On Thu, Sep 01, 2022 at 05:45:08PM +0000, Ashok Raj wrote:
> Hi Kirill,
>
> On Tue, Aug 30, 2022 at 04:00:53AM +0300, Kirill A. Shutemov wrote:
> > Linear Address Masking[1] (LAM) modifies the checking that is applied to
> > 64-bit linear addresses, allowing software to use of the untranslated
> > address bits for metadata.
>
> We discussed this internally, but didn't bubble up here.
>
> Given that we are working on enabling Shared Virtual Addressing (SVA)
> within the IOMMU. This permits user to share VA directly with the device,
> and the device can participate even in fixing page-faults and such.
>
> IOMMU enforces canonical addressing, since we are hijacking the top order
> bits for meta-data, it will fail sanity check and we would return a failure
> back to device on any page-faults from device.
>
> It also complicates how device TLB and ATS work, and needs some major
> improvements to detect device capability to accept tagged pointers, adjust
> the devtlb to act accordingly.
>
>
> Both are orthogonal features, but there is an intersection of both
> that are fundamentally incompatible.
>
> Its even more important, since an application might be using SVA under the
> cover provided by some library that's used without their knowledge.
>
> The path would be:
>
> 1. Ensure both LAM and SVM are incompatible by design, without major
> changes.
> - If LAM is enabled already and later SVM enabling is requested by
> user, that should fail. and Vice versa.
> - Provide an API to user to ask for opt-out. Now they know they
> must sanitize the pointers before sending to device, or the
> working set is already isolated and needs no work.
The patch below implements something like this. It is PoC, build-tested only.
To be honest, I hate it. It is clearly a layering violation. It feels
dirty. But I don't see any better way as we tie orthogonal features
together.
Also I have no idea how to make forced PASID allocation if LAM enabled.
What the API has to look like?
Any comments?
> 2. I suppose for any syscalls that take tagged pointers you would maybe
> relax checks for how many bits to ignore for canonicallity. This is
> required so user don't need to do the same for everything sanitization
> before every syscall.
I'm not quite follow this. For syscalls that allow tagged pointers, we do
untagged_addr() now. Not sure what else needed.
> If you have it fail, the library might choose a less optimal path if one is
> available.
>
> Cheers,
> Ashok
diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index a31e27b95b19..e5c04ced36c9 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -23,5 +23,6 @@
#define ARCH_GET_UNTAG_MASK 0x4001
#define ARCH_ENABLE_TAGGED_ADDR 0x4002
#define ARCH_GET_MAX_TAG_BITS 0x4003
+#define ARCH_ENABLE_TAGGED_ADDR_FORCED 0x4004
#endif /* _ASM_X86_PRCTL_H */
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 337f80a0862f..7d89a2fd1a55 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -774,7 +774,8 @@ static bool lam_u48_allowed(void)
#define LAM_U48_BITS 15
#define LAM_U57_BITS 6
-static int prctl_enable_tagged_addr(struct mm_struct *mm, unsigned long nr_bits)
+static int prctl_enable_tagged_addr(struct mm_struct *mm, unsigned long nr_bits,
+ bool forced)
{
int ret = 0;
@@ -793,6 +794,11 @@ static int prctl_enable_tagged_addr(struct mm_struct *mm, unsigned long nr_bits)
goto out;
}
+ if (pasid_valid(mm->pasid) && !forced) {
+ ret = -EBUSY;
+ goto out;
+ }
+
if (!nr_bits) {
ret = -EINVAL;
goto out;
@@ -910,7 +916,9 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
return put_user(task->mm->context.untag_mask,
(unsigned long __user *)arg2);
case ARCH_ENABLE_TAGGED_ADDR:
- return prctl_enable_tagged_addr(task->mm, arg2);
+ return prctl_enable_tagged_addr(task->mm, arg2, false);
+ case ARCH_ENABLE_TAGGED_ADDR_FORCED:
+ return prctl_enable_tagged_addr(task->mm, arg2, true);
case ARCH_GET_MAX_TAG_BITS: {
int nr_bits;
diff --git a/drivers/iommu/iommu-sva-lib.c b/drivers/iommu/iommu-sva-lib.c
index 106506143896..a6ec17de1937 100644
--- a/drivers/iommu/iommu-sva-lib.c
+++ b/drivers/iommu/iommu-sva-lib.c
@@ -4,6 +4,7 @@
*/
#include <linux/mutex.h>
#include <linux/sched/mm.h>
+#include <asm/mmu_context.h>
#include "iommu-sva-lib.h"
@@ -32,6 +33,15 @@ int iommu_sva_alloc_pasid(struct mm_struct *mm, ioasid_t min, ioasid_t max)
return -EINVAL;
mutex_lock(&iommu_sva_lock);
+
+ /* Serialize against LAM enabling */
+ mutex_lock(&mm->context.lock);
+
+ if (mm_lam_cr3_mask(mm)) {
+ ret = -EBUSY;
+ goto out;
+ }
+
/* Is a PASID already associated with this mm? */
if (pasid_valid(mm->pasid)) {
if (mm->pasid < min || mm->pasid >= max)
@@ -45,6 +55,7 @@ int iommu_sva_alloc_pasid(struct mm_struct *mm, ioasid_t min, ioasid_t max)
else
mm_pasid_set(mm, pasid);
out:
+ mutex_unlock(&mm->context.lock);
mutex_unlock(&iommu_sva_lock);
return ret;
}
--
Kiryl Shutsemau / Kirill A. Shutemov
