Re: [PATCH] arm64: KVM: marking pages as XN in Stage-2 does not care about CTR_EL0.DIC
From: Marc Zyngier
Date: Tue Oct 13 2020 - 10:56:09 EST
On 2020-10-13 13:56, limingwang (A) wrote:
Hi Li,
On 2020-10-12 02:08, l00484210 wrote:
From: MingWang Li <limingwang@xxxxxxxxxx>
When testing the ARMv8.2-TTS2UXN feature, setting bits of XN is
unavailable.
Because the control bit CTR_EL0.DIC is set by default on system.
But when CTR_EL0.DIC is set, software does not need to flush icache
actively, instead of clearing XN bits.The patch, the commit id of
which is 6ae4b6e0578886eb36cedbf99f04031d93f9e315, has implemented
the
function of CTR_EL0.DIC.
Signed-off-by: MingWang Li <limingwang@xxxxxxxxxx>
Signed-off-by: Henglong Fan <fanhenglong@xxxxxxxxxx>
---
arch/arm64/include/asm/pgtable-prot.h | 12 +-----------
1 file changed, 1 insertion(+), 11 deletions(-)
diff --git a/arch/arm64/include/asm/pgtable-prot.h
b/arch/arm64/include/asm/pgtable-prot.h
index 4d867c6446c4..5feb94882bf7 100644
--- a/arch/arm64/include/asm/pgtable-prot.h
+++ b/arch/arm64/include/asm/pgtable-prot.h
@@ -79,17 +79,7 @@ extern bool arm64_use_ng_mappings;
__val; \
})
-#define PAGE_S2_XN \
- ({ \
- u64 __val; \
- if (cpus_have_const_cap(ARM64_HAS_CACHE_DIC)) \
- __val = 0; \
- else \
- __val = PTE_S2_XN; \
- __val; \
- })
-
-#define PAGE_S2 __pgprot(_PROT_DEFAULT | PAGE_S2_MEMATTR(NORMAL) |
PTE_S2_RDONLY | PAGE_S2_XN)
+#define PAGE_S2 __pgprot(_PROT_DEFAULT | PAGE_S2_MEMATTR(NORMAL) |
PTE_S2_RDONLY | PTE_S2_XN)
#define PAGE_S2_DEVICE __pgprot(_PROT_DEFAULT |
PAGE_S2_MEMATTR(DEVICE_nGnRE) | PTE_S2_RDONLY | PTE_S2_XN)
#define PAGE_NONE __pgprot(((_PAGE_DEFAULT) & ~PTE_VALID) |
PTE_PROT_NONE | PTE_RDONLY | PTE_NG | PTE_PXN | PTE_UXN)
I don't understand what you are trying to achieve here.
This whole point of not setting XN in the page tables when DIC is
present is to avoid a pointless permission fault at run time. At you
noticed above, no icache invalidation is necessary. So why would you
ever want to take a fault on exec the first place?
M.
--
Jazz is not dead. It just smells funny...
Hi Marc,
According to ARMv8.2-TTS2UXN feature, which extends the stage 2
translation table access
permissions to provide control of whether memory is executable at EL0
independent of whether
it is executable at EL1.
Testing this feature in some security scenario, for example, if I want
to grant execute permission
to some memory only for EL0, but it will failed. Because KVM clears XN
bits at first, this means that
the memory can be executable in both EL0 and El1.
KVM currently offers no support for this, and the only use we have for
XN so far is to to ensure Data/Instruction coherency when the CPU
doesn't offer it in HW.
So the execute permission is not granted when the page table is
created for the first time, then
grant the execute permission by setting xn, based on the actual
requirements.
And according to spec:
DIC, bit [29]
Instruction cache invalidation requirements for data to instruction
coherence.
0b0 Instruction cache invalidation to the Point of Unification is
required for data to instruction coherence.
0b1 Instruction cache invalidation to the Point of Unification is not
required for data to instruction coherence.
So when DIC is set, if the memory is changed to executable, the
hardware will flush icache.
No. The Icache *snoops* the Dcache at all times. Which is why we don't
need to trap on execution, and we can leave the guest run without
any intervention.
If as you said, I feel that DIC conflicts with ARMv8.2-TTS2UXN feature.
There is no conflict. KVM doesn't make use of all the bells and whistle
in the architecture, which is probably a good thing. If you feel that
there
is a need for S2UXN as a security feature, we can discuss how to expose
this
to the guest (because it definitely needs to know about that).
But setting XN when DIC is present for no other reason than "it may be
useful one day" doesn't make sense, and results in a massive performance
drop on the platforms that have DIC (and I really wish they all had it).
M.
--
Jazz is not dead. It just smells funny...