject: Re: stack buffer overflow in fbdev
On 7/21/19 10:09 PM, Daniel Vetter wrote:
> On Sun, Jul 21, 2019 at 11:03:01AM -0700, Linus Torvalds wrote:
>> Completely untested patch attached. There are probably better ways to do this.
>>
>> Adding the proper people to the cc, and quoting Tavis' email in its entirety.
>>
>> Daniel - you got added despite not being explicitly listed as
>> maintainer because you've touched fbdev/core/ more than most lately,
>> plus you know edid anyway. As such: "tag, you're it, sucker".
>
> Yeah I also realized with regrets that get_maintainers thinks I'm
> responsible for fbdev core :-/
Well, I've been thinking lately about officially adding you as
a co-maintainer to MAINTAINERS file. 8)
The patch documenting (already agreed) moving of fbdev handling to
drm-misc tree seems like a perfect occasion..
Ack?
Best regards,
--
Bartlomiej Zolnierkiewicz
Samsung R&D Institute Poland
Samsung Electronics
> Wrt the bug: I had a multi-paragraph explanation here about how fbmon.c
> edid parser is only used by old crap drivers, and not when you have a
> drm-kms driver providing the fbdev emulation (like pretty much every
> modern system). Also that the version in fbmon.c seriously lacks compared
> to the drm_edid.c one.
>
> And then I ran grep and noticed it's dead code. The last user disappeared
> in 34280340b1dc ("fbdev: Remove unused SH-Mobile HDMI driver") from 2015.
> I'll type a patch for 5.4 to remove this outright.
>
> Cheers, Daniel
>
> PS: git log -G disappoints by not using all the cores I have here ..
>
>>
>> Linus
>>
>> On Sat, Jul 20, 2019 at 5:35 PM Tavis Ormandy <taviso@...il.com> wrote:
>>>
>>> Hello, during a conversation on twitter we noticed a stack buffer
>>> overflow in fbdev with malicious edid data:
>>>
>>> https://github.com/torvalds/linux/blob/22051d9c4a57d3b4a8b5a7407efc80c71c7bfb16/drivers/video/fbdev/core/fbmon.c#L1033
>>>
>>> There is enough space to have 52 1-byte length values, which makes svd_n
>>> 52, then make the final value length 0x1f (the maximum), which makes
>>> svd_n 83 and overflows the 64 byte stack buffer svd[] with controlled
>>> data.
>>>
>>> This requires a malicious monitor / projector / etc, so pretty low impact.
>>>
>>> I pulled out the code to make a demo (I removed the checksum, but it
>>> doesnt prevent the bug):
>>>
>>> https://gist.github.com/taviso/923776e633cb8fb1ab847cce761a0f10
>>>
>>> This was discovered by Nico Waisman of Semmle.
>>>
>>> Tavis.
>>>
>>> --
>>> -------------------------------------
>>> taviso@....lonestar.org | finger me for my pgp key.
>>> -------------------------------------------------------
>
>> drivers/video/fbdev/core/fbmon.c | 8 +++++++-
>> 1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/video/fbdev/core/fbmon.c b/drivers/video/fbdev/core/fbmon.c
>> index 3558a70a6664..2ab1fd6e33b7 100644
>> --- a/drivers/video/fbdev/core/fbmon.c
>> +++ b/drivers/video/fbdev/core/fbmon.c
>> @@ -1030,7 +1030,9 @@ void fb_edid_add_monspecs(unsigned char *edid, struct fb_monspecs *specs)
>> if (type == 2) {
>> for (i = pos; i < pos + len; i++) {
>> u8 idx = edid[pos + i] & 0x7f;
>> - svd[svd_n++] = idx;
>> + if (svd_n < sizeof(svd))
>> + svd[svd_n] = idx;
>> + svd_n++;
>> pr_debug("N%sative mode #%d\n",
>> edid[pos + i] & 0x80 ? "" : "on-n", idx);
>> }
>> @@ -1044,6 +1046,10 @@ void fb_edid_add_monspecs(unsigned char *edid, struct fb_monspecs *specs)
>> pos += len + 1;
>> }
>>
>> + /* Evil monitor? */
>> + if (WARN_ON_ONCE(svd_n > sizeof(svd)))
>> + return;
>> +
>> block = edid + edid[2];
>>
>> DPRINTK(" Extended Detailed Timings\n");
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