Re: [PATCH 1/1] iommu: Avoid races around default domain allocations

[Nikhil V]

On 1/18/2024 3:41 PM, Nikhil V wrote:
> From: Charan Teja Kalla <quic_charante@xxxxxxxxxxx>
>
> This fix is applicable for 6.1 kernel. In latest kernels, this race
> issue is fixed by the patch series [1] and [2]. This fix can be taken
> as alternative instead of backporting the series of patches as these
> seems too intrusive to be picked for stable branches.
> [1] https://lore.kernel.org/all/0-v8-81230027b2fa+9d-iommu_all_defdom_jgg@xxxxxxxxxx/
> [2] https://lore.kernel.org/all/0-v5-1b99ae392328+44574-iommu_err_unwind_jgg@xxxxxxxxxx/
>
> A race condition is observed when arm_smmu_device_probe and
> modprobe of client devices happens in parallel. This results
> in the allocation of a new default domain for the iommu group
> even though it was previously allocated and the respective iova
> domain(iovad) was initialized. However, for this newly allocated
> default domain, iovad will not be initialized. As a result, for
> devices requesting dma allocations, this uninitialized iovad will
> be used, thereby causing NULL pointer dereference issue.
>
> Flow:
> - During arm_smmu_device_probe, bus_iommu_probe() will be called
> as part of iommu_device_register(). This results in the device probe,
> __iommu_probe_device().
>
> - When the modprobe of the client device happens in parallel, it
> sets up the DMA configuration for the device using of_dma_configure_id(),
> which inturn calls iommu_probe_device(). Later, default domain is
> allocated and attached using iommu_alloc_default_domain() and
> __iommu_attach_device() respectively. It then ends up initializing a
> mapping domain(IOVA domain) and rcaches for the device via
> arch_setup_dma_ops()->iommu_setup_dma_ops().
>
> - Now, in the bus_iommu_probe() path, it again tries to allocate
> a default domain via probe_alloc_default_domain(). This results in
> allocating a new default domain(along with IOVA domain) via
> __iommu_domain_alloc(). However, this newly allocated IOVA domain
> will not be initialized.
>
> - Now, when the same client device tries dma allocations via
> iommu_dma_alloc(), it ends up accessing the rcaches of the newly
> allocated IOVA domain, which is not initialized. This results
> into NULL pointer dereferencing.
>
> Fix this issue by adding a check in iommu_group_alloc_default_domain()
> to see if the iommu_group already has a default domain allocated
> and initialized.
>
> Signed-off-by: Charan Teja Kalla <quic_charante@xxxxxxxxxxx>
> Co-developed-by: Nikhil V <quic_nprakash@xxxxxxxxxxx>
> Signed-off-by: Nikhil V <quic_nprakash@xxxxxxxxxxx>
> ---
>   drivers/iommu/iommu.c | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 8b3897239477..99f8cd5af497 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -1594,6 +1594,9 @@ static int iommu_group_alloc_default_domain(struct bus_type *bus,
>   {
>   	struct iommu_domain *dom;
>   
> +	if (group->default_domain)
> +		return 0;
> +
>   	dom = __iommu_domain_alloc(bus, type);
>   	if (!dom && type != IOMMU_DOMAIN_DMA) {
>   		dom = __iommu_domain_alloc(bus, IOMMU_DOMAIN_DMA);

Hi,

Gentle ping to have your valuable feedback. This fix is helping us 
downstream without which we see a bunch of kernel crashes.

Thanks
Nikhil V