KASAN: use-after-free Read in vgem_gem_dumb_create

[Christian König]

Am 03.02.20 um 10:06 schrieb Dan Carpenter:
> On Sun, Feb 02, 2020 at 02:19:18PM +0100, Daniel Vetter wrote:
>> On Fri, Jan 31, 2020 at 11:28 PM syzbot
>> <syzbot+0dc4444774d419e916c8 at syzkaller.appspotmail.com> wrote:
>>> Hello,
>>>
>>> syzbot found the following crash on:
>>>
>>> HEAD commit:    39bed42d Merge tag 'for-linus-hmm' of git://git.kernel.org..
>>> git tree:       upstream
>>> console output: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fsyzkaller.appspot.com%2Fx%2Flog.txt%3Fx%3D179465bee00000&data=02%7C01%7Cchristian.koenig%40amd.com%7C529f2273b8374f38560108d7a88862eb%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637163176051177627&sdata=3goGqBs4%2BjkjCeV2bX5VTB%2F1PRLEP5bzq5Ec%2BN7fKHs%3D&reserved=0
>>> kernel config:  https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fsyzkaller.appspot.com%2Fx%2F.config%3Fx%3D2646535f8818ae25&data=02%7C01%7Cchristian.koenig%40amd.com%7C529f2273b8374f38560108d7a88862eb%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637163176051177627&sdata=SnlKln%2FAG%2BVRVjSrOSJjUE%2BhSDf35wTqzWLCAyGQVss%3D&reserved=0
>>> dashboard link: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fsyzkaller.appspot.com%2Fbug%3Fextid%3D0dc4444774d419e916c8&data=02%7C01%7Cchristian.koenig%40amd.com%7C529f2273b8374f38560108d7a88862eb%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637163176051177627&sdata=33EJNAWjTm6Edi1J0oPBfs8epb%2BQ2cpAKlzl1sT40CQ%3D&reserved=0
>>> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
>>> syz repro:      https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fsyzkaller.appspot.com%2Fx%2Frepro.syz%3Fx%3D16251279e00000&data=02%7C01%7Cchristian.koenig%40amd.com%7C529f2273b8374f38560108d7a88862eb%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637163176051177627&sdata=zmUyyp7znqQfLzzNZ80bNgCILAjeMeCVVr7xf7CHaWk%3D&reserved=0
>>>
>>> The bug was bisected to:
>>>
>>> commit 7611750784664db46d0db95631e322aeb263dde7
>>> Author: Alex Deucher <alexander.deucher at amd.com>
>>> Date:   Wed Jun 21 16:31:41 2017 +0000
>>>
>>>      drm/amdgpu: use kernel is_power_of_2 rather than local version
>>>
>>> bisection log:  https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fsyzkaller.appspot.com%2Fx%2Fbisect.txt%3Fx%3D11628df1e00000&data=02%7C01%7Cchristian.koenig%40amd.com%7C529f2273b8374f38560108d7a88862eb%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637163176051177627&sdata=5QpTG4iU%2FOt22L3jxRbNxtVPZZ2EvBAcFGZdqVnVCbU%3D&reserved=0
>>> final crash:    https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fsyzkaller.appspot.com%2Fx%2Freport.txt%3Fx%3D13628df1e00000&data=02%7C01%7Cchristian.koenig%40amd.com%7C529f2273b8374f38560108d7a88862eb%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637163176051177627&sdata=hN6UZnFR2nIMPMspjIF7S82oXstaRl%2BLAzmz5yujPac%3D&reserved=0
>>> console output: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fsyzkaller.appspot.com%2Fx%2Flog.txt%3Fx%3D15628df1e00000&data=02%7C01%7Cchristian.koenig%40amd.com%7C529f2273b8374f38560108d7a88862eb%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637163176051177627&sdata=LHXMANOURDv3EsqTSvHSBZnPEzGQoJU1RbeqYExCaGk%3D&reserved=0
>>>
>>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>>> Reported-by: syzbot+0dc4444774d419e916c8 at syzkaller.appspotmail.com
>>> Fixes: 761175078466 ("drm/amdgpu: use kernel is_power_of_2 rather than local version")
>> Aside: This bisect line is complete nonsense ... I'm kinda at the
>> point where I'm assuming that syzbot bisect results are garbage, which
>> is maybe not what we want. I guess much stricter filtering for noise
>> is needed, dunno.
>> -Danile
> With race conditions the git bisect is often nonsense.

Which makes sense, but we can still try to sanitize the result. I'm not 
familiar with the test case, but I think it doesn't even compile the 
amdgpu driver.

So skipping all patches of stuff you don't even compile would make not 
only the result of bisecting quite a bit more reliable, but also speed 
the process up quite a bit.

But no good idea to how teach that to a compile bot or the git bisect 
command.

Regards,
Christian.

>
> regards,
> dan carpenter
>