Hi Christoffer,
On 02/11/18 13:43, Christoffer Dall wrote:
On Fri, Nov 02, 2018 at 10:26:37AM +0000, Suzuki K Poulose wrote:
Hi Christoffer,
On 02/11/18 07:53, Christoffer Dall wrote:
There are two things we need to take care of when we create block
mappings in the stage 2 page tables:
(1) The alignment within a PMD between the host address range and the
guest IPA range must be the same, since otherwise we end up mapping
pages with the wrong offset.
(2) The head and tail of a memory slot may not cover a full block
size, and we have to take care to not map those with block
descriptors, since we could expose memory to the guest that the host
did not intend to expose.
So far, we have been taking care of (1), but not (2), and our commentary
describing (1) was somewhat confusing.
This commit attempts to factor out the checks of both into a common
function, and if we don't pass the check, we won't attempt any PMD
mappings for neither hugetlbfs nor THP.
Note that we used to only check the alignment for THP, not for
hugetlbfs, but as far as I can tell the check needs to be applied to
both scenarios.
Cc: Ralph Palutke <ralph.palutke@xxxxxx>
Cc: Lukas Braun <koomi@xxxxxxxxxxx>
Reported-by: Lukas Braun <koomi@xxxxxxxxxxx>
Signed-off-by: Christoffer Dall <christoffer.dall@xxxxxxx>
---
virt/kvm/arm/mmu.c | 79 +++++++++++++++++++++++++++++++++++++---------
1 file changed, 64 insertions(+), 15 deletions(-)
diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
index 5eca48bdb1a6..4e7572656b5c 100644
--- a/virt/kvm/arm/mmu.c
+++ b/virt/kvm/arm/mmu.c
@@ -1470,6 +1470,63 @@ static void kvm_send_hwpoison_signal(unsigned long address,
send_sig_mceerr(BUS_MCEERR_AR, (void __user *)address, lsb, current);
}
+static bool fault_supports_stage2_pmd_mappings(struct kvm_memory_slot *memslot,
+ unsigned long hva)
+{
+ gpa_t gpa_start, gpa_end;
+ hva_t uaddr_start, uaddr_end;
+ size_t size;
+
+ size = memslot->npages * PAGE_SIZE;
+
+ gpa_start = memslot->base_gfn << PAGE_SHIFT;
+ gpa_end = gpa_start + size;
+
+ uaddr_start = memslot->userspace_addr;
+ uaddr_end = uaddr_start + size;
+
+ /*
+ * Pages belonging to memslots that don't have the same alignment
+ * within a PMD for userspace and IPA cannot be mapped with stage-2
+ * PMD entries, because we'll end up mapping the wrong pages.
+ *
+ * Consider a layout like the following:
+ *
+ * memslot->userspace_addr:
+ * +-----+--------------------+--------------------+---+
+ * |abcde|fgh Stage-1 PMD | Stage-1 PMD tv|xyz|
+ * +-----+--------------------+--------------------+---+
+ *
+ * memslot->base_gfn << PAGE_SIZE:
+ * +---+--------------------+--------------------+-----+
+ * |abc|def Stage-2 PMD | Stage-2 PMD |tvxyz|
+ * +---+--------------------+--------------------+-----+
+ *
+ * If we create those stage-2 PMDs, we'll end up with this incorrect
+ * mapping:
+ * d -> f
+ * e -> g
+ * f -> h
+ */
Nice ! The comment makes it so easy to understand the problem.
+ if ((gpa_start & ~S2_PMD_MASK) != (uaddr_start & ~S2_PMD_MASK))
+ return false;
+
+ /*
+ * Next, let's make sure we're not trying to map anything not covered
+ * by the memslot. This means we have to prohibit PMD size mappings
+ * for the beginning and end of a non-PMD aligned and non-PMD sized
+ * memory slot (illustrated by the head and tail parts of the
+ * userspace view above containing pages 'abcde' and 'xyz',
+ * respectively).
+ *
+ * Note that it doesn't matter if we do the check using the
+ * userspace_addr or the base_gfn, as both are equally aligned (per
+ * the check above) and equally sized.
+ */
+ return (hva & S2_PMD_MASK) >= uaddr_start &&
+ (hva & S2_PMD_MASK) + S2_PMD_SIZE <= uaddr_end;
Shouldn't this be :
(hva & S2_PMD_MASK) + S2_PMD_SIZE < uaddr_end;
or
(hva & S2_PMD_MASK) + S2_PMD_SIZE <= (uaddr_end - 1);
Let's consider an example. Imagine a 4MB region (1024 4K pages) start
at 0, and we want to ensure that the address is within one of the 2MB
pages.
That means, size will be:
size = 1024 * 4096 = 4194304
uaddr_start = 0
uaddr_end = 0 + 4194304 = 4194304
and the comparison check for anything in the first 2MB region becomes
return 0 >= 0 && 2097152 <= 4194304;
which is what we want.
The comparison check for anything in the second 2MB region becomes
return 2097152 >= 0 && 4194304 <= 4194304;
which is also what we want.
If we had done a stricly less than or subtracted one from uaddr_end this
check would fail, which we don't want.
Am I missing something?
You're right. Sorry for the noise. I ignored the (& S2_PMD_MASK) which
should make sure we are comparing the outer-boundary of the hugepage.
+}
+
static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
struct kvm_memory_slot *memslot, unsigned long hva,
unsigned long fault_status)
@@ -1495,6 +1552,12 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
return -EFAULT;
}
+ if (!fault_supports_stage2_pmd_mappings(memslot, hva))
+ force_pte = true;
+
+ if (logging_active)
+ force_pte = true;
super minor nit: If we combine the checks, may be we could skip checking
the alignments, when logging is active.
if (logging_active ||
!fault_supports_stage2_pmd_mappings(memslot, hva))
force_pte = true;
I prefer the way the code is written now, because logging active is a
logically very separate concept from checking the boundaries, and this
is realy the slow path anyway, so not sure there's any measurable
benefit.
More generally, I'd like user_mem_abort() to become more like:
if (check1())
conditionr1 = value;
if (check2())
condition2 = value;
if (check3())
condition3 = value;
manipulate_page_tables(condition1, condition2, condition3, ...);
And I'll have a got at that following Punit's PUD huge pages.
Ok.
FWIW,
Reviewed-by: Suzuki K Poulose <suzuki.poulos@xxxxxxx>
Suzuki
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