Re: [RFC 2/3] blktrace: fix debugfs use after free

[Eric Sandeen]

On 4/1/20 7:00 PM, Luis Chamberlain wrote:
> On commit 6ac93117ab00 ("blktrace: use existing disk debugfs directory")
> Omar fixed the original blktrace code for multiqueue use. This however
> left in place a possible crash, if you happen to abuse blktrace in a way
> it was not intended.
> 
> Namely, if you loop adding a device, setup the blktrace with BLKTRACESETUP,
> forget to BLKTRACETEARDOWN, and then just remove the device you end up
> with a panic:

Weird, I swear I tested that and didn't hit it, but ...
 

> This issue can be reproduced with break-blktrace [2] using:
> 
>   break-blktrace -c 10 -d

+ -s, right?
 
> This patch fixes this issue. Note that there is also another
> respective UAF but from the ioctl path [3], this should also fix
> that issue.
> 
> This patch then also contends the severity of CVE-2019-19770 as
> this issue is only possible using root to shoot yourself in the
> foot by also misuing blktrace.
> 
> [0] https://bugzilla.kernel.org/show_bug.cgi?id=205713
> [1] https://nvd.nist.gov/vuln/detail/CVE-2019-19770
> [2] https://github.com/mcgrof/break-blktrace

I verified that this does reproduce the exact same KASAN splat on
kernel 4.19.83 as reported in the original bug, thanks!

-Eric