Re: [PATCH v3] fbdev/simplefb: Fix use after free in simplefb_detach_genpds()

[Janne Grunau <j@jannau.net>]

On Tue, Sep 30, 2025 at 09:12:21PM +0200, Helge Deller wrote:
> On 9/15/25 08:36, Janne Grunau wrote:
> > The pm_domain cleanup can not be devres managed as it uses struct
> > simplefb_par which is allocated within struct fb_info by
> > framebuffer_alloc(). This allocation is explicitly freed by
> > unregister_framebuffer() in simplefb_remove().
> > Devres managed cleanup runs after the device remove call and thus can no
> > longer access struct simplefb_par.
> > Call simplefb_detach_genpds() explicitly from simplefb_destroy() like
> > the cleanup functions for clocks and regulators.
> > 
> > Fixes an use after free on M2 Mac mini during
> > aperture_remove_conflicting_devices() using the downstream asahi kernel
> > with Debian's kernel config. For unknown reasons this started to
> > consistently dereference an invalid pointer in v6.16.3 based kernels.
> > 
> > [    6.736134] BUG: KASAN: slab-use-after-free in simplefb_detach_genpds+0x58/0x220
> > [    6.743545] Read of size 4 at addr ffff8000304743f0 by task (udev-worker)/227
> > [    6.750697]
> > [    6.752182] CPU: 6 UID: 0 PID: 227 Comm: (udev-worker) Tainted: G S                  6.16.3-asahi+ #16 PREEMPTLAZY
> > [    6.752186] Tainted: [S]=CPU_OUT_OF_SPEC
> > [    6.752187] Hardware name: Apple Mac mini (M2, 2023) (DT)
> > [    6.752189] Call trace:
> > [    6.752190]  show_stack+0x34/0x98 (C)
> > [    6.752194]  dump_stack_lvl+0x60/0x80
> > [    6.752197]  print_report+0x17c/0x4d8
> > [    6.752201]  kasan_report+0xb4/0x100
> > [    6.752206]  __asan_report_load4_noabort+0x20/0x30
> > [    6.752209]  simplefb_detach_genpds+0x58/0x220
> > [    6.752213]  devm_action_release+0x50/0x98
> > [    6.752216]  release_nodes+0xd0/0x2c8
> > [    6.752219]  devres_release_all+0xfc/0x178
> > [    6.752221]  device_unbind_cleanup+0x28/0x168
> > [    6.752224]  device_release_driver_internal+0x34c/0x470
> > [    6.752228]  device_release_driver+0x20/0x38
> > [    6.752231]  bus_remove_device+0x1b0/0x380
> > [    6.752234]  device_del+0x314/0x820
> > [    6.752238]  platform_device_del+0x3c/0x1e8
> > [    6.752242]  platform_device_unregister+0x20/0x50
> > [    6.752246]  aperture_detach_platform_device+0x1c/0x30
> > [    6.752250]  aperture_detach_devices+0x16c/0x290
> > [    6.752253]  aperture_remove_conflicting_devices+0x34/0x50
> > ...
> > [    6.752343]
> > [    6.967409] Allocated by task 62:
> > [    6.970724]  kasan_save_stack+0x3c/0x70
> > [    6.974560]  kasan_save_track+0x20/0x40
> > [    6.978397]  kasan_save_alloc_info+0x40/0x58
> > [    6.982670]  __kasan_kmalloc+0xd4/0xd8
> > [    6.986420]  __kmalloc_noprof+0x194/0x540
> > [    6.990432]  framebuffer_alloc+0xc8/0x130
> > [    6.994444]  simplefb_probe+0x258/0x2378
> > ...
> > [    7.054356]
> > [    7.055838] Freed by task 227:
> > [    7.058891]  kasan_save_stack+0x3c/0x70
> > [    7.062727]  kasan_save_track+0x20/0x40
> > [    7.066565]  kasan_save_free_info+0x4c/0x80
> > [    7.070751]  __kasan_slab_free+0x6c/0xa0
> > [    7.074675]  kfree+0x10c/0x380
> > [    7.077727]  framebuffer_release+0x5c/0x90
> > [    7.081826]  simplefb_destroy+0x1b4/0x2c0
> > [    7.085837]  put_fb_info+0x98/0x100
> > [    7.089326]  unregister_framebuffer+0x178/0x320
> > [    7.093861]  simplefb_remove+0x3c/0x60
> > [    7.097611]  platform_remove+0x60/0x98
> > [    7.101361]  device_remove+0xb8/0x160
> > [    7.105024]  device_release_driver_internal+0x2fc/0x470
> > [    7.110256]  device_release_driver+0x20/0x38
> > [    7.114529]  bus_remove_device+0x1b0/0x380
> > [    7.118628]  device_del+0x314/0x820
> > [    7.122116]  platform_device_del+0x3c/0x1e8
> > [    7.126302]  platform_device_unregister+0x20/0x50
> > [    7.131012]  aperture_detach_platform_device+0x1c/0x30
> > [    7.136157]  aperture_detach_devices+0x16c/0x290
> > [    7.140779]  aperture_remove_conflicting_devices+0x34/0x50
> > ...
> > 
> > Reported-by: Daniel Huhardeaux <tech@tootai.net>
> > Cc: stable@vger.kernel.org
> > Fixes: 92a511a568e44 ("fbdev/simplefb: Add support for generic power-domains")
> > Signed-off-by: Janne Grunau <j@jannau.net>
> > ---
> > Changes in v3:
> > - release power-domains on probe errors
> > - set par->num_genpds when it's <= 1
> > - set par->num_genpds to 0 after detaching
> > - Link to v2: https://lore.kernel.org/r/20250908-simplefb-genpd-uaf-v2-1-f88a0d9d880f@jannau.net
> > 
> > Changes in v2:
> > - reworked change due to missed use of `par->num_genpds` before setting
> >    it. Missed in testing due to mixing up FB_SIMPLE and SYSFB_SIMPLEFB.
> > - Link to v1: https://lore.kernel.org/r/20250901-simplefb-genpd-uaf-v1-1-0d9f3a34c4dc@jannau.net
> > ---
> >   drivers/video/fbdev/simplefb.c | 31 +++++++++++++++++++++++--------
> >   1 file changed, 23 insertions(+), 8 deletions(-)
> 
> applied to fbdev git tree.

Thanks.
> 
> PS: Janne, if you want to push yourself via drm-misc, just let me know and I drop it...

I won't. My request for commit access is still pending. I should fine
someone at xdc to ask about that.

Janne