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

[Dmitry Osipenko <digetx-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On 14.05.2017 15:27, Mikko Perttunen wrote:
> 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-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
>> ---
>>
>> 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.
> 
Good point, unfortunately there are a lot of ways to abuse the staging API on
the IOMMU-less Tegra20 right now.

-- 
Dmitry