[PATCH rc v2] iommufd: Explicitize struct iommu_hwpt_pgfault padding

[PATCH rc v2] iommufd: Explicitize struct iommu_hwpt_pgfault padding

[Nicolin Chen]

Though the padding could be done by the compiler, add a 32-bit padding
explicitly. Update iommufd_compose_fault_message to set it explicitly.
Also, change the __u64 addr to __aligned_u64.

pahole result, before:
struct iommu_hwpt_pgfault {
        __u32     flags;                /*     0     4 */
        __u32     dev_id;               /*     4     4 */
        __u32     pasid;                /*     8     4 */
        __u32     grpid;                /*    12     4 */
        __u32     perm;                 /*    16     4 */

        /* XXX 4 bytes hole, try to pack */

        __u64     addr;                 /*    24     8 */
        __u32     length;               /*    32     4 */
        __u32     cookie;               /*    36     4 */

        /* size: 40, cachelines: 1, members: 8 */
        /* sum members: 36, holes: 1, sum holes: 4 */
        /* last cacheline: 40 bytes */
};

pahole result, after:
struct iommu_hwpt_pgfault {
        __u32      flags;                /*     0     4 */
        __u32      dev_id;               /*     4     4 */
        __u32      pasid;                /*     8     4 */
        __u32      grpid;                /*    12     4 */
        __u32      perm;                 /*    16     4 */
        __u32      __reserved;           /*    20     4 */
        __u64      addr __attribute__((__aligned__(8))); /*    24     8 */
        __u32      length;               /*    32     4 */
        __u32      cookie;               /*    36     4 */

        /* size: 40, cachelines: 1, members: 9 */
        /* forced alignments: 1 */
        /* last cacheline: 40 bytes */
} __attribute__((__aligned__(8)));

Fixes: c714f15860fc ("iommufd: Add fault and response message definitions")
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
---
 drivers/iommu/iommufd/fault.c | 1 +
 include/uapi/linux/iommufd.h  | 4 +++-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/iommufd/fault.c b/drivers/iommu/iommufd/fault.c
index a9160f4443d2..52003bf1ba68 100644
--- a/drivers/iommu/iommufd/fault.c
+++ b/drivers/iommu/iommufd/fault.c
@@ -253,6 +253,7 @@ static void iommufd_compose_fault_message(struct iommu_fault *fault,
 	hwpt_fault->pasid = fault->prm.pasid;
 	hwpt_fault->grpid = fault->prm.grpid;
 	hwpt_fault->perm = fault->prm.perm;
+	hwpt_fault->__reserved = 0;
 	hwpt_fault->addr = fault->prm.addr;
 	hwpt_fault->length = 0;
 	hwpt_fault->cookie = cookie;
diff --git a/include/uapi/linux/iommufd.h b/include/uapi/linux/iommufd.h
index 34810f6ae2b5..78747b24bd0f 100644
--- a/include/uapi/linux/iommufd.h
+++ b/include/uapi/linux/iommufd.h
@@ -868,6 +868,7 @@ enum iommu_hwpt_pgfault_perm {
  * @pasid: Process Address Space ID
  * @grpid: Page Request Group Index
  * @perm: Combination of enum iommu_hwpt_pgfault_perm
+ * @__reserved: Must be 0.
  * @addr: Fault address
  * @length: a hint of how much data the requestor is expecting to fetch. For
  *          example, if the PRI initiator knows it is going to do a 10MB
@@ -883,7 +884,8 @@ struct iommu_hwpt_pgfault {
 	__u32 pasid;
 	__u32 grpid;
 	__u32 perm;
-	__u64 addr;
+	__u32 __reserved;
+	__aligned_u64 addr;
 	__u32 length;
 	__u32 cookie;
 };
-- 
2.34.1

Re: [PATCH rc v2] iommufd: Explicitize struct iommu_hwpt_pgfault padding

[Jason Gunthorpe]

On Sun, Jan 19, 2025 at 10:24:11PM -0800, Nicolin Chen wrote:
> --- a/drivers/iommu/iommufd/fault.c
> +++ b/drivers/iommu/iommufd/fault.c
> @@ -253,6 +253,7 @@ static void iommufd_compose_fault_message(struct iommu_fault *fault,
>  	hwpt_fault->pasid = fault->prm.pasid;
>  	hwpt_fault->grpid = fault->prm.grpid;
>  	hwpt_fault->perm = fault->prm.perm;
> +	hwpt_fault->__reserved = 0;
>  	hwpt_fault->addr = fault->prm.addr;
>  	hwpt_fault->length = 0;
>  	hwpt_fault->cookie = cookie;

Yikes, so it was leaking kernel stack memory through the padding too.

We should zero init the stack struct to be safe:

@@ -247,7 +247,7 @@ static ssize_t iommufd_fault_fops_read(struct file *filep, char __user *buf,
 {
        size_t fault_size = sizeof(struct iommu_hwpt_pgfault);
        struct iommufd_fault *fault = filep->private_data;
-       struct iommu_hwpt_pgfault data;
+       struct iommu_hwpt_pgfault data = {};
        struct iommufd_device *idev;
        struct iopf_group *group;
        struct iopf_fault *iopf;

I can fix it up if that is the only change

Jason

Re: [PATCH rc v2] iommufd: Explicitize struct iommu_hwpt_pgfault padding

[Jason Gunthorpe]

On Sun, Jan 19, 2025 at 10:24:11PM -0800, Nicolin Chen wrote:
> Though the padding could be done by the compiler, add a 32-bit padding
> explicitly. Update iommufd_compose_fault_message to set it explicitly.
> Also, change the __u64 addr to __aligned_u64.

Mention that this also leaks the content of kernel stack memory to
userspace since it was never zeroing the padding.

> Fixes: c714f15860fc ("iommufd: Add fault and response message definitions")
> Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
> ---
>  drivers/iommu/iommufd/fault.c | 1 +
>  include/uapi/linux/iommufd.h  | 4 +++-
>  2 files changed, 4 insertions(+), 1 deletion(-)

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Jason

Re: [PATCH rc v2] iommufd: Explicitize struct iommu_hwpt_pgfault padding

[Nicolin Chen]

On Mon, Jan 20, 2025 at 10:37:19AM -0400, Jason Gunthorpe wrote:
> On Sun, Jan 19, 2025 at 10:24:11PM -0800, Nicolin Chen wrote:
> > --- a/drivers/iommu/iommufd/fault.c
> > +++ b/drivers/iommu/iommufd/fault.c
> > @@ -253,6 +253,7 @@ static void iommufd_compose_fault_message(struct iommu_fault *fault,
> >  	hwpt_fault->pasid = fault->prm.pasid;
> >  	hwpt_fault->grpid = fault->prm.grpid;
> >  	hwpt_fault->perm = fault->prm.perm;
> > +	hwpt_fault->__reserved = 0;
> >  	hwpt_fault->addr = fault->prm.addr;
> >  	hwpt_fault->length = 0;
> >  	hwpt_fault->cookie = cookie;
> 
> Yikes, so it was leaking kernel stack memory through the padding too.
> 
> We should zero init the stack struct to be safe:
> 
> @@ -247,7 +247,7 @@ static ssize_t iommufd_fault_fops_read(struct file *filep, char __user *buf,
>  {
>         size_t fault_size = sizeof(struct iommu_hwpt_pgfault);
>         struct iommufd_fault *fault = filep->private_data;
> -       struct iommu_hwpt_pgfault data;
> +       struct iommu_hwpt_pgfault data = {};
>         struct iommufd_device *idev;
>         struct iopf_group *group;
>         struct iopf_fault *iopf;
> 
> I can fix it up if that is the only change

Sending a v3.

I think having the zeroing in iommufd_fault_fops_read can drop
the "__reserved = 0" iommufd_compose_fault_message.

Thanks
Nicolin