[PATCH v2] drm/tegra: Check size of a submitted command buffer

[Mikko Perttunen]

On 05/12/2017 10:29 PM, Dmitry Osipenko wrote:
> If command buffer claims a number of words that is higher than its BO can
> fit and a relocation lays past the BO, a kernel OOPS will be fired on that
> relocation address patching. This was triggered by an opentegra Xorg driver
> that erroneously pushed too many commands to the pushbuf.
>
> [   46.829393] Unable to handle kernel paging request at virtual address f09b2000
> ...
> [<c04a3ba4>] (host1x_job_pin) from [<c04dfcd0>] (tegra_drm_submit+0x474/0x510)
> [<c04dfcd0>] (tegra_drm_submit) from [<c04deea0>] (tegra_submit+0x50/0x6c)
> [<c04deea0>] (tegra_submit) from [<c04c07c0>] (drm_ioctl+0x1e4/0x3ec)
> [<c04c07c0>] (drm_ioctl) from [<c02541a0>] (do_vfs_ioctl+0x9c/0x8e4)
> [<c02541a0>] (do_vfs_ioctl) from [<c0254a1c>] (SyS_ioctl+0x34/0x5c)
> [<c0254a1c>] (SyS_ioctl) from [<c0107640>] (ret_fast_syscall+0x0/0x3c)
>
> Signed-off-by: Dmitry Osipenko <digetx at gmail.com>
> ---
>
> v2: Take into account the cmdbuf.offset
>
>  drivers/gpu/drm/tegra/drm.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/tegra/drm.c b/drivers/gpu/drm/tegra/drm.c
> index 732c8d98044f..9ad4ac7c08d1 100644
> --- a/drivers/gpu/drm/tegra/drm.c
> +++ b/drivers/gpu/drm/tegra/drm.c
> @@ -361,20 +361,30 @@ int tegra_drm_submit(struct tegra_drm_context *context,
>
>  	while (num_cmdbufs) {
>  		struct drm_tegra_cmdbuf cmdbuf;
> -		struct host1x_bo *bo;
> +		struct drm_gem_object *gem;
> +		struct tegra_bo *bo;
>
>  		if (copy_from_user(&cmdbuf, cmdbufs, sizeof(cmdbuf))) {
>  			err = -EFAULT;
>  			goto fail;
>  		}
>
> -		bo = host1x_bo_lookup(file, cmdbuf.handle);
> -		if (!bo) {
> +		gem = drm_gem_object_lookup(file, cmdbuf.handle);
> +		if (!gem) {
>  			err = -ENOENT;
>  			goto fail;
>  		}
>
> -		host1x_job_add_gather(job, bo, cmdbuf.words, cmdbuf.offset);
> +		drm_gem_object_unreference_unlocked(gem);
> +
> +		if (cmdbuf.offset + cmdbuf.words * 4 > gem->size) {
> +			err = -EINVAL;
> +			goto fail;
> +		}

Nasty bug! Well found. Two points: the arithmetic here could overflow, 
so userspace could supply some large values for offset/words and this 
check would not catch it. A fix would be to do the arithmetic in 64-bit. 
Also, looking at the code closer, I can't see any bounds checking for 
relocs either.. That code (host1x_reloc_copy_from_user) is the other 
place using host1x_bo_lookup, so we could e.g. change host1x_bo_lookup 
to take offset and words parameters and verify those at the same time.

Cheers,
Mikko

> +
> +		bo = to_tegra_bo(gem);
> +		host1x_job_add_gather(job, &bo->base,
> +				      cmdbuf.words, cmdbuf.offset);
>  		num_cmdbufs--;
>  		cmdbufs++;
>  	}
>