Subject: Re: KASAN: use-after-free Read in vgem_gem_dumb_create

From: Dan Carpenter <[email-masked]>
Date: Sat, 1 Feb 2020 09:17:57 +0300

On Sat, Feb 01, 2020 at 12:32:09PM +0800, Hillf Danton wrote:
> 
> Fri, 31 Jan 2020 14:28:10 -0800 (PST)
> > 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://syzkaller.appspot.com/x/log.txt?x=179465bee00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=2646535f8818ae25
> > dashboard link: https://syzkaller.appspot.com/bug?extid=0dc4444774d419e916c8
> > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=16251279e00000
> > 
> > The bug was bisected to:
> > 
> > commit 7611750784664db46d0db95631e322aeb263dde7
> > Author: Alex Deucher <[email-masked]>
> > Date:   Wed Jun 21 16:31:41 2017 +0000
> > 
> >     drm/amdgpu: use kernel is_power_of_2 rather than local version
> > 
> > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=11628df1e00000
> > final crash:    https://syzkaller.appspot.com/x/report.txt?x=13628df1e00000
> > console output: https://syzkaller.appspot.com/x/log.txt?x=15628df1e00000
> > 
> > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > Reported-by: [email-masked]
> > Fixes: 761175078466 ("drm/amdgpu: use kernel is_power_of_2 rather than local version")
> > 
> > ==================================================================
> > BUG: KASAN: use-after-free in vgem_gem_dumb_create+0x238/0x250 drivers/gpu/drm/vgem/vgem_drv.c:221
> > Read of size 8 at addr ffff88809fa67908 by task syz-executor.0/14871
> > 
> > CPU: 0 PID: 14871 Comm: syz-executor.0 Not tainted 5.5.0-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > Call Trace:
> >  __dump_stack lib/dump_stack.c:77 [inline]
> >  dump_stack+0x197/0x210 lib/dump_stack.c:118
> >  print_address_description.constprop.0.cold+0xd4/0x30b mm/kasan/report.c:374
> >  __kasan_report.cold+0x1b/0x32 mm/kasan/report.c:506
> >  kasan_report+0x12/0x20 mm/kasan/common.c:639
> >  __asan_report_load8_noabort+0x14/0x20 mm/kasan/generic_report.c:135
> >  vgem_gem_dumb_create+0x238/0x250 drivers/gpu/drm/vgem/vgem_drv.c:221
> >  drm_mode_create_dumb+0x282/0x310 drivers/gpu/drm/drm_dumb_buffers.c:94
> >  drm_mode_create_dumb_ioctl+0x26/0x30 drivers/gpu/drm/drm_dumb_buffers.c:100
> >  drm_ioctl_kernel+0x244/0x300 drivers/gpu/drm/drm_ioctl.c:786
> >  drm_ioctl+0x54e/0xa60 drivers/gpu/drm/drm_ioctl.c:886
> >  vfs_ioctl fs/ioctl.c:47 [inline]
> >  ksys_ioctl+0x123/0x180 fs/ioctl.c:747
> >  __do_sys_ioctl fs/ioctl.c:756 [inline]
> >  __se_sys_ioctl fs/ioctl.c:754 [inline]
> >  __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:754
> >  do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
> >  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > RIP: 0033:0x45b349
> > Code: ad b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 7b b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
> > RSP: 002b:00007f871af46c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> > RAX: ffffffffffffffda RBX: 00007f871af476d4 RCX: 000000000045b349
> > RDX: 0000000020000180 RSI: 00000000c02064b2 RDI: 0000000000000003
> > RBP: 000000000075bf20 R08: 0000000000000000 R09: 0000000000000000
> > R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
> > R13: 0000000000000285 R14: 00000000004d14d0 R15: 000000000075bf2c
> > 
> > Allocated by task 14871:
> >  save_stack+0x23/0x90 mm/kasan/common.c:72
> >  set_track mm/kasan/common.c:80 [inline]
> >  __kasan_kmalloc mm/kasan/common.c:513 [inline]
> >  __kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:486
> >  kasan_kmalloc+0x9/0x10 mm/kasan/common.c:527
> >  kmem_cache_alloc_trace+0x158/0x790 mm/slab.c:3551
> >  kmalloc include/linux/slab.h:556 [inline]
> >  kzalloc include/linux/slab.h:670 [inline]
> >  __vgem_gem_create+0x49/0x100 drivers/gpu/drm/vgem/vgem_drv.c:165
> >  vgem_gem_create drivers/gpu/drm/vgem/vgem_drv.c:194 [inline]
> >  vgem_gem_dumb_create+0xd7/0x250 drivers/gpu/drm/vgem/vgem_drv.c:217
> >  drm_mode_create_dumb+0x282/0x310 drivers/gpu/drm/drm_dumb_buffers.c:94
> >  drm_mode_create_dumb_ioctl+0x26/0x30 drivers/gpu/drm/drm_dumb_buffers.c:100
> >  drm_ioctl_kernel+0x244/0x300 drivers/gpu/drm/drm_ioctl.c:786
> >  drm_ioctl+0x54e/0xa60 drivers/gpu/drm/drm_ioctl.c:886
> >  vfs_ioctl fs/ioctl.c:47 [inline]
> >  ksys_ioctl+0x123/0x180 fs/ioctl.c:747
> >  __do_sys_ioctl fs/ioctl.c:756 [inline]
> >  __se_sys_ioctl fs/ioctl.c:754 [inline]
> >  __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:754
> >  do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
> >  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > 
> > Freed by task 14871:
> >  save_stack+0x23/0x90 mm/kasan/common.c:72
> >  set_track mm/kasan/common.c:80 [inline]
> >  kasan_set_free_info mm/kasan/common.c:335 [inline]
> >  __kasan_slab_free+0x102/0x150 mm/kasan/common.c:474
> >  kasan_slab_free+0xe/0x10 mm/kasan/common.c:483
> >  __cache_free mm/slab.c:3426 [inline]
> >  kfree+0x10a/0x2c0 mm/slab.c:3757
> >  vgem_gem_free_object+0xbe/0xe0 drivers/gpu/drm/vgem/vgem_drv.c:68
> >  drm_gem_object_free+0x100/0x220 drivers/gpu/drm/drm_gem.c:983
> >  kref_put include/linux/kref.h:65 [inline]
> >  drm_gem_object_put_unlocked drivers/gpu/drm/drm_gem.c:1017 [inline]
> >  drm_gem_object_put_unlocked+0x196/0x1c0 drivers/gpu/drm/drm_gem.c:1002
> >  vgem_gem_create drivers/gpu/drm/vgem/vgem_drv.c:199 [inline]
> >  vgem_gem_dumb_create+0x115/0x250 drivers/gpu/drm/vgem/vgem_drv.c:217
> >  drm_mode_create_dumb+0x282/0x310 drivers/gpu/drm/drm_dumb_buffers.c:94
> >  drm_mode_create_dumb_ioctl+0x26/0x30 drivers/gpu/drm/drm_dumb_buffers.c:100
> >  drm_ioctl_kernel+0x244/0x300 drivers/gpu/drm/drm_ioctl.c:786
> >  drm_ioctl+0x54e/0xa60 drivers/gpu/drm/drm_ioctl.c:886
> >  vfs_ioctl fs/ioctl.c:47 [inline]
> >  ksys_ioctl+0x123/0x180 fs/ioctl.c:747
> >  __do_sys_ioctl fs/ioctl.c:756 [inline]
> >  __se_sys_ioctl fs/ioctl.c:754 [inline]
> >  __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:754
> >  do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
> >  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > 
> > The buggy address belongs to the object at ffff88809fa67800
> >  which belongs to the cache kmalloc-1k of size 1024
> > The buggy address is located 264 bytes inside of
> >  1024-byte region [ffff88809fa67800, ffff88809fa67c00)
> > The buggy address belongs to the page:
> > page:ffffea00027e99c0 refcount:1 mapcount:0 mapping:ffff8880aa400c40 index:0x0
> > raw: 00fffe0000000200 ffffea0002293548 ffffea00023e1f08 ffff8880aa400c40
> > raw: 0000000000000000 ffff88809fa67000 0000000100000002 0000000000000000
> > page dumped because: kasan: bad access detected
> > 
> > Memory state around the buggy address:
> >  ffff88809fa67800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >  ffff88809fa67880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> > >ffff88809fa67900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >                       ^
> >  ffff88809fa67980: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >  ffff88809fa67a00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> > ==================================================================
> 
> Release obj in error path.
> 
> --- a/drivers/gpu/drm/vgem/vgem_drv.c
> +++ b/drivers/gpu/drm/vgem/vgem_drv.c
> @@ -196,10 +196,10 @@ static struct drm_gem_object *vgem_gem_c
>  		return ERR_CAST(obj);
>  
>  	ret = drm_gem_handle_create(file, &obj->base, handle);
> -	drm_gem_object_put_unlocked(&obj->base);
> -	if (ret)
> +	if (ret) {
> +		drm_gem_object_put_unlocked(&obj->base);
>  		return ERR_PTR(ret);
> -
> +	}
>  	return &obj->base;

Oh yeah.  It's weird that we never noticed the success path was broken.
It's been that way for three years and no one noticed at all.  Very
strange.

Anyway, it already gets freed on error in drm_gem_handle_create() so
we should just delete the drm_gem_object_put_unlocked() here it looks
like.

regards,
dan carpenter