Subject: Re: [PATCH kernel v6 29/29] vfio: powerpc/spapr: Support Dynamic DMA windows

From: Alex Williamson <[email-masked]>
Date: Mon, 16 Mar 2015 20:49:50 -0600

On Tue, 2015-03-17 at 12:02 +1100, Alexey Kardashevskiy wrote:
> On 03/17/2015 06:38 AM, Alex Williamson wrote:
> > On Fri, 2015-03-13 at 19:07 +1100, Alexey Kardashevskiy wrote:
> >> This adds create/remove window ioctls to create and remove DMA windows.
> >> sPAPR defines a Dynamic DMA windows capability which allows
> >> para-virtualized guests to create additional DMA windows on a PCI bus.
> >> The existing linux kernels use this new window to map the entire guest
> >> memory and switch to the direct DMA operations saving time on map/unmap
> >> requests which would normally happen in a big amounts.
> >>
> >> This adds 2 ioctl handlers - VFIO_IOMMU_SPAPR_TCE_CREATE and
> >> VFIO_IOMMU_SPAPR_TCE_REMOVE - to create and remove windows.
> >> Up to 2 windows are supported now by the hardware and by this driver.
> >>
> >> This changes VFIO_IOMMU_SPAPR_TCE_GET_INFO handler to return additional
> >> information such as a number of supported windows and maximum number
> >> levels of TCE tables.
> >>
> >> DDW is added as a capability, not as a SPAPR TCE IOMMU v2 unique feature
> >> as we still want to support v2 on platforms which cannot do DDW for
> >> the sake of TCE acceleration in KVM (coming soon).
> >>
> >> Signed-off-by: Alexey Kardashevskiy <[email-masked]>
> >> ---
> >> Changes:
> >> v6:
> >> * added explicit VFIO_IOMMU_INFO_DDW flag to vfio_iommu_spapr_tce_info,
> >> it used to be page mask flags from platform code
> >> * added explicit pgsizes field
> >> * added cleanup if tce_iommu_create_window() failed in a middle
> >> * added checks for callbacks in tce_iommu_create_window and remove those
> >> from tce_iommu_remove_window when it is too late to test anyway
> >> * spapr_tce_find_free_table returns sensible error code now
> >> * updated description of VFIO_IOMMU_SPAPR_TCE_CREATE/
> >> VFIO_IOMMU_SPAPR_TCE_REMOVE
> >>
> >> v4:
> >> * moved code to tce_iommu_create_window()/tce_iommu_remove_window()
> >> helpers
> >> * added docs
> >> ---
> >>   Documentation/vfio.txt              |  19 ++++
> >>   arch/powerpc/include/asm/iommu.h    |   2 +-
> >>   drivers/vfio/vfio_iommu_spapr_tce.c | 206 +++++++++++++++++++++++++++++++++++-
> >>   include/uapi/linux/vfio.h           |  41 ++++++-
> >>   4 files changed, 265 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/Documentation/vfio.txt b/Documentation/vfio.txt
> >> index 791e85c..61ce393 100644
> >> --- a/Documentation/vfio.txt
> >> +++ b/Documentation/vfio.txt
> >> @@ -446,6 +446,25 @@ the memory block.
> >>   The user space is not expected to call these often and the block descriptors
> >>   are stored in a linked list in the kernel.
> >>
> >> +6) sPAPR specification allows guests to have an ddditional DMA window(s) on
> >
> >
> > s/ddditional/additional/
> >
> >> +a PCI bus with a variable page size. Two ioctls have been added to support
> >> +this: VFIO_IOMMU_SPAPR_TCE_CREATE and VFIO_IOMMU_SPAPR_TCE_REMOVE.
> >> +The platform has to support the functionality or error will be returned to
> >> +the userspace. The existing hardware supports up to 2 DMA windows, one is
> >> +2GB long, uses 4K pages and called "default 32bit window"; the other can
> >> +be as big as entire RAM, use different page size, it is optional - guests
> >> +create those in run-time if the guest driver supports 64bit DMA.
> >> +
> >> +VFIO_IOMMU_SPAPR_TCE_CREATE receives a page shift, a DMA window size and
> >> +a number of TCE table levels (if a TCE table is going to be big enough and
> >> +the kernel may not be able to allocate enough of physicall contiguous memory).
> >
> > s/physicall/physically/
> >
> >> +It creates a new window in the available slot and returns the bus address where
> >> +the new window starts. Due to hardware limitation, the user space cannot choose
> >> +the location of DMA windows.
> >> +
> >> +VFIO_IOMMU_SPAPR_TCE_REMOVE receives the bus start address of the window
> >> +and removes it.
> >> +
> >>   -------------------------------------------------------------------------------
> >>
> >>   [1] VFIO was originally an acronym for "Virtual Function I/O" in its
> >> diff --git a/arch/powerpc/include/asm/iommu.h b/arch/powerpc/include/asm/iommu.h
> >> index 13145a2..bac02bf 100644
> >> --- a/arch/powerpc/include/asm/iommu.h
> >> +++ b/arch/powerpc/include/asm/iommu.h
> >> @@ -138,7 +138,7 @@ extern void iommu_free_table(struct iommu_table *tbl, const char *node_name);
> >>   extern struct iommu_table *iommu_init_table(struct iommu_table * tbl,
> >>   					    int nid);
> >>
> >> -#define IOMMU_TABLE_GROUP_MAX_TABLES	1
> >> +#define IOMMU_TABLE_GROUP_MAX_TABLES	2
> >>
> >>   struct iommu_table_group;
> >>
> >> diff --git a/drivers/vfio/vfio_iommu_spapr_tce.c b/drivers/vfio/vfio_iommu_spapr_tce.c
> >> index d94116b..0129a4f 100644
> >> --- a/drivers/vfio/vfio_iommu_spapr_tce.c
> >> +++ b/drivers/vfio/vfio_iommu_spapr_tce.c
> >> @@ -600,11 +600,137 @@ static long tce_iommu_build(struct tce_container *container,
> >>   	return ret;
> >>   }
> >>
> >> +static int spapr_tce_find_free_table(struct tce_container *container)
> >> +{
> >> +	int i;
> >> +
> >> +	for (i = 0; i < IOMMU_TABLE_GROUP_MAX_TABLES; ++i) {
> >> +		struct iommu_table *tbl = &container->tables[i];
> >> +
> >> +		if (!tbl->it_size)
> >> +			return i;
> >> +	}
> >> +
> >> +	return -ENOSPC;
> >> +}
> >> +
> >> +static long tce_iommu_create_window(struct tce_container *container,
> >> +		__u32 page_shift, __u64 window_size, __u32 levels,
> >> +		__u64 *start_addr)
> >> +{
> >> +	struct tce_iommu_group *tcegrp;
> >> +	struct iommu_table_group *table_group;
> >> +	struct iommu_table *tbl;
> >> +	int num;
> >> +	long ret;
> >> +
> >> +	num = spapr_tce_find_free_table(container);
> >> +	if (num < 0)
> >> +		return num;
> >> +
> >> +	tbl = &container->tables[num];
> >> +
> >> +	/* Get the first group for ops::create_table */
> >> +	tcegrp = list_first_entry(&container->group_list,
> >> +			struct tce_iommu_group, next);
> >> +	table_group = iommu_group_get_iommudata(tcegrp->grp);
> >> +	if (!table_group)
> >> +		return -EFAULT;
> >> +
> >> +	if (!(table_group->pgsizes & (1ULL << page_shift)))
> >> +		return -EINVAL;
> >> +
> >> +	if (!table_group->ops->set_window || !table_group->ops->unset_window ||
> >> +			!table_group->ops->create_table)
> >> +		return -EPERM;
> >> +
> >> +	/* Create TCE table */
> >> +	ret = table_group->ops->create_table(table_group, num,
> >> +			page_shift, window_size, levels, tbl);
> >> +	if (ret)
> >> +		return ret;
> >> +
> >> +	BUG_ON(!tbl->it_ops->free);
> >> +
> >> +	/*
> >> +	 * Program the table to every group.
> >> +	 * Groups have been tested for compatibility at the attach time.
> >> +	 */
> >> +	list_for_each_entry(tcegrp, &container->group_list, next) {
> >> +		table_group = iommu_group_get_iommudata(tcegrp->grp);
> >> +
> >> +		ret = table_group->ops->set_window(table_group, num, tbl);
> >> +		if (ret)
> >> +			goto unset_exit;
> >> +	}
> >> +
> >> +	/* Return start address assigned by platform in create_table() */
> >> +	*start_addr = tbl->it_offset << tbl->it_page_shift;
> >> +
> >> +	return 0;
> >> +
> >> +unset_exit:
> >> +	list_for_each_entry(tcegrp, &container->group_list, next) {
> >> +		table_group = iommu_group_get_iommudata(tcegrp->grp);
> >> +		table_group->ops->unset_window(table_group, num);
> >> +	}
> >> +	tbl->it_ops->free(tbl);
> >> +	memset(tbl, 0, sizeof(*tbl));
> >> +
> >> +	return ret;
> >> +}
> >> +
> >> +static long tce_iommu_remove_window(struct tce_container *container,
> >> +		__u64 start_addr)
> >> +{
> >> +	struct iommu_table_group *table_group = NULL;
> >> +	struct iommu_table *tbl;
> >> +	struct tce_iommu_group *tcegrp;
> >> +	int num;
> >> +
> >> +	tbl = spapr_tce_find_table(container, start_addr);
> >> +	if (!tbl)
> >> +		return -EINVAL;
> >> +
> >> +	/* Detach groups from IOMMUs */
> >> +	num = tbl - container->tables;
> >> +	list_for_each_entry(tcegrp, &container->group_list, next) {
> >> +		table_group = iommu_group_get_iommudata(tcegrp->grp);
> >> +
> >> +		/*
> >> +		 * When a group is attached, a window might be set if DDW
> >> +		 * is supported by the platform. If it is not, VFIO will
> >> +		 * use the existing window created by the platform code
> >> +		 * so spapr_tce_find_table() will find a table but
> >> +		 * since it is owned by the platform code, ops or unset_window()
> >> +		 * will not be defined. If in this situation a broken
> >> +		 * guest calls VFIO_IOMMU_SPAPR_TCE_REMOVE, we crash here
> >> +		 * without the check.
> >> +		 */
> >
> > Doesn't this indicate a more serious issue if a user can remove windows
> > that they didn't create?  It seems like that would open us to all sorts
>  > of inconsistent state, if not exploits.
> 
> 
> The existing upstream QEMU expects the default window to exits. So there is 
> at least one situation when there is a window which the user did not create.
> 
> 1. A window can be owned by the "powernv" platform or VFIO. If it is 
> "powernv" (old power7 IO chips) - removal of a window is not enabled, 
> otherwise (new power8 chip) it is.
> 
> 2. Old QEMU expects the default window to exist. If it is "powernv" owned 
> window, then VFIO just reuses it, otherwise it creates one.
> 
> What is inconsistent about the state? Or I just document it in 
> Documentation/vfio.txt and that's it?

Perhaps the comment simply needs to be made more clear.  I think what
you're saying is that dma32_window_start and size can be exposed through
vfio_iommu_spapr_tce_info and some platforms allow the user to remove
this TCE window.  On platforms that do not support that, the user is not
permitted to remove the window and we check that platform capability
here.

> >> +		if (!table_group->ops || !table_group->ops->unset_window)
> >> +			return -EPERM;
> >> +
> >> +		if (container->tables[num].it_size)
> >> +			table_group->ops->unset_window(table_group, num);
> >> +	}
> >> +
> >> +	/* Free table */
> >> +	tcegrp = list_first_entry(&container->group_list,
> >> +			struct tce_iommu_group, next);
> >> +	table_group = iommu_group_get_iommudata(tcegrp->grp);
> >> +
> >> +	tce_iommu_clear(container, tbl, tbl->it_offset, tbl->it_size);
> >> +	tbl->it_ops->free(tbl);
> >> +	memset(tbl, 0, sizeof(*tbl));
> >> +
> >> +	return 0;
> >> +}
> >> +
> >>   static long tce_iommu_ioctl(void *iommu_data,
> >>   				 unsigned int cmd, unsigned long arg)
> >>   {
> >>   	struct tce_container *container = iommu_data;
> >> -	unsigned long minsz;
> >> +	unsigned long minsz, ddwsz;
> >>   	long ret;
> >>
> >>   	switch (cmd) {
> >> @@ -645,9 +771,24 @@ static long tce_iommu_ioctl(void *iommu_data,
> >>   		if (info.argsz < minsz)
> >>   			return -EINVAL;
> >>
> >> +		info.flags = 0;
> >
> > This should not have been removed in the previous patch
> 
> Ouch. Multiple rebase result :)
> 
> 
> >>   		info.dma32_window_start = table_group->tce32_start;
> >>   		info.dma32_window_size = table_group->tce32_size;
> >>
> >> +		if (table_group->max_dynamic_windows_supported) {
> >> +			info.flags = VFIO_IOMMU_INFO_DDW;
> >
> > Shouldn't this be |= ?
> >
> >> +			info.pgsizes = table_group->pgsizes;
> >> +			info.max_dynamic_windows_supported =
> >> +				table_group->max_dynamic_windows_supported;
> >> +			info.levels = table_group->max_levels;
> >> +		}
> >> +
> >> +		ddwsz = offsetofend(struct vfio_iommu_spapr_tce_info,
> >> +				levels);
> >> +
> >> +		if (info.argsz == ddwsz)
> >> +			minsz = ddwsz;
> >> +
> >
> > How about >= ?  Requiring == means that a new userspace that might know
> > about a new field added to a later kernel will break on this kernel.
> >
> > Also, if DDW is not supported, you're just about to write uninitialized
> > kernel data to userspace :-\
> >
> >>   		if (copy_to_user((void __user *)arg, &info, minsz))
> 
> 
> then copy_to_user(..., info.argsz) is the answer, no? I do not really want 
> to touch the area of the struct I do not support in this call, unless there 
> is some code guideline for ioctl implementation.

What you write back to the user should be a) no bigger than they've told
you they've allocated and b) only initialized data, zero or actual
values.  Copying uninitialized data to userspace can have security
consequences.  I don't particularly care if you write short when ddw is
not supported or write zeros, but the docs need to make it clear that
those fields are only valid when the ddw flag is set.  If this structure
gets extended again by another flag in the future, we'll probably just
want to zero initialize fields rather than attempt sparse writes to the
structure, so maybe it makes sense to do that now.

> >>   			return -EFAULT;
> >>
> >> @@ -830,6 +971,69 @@ static long tce_iommu_ioctl(void *iommu_data,
> >>   		return ret;
> >>   	}
> >>
> >> +	case VFIO_IOMMU_SPAPR_TCE_CREATE: {
> >> +		struct vfio_iommu_spapr_tce_create create;
> >> +
> >> +		if (!tce_preregistered(container))
> >> +			return -EPERM;
> >> +
> >> +		if (!tce_groups_attached(container))
> >> +			return -ENXIO;
> >> +
> >> +		minsz = offsetofend(struct vfio_iommu_spapr_tce_create,
> >> +				start_addr);
> >> +
> >> +		if (copy_from_user(&create, (void __user *)arg, minsz))
> >> +			return -EFAULT;
> >> +
> >> +		if (create.argsz < minsz)
> >> +			return -EINVAL;
> >> +
> >> +		if (create.flags)
> >> +			return -EINVAL;
> >> +
> >> +		mutex_lock(&container->lock);
> >> +
> >> +		ret = tce_iommu_create_window(container, create.page_shift,
> >> +				create.window_size, create.levels,
> >> +				&create.start_addr);
> >> +
> >> +		if (!ret && copy_to_user((void __user *)arg, &create, minsz))
> >> +			ret = -EFAULT;
> >> +
> >> +		mutex_unlock(&container->lock);
> >
> > Even easier would be to move the unlock up, we probably don't need to
> > protect the copy_to_user().
>  >
> >> +
> >> +		return ret;
> >> +	}
> >> +	case VFIO_IOMMU_SPAPR_TCE_REMOVE: {
> >> +		struct vfio_iommu_spapr_tce_remove remove;
> >> +
> >> +		if (!tce_preregistered(container))
> >> +			return -EPERM;
> >> +
> >> +		if (!tce_groups_attached(container))
> >> +			return -ENXIO;
> >> +
> >> +		minsz = offsetofend(struct vfio_iommu_spapr_tce_remove,
> >> +				start_addr);
> >> +
> >> +		if (copy_from_user(&remove, (void __user *)arg, minsz))
> >> +			return -EFAULT;
> >> +
> >> +		if (remove.argsz < minsz)
> >> +			return -EINVAL;
> >> +
> >> +		if (remove.flags)
> >> +			return -EINVAL;
> >> +
> >> +		mutex_lock(&container->lock);
> >> +
> >> +		ret = tce_iommu_remove_window(container, remove.start_addr);
> >> +
> >> +		mutex_unlock(&container->lock);
> >> +
> >> +		return ret;
> >> +	}
> >>   	}
> >>
> >>   	return -ENOTTY;
> >> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> >> index fbc5286..7fa02e1 100644
> >> --- a/include/uapi/linux/vfio.h
> >> +++ b/include/uapi/linux/vfio.h
> >> @@ -457,9 +457,13 @@ struct vfio_iommu_type1_dma_unmap {
> >>    */
> >>   struct vfio_iommu_spapr_tce_info {
> >>   	__u32 argsz;
> >> -	__u32 flags;			/* reserved for future use */
> >> +	__u32 flags;
> >> +#define VFIO_IOMMU_INFO_DDW	(1 << 0)	/* DDW supported */
> >
> > s/VFIO_IOMMU_INFO_DDW/VFIO_IOMMU_SPAPR_INFO_DDW/
> 
> VFIO_IOMMU_INFO_PGSIZES is not VFIO_IOMMU_TYPE1_INFO_PGSIZES ;-) but I'll fix.

Too late to complain about that one now.

> >
> >>   	__u32 dma32_window_start;	/* 32 bit window start (bytes) */
> >>   	__u32 dma32_window_size;	/* 32 bit window size (bytes) */
> >> +	__u64 pgsizes;			/* Bitmap of supported page sizes */
> >> +	__u32 max_dynamic_windows_supported;
> >> +	__u32 levels;
> >
> > Please indicate that the last 3 fields are only supported with DDW.
> 
> 
> Like this?
> 
> ===
> - * A flag will need to be added if other page sizes are supported,
> - * so as defined here, it is always 4k.
> + * Flags supported:
> + * - VFIO_IOMMU_INFO_DDW: informs the userspace that dynamic DMA windows
> + *   (DDW) support is present. @pgsizes, @max_dynamic_windows_supported,
> + *   @levels are only supported when DDW is present.
>    */
>   struct vfio_iommu_spapr_tce_info {
>          __u32 argsz;

Yeah, I might try to say that the value of those fields is only valid
when the ddw flag is set.  Additionally, the comment next to the field
could say "Only with DDW".

You might also want to consider if you want all 3 of these fields
toggled by a single flag.  Does it make sense to advertise pgsize,
max-ddw, and levels each as a separate flag?  Alternatively, you could
embed at struct to make the association to the flag more clear:

struct blah_blah_ddw_info {
	__u64 pgsizes;
	__u32 max_ddws;
	__u32 levels;
};

struct vfio_iommu_spapr_tce_info {
	...
	struct blah_blah_ddw_info ddw; /* only valid with DDW flag */

> ===
> 
> 
> >
> >>   };
> >>
> >>   #define VFIO_IOMMU_SPAPR_TCE_GET_INFO	_IO(VFIO_TYPE, VFIO_BASE + 12)
> >> @@ -520,6 +524,41 @@ struct vfio_iommu_spapr_register_memory {
> >>    */
> >>   #define VFIO_IOMMU_SPAPR_UNREGISTER_MEMORY	_IO(VFIO_TYPE, VFIO_BASE + 18)
> >>
> >> +/**
> >> + * VFIO_IOMMU_SPAPR_TCE_CREATE - _IOWR(VFIO_TYPE, VFIO_BASE + 19, struct vfio_iommu_spapr_tce_create)
> >> + *
> >> + * Creates an additional TCE table and programs it (sets a new DMA window)
> >> + * to every IOMMU group in the container. It receives page shift, window
> >> + * size and number of levels in the TCE table being created.
> >> + *
> >> + * It allocates and returns an offset on a PCI bus of the new DMA window.
> >> + */
> >> +struct vfio_iommu_spapr_tce_create {
> >> +	__u32 argsz;
> >> +	__u32 flags;
> >> +	/* in */
> >> +	__u32 page_shift;
> >> +	__u64 window_size;
> >> +	__u32 levels;
> >> +	/* out */
> >> +	__u64 start_addr;
> >> +};
> >> +#define VFIO_IOMMU_SPAPR_TCE_CREATE	_IO(VFIO_TYPE, VFIO_BASE + 19)
> >> +
> >> +/**
> >> + * VFIO_IOMMU_SPAPR_TCE_REMOVE - _IOW(VFIO_TYPE, VFIO_BASE + 20, struct vfio_iommu_spapr_tce_remove)
> >> + *
> >> + * Unprograms a TCE table from all groups in the container and destroys it.
> >> + * It receives a PCI bus offset as a window id.
> >> + */
> >> +struct vfio_iommu_spapr_tce_remove {
> >> +	__u32 argsz;
> >> +	__u32 flags;
> >> +	/* in */
> >> +	__u64 start_addr;
> >> +};
> >> +#define VFIO_IOMMU_SPAPR_TCE_REMOVE	_IO(VFIO_TYPE, VFIO_BASE + 20)
> >> +
> >>   /* ***************************************************************** */
> >>
> >>   #endif /* _UAPIVFIO_H */
> >
> >
> >
> 
>