The slbmte instruction is legal in radix mode, including radix guest
mode. This means radix guests can load the SLB with arbitrary data.
KVM host does not clear the SLB when exiting a guest if it was a
radix guest, which would allow a rogue radix guest to use the SLB as
a side channel to communicate with other guests.
Fix this by ensuring the SLB is cleared when coming out of a radix
guest. Only the first 4 entries are a concern, because radix guests
always run with LPCR[UPRT]=1, which limits the reach of slbmte. slbia
is not used (except in a non-performance-critical path) because it
can clear cached translations.
Signed-off-by: Nicholas Piggin <npig...@gmail.com>
---
arch/powerpc/kvm/book3s_hv_rmhandlers.S | 39 ++++++++++++++++++++-----
1 file changed, 31 insertions(+), 8 deletions(-)
diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
index d5a9b57ec129..0e1f5bf168a1 100644
--- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
+++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
@@ -1157,6 +1157,20 @@ EXPORT_SYMBOL_GPL(__kvmhv_vcpu_entry_p9)
mr r4, r3
b fast_guest_entry_c
guest_exit_short_path:
+ /*
+ * Malicious or buggy radix guests may have inserted SLB entries
+ * (only 0..3 because radix always runs with UPRT=1), so these must
+ * be cleared here to avoid side-channels. slbmte is used rather
+ * than slbia, as it won't clear cached translations.
+ */
+ li r0,0
+ slbmte r0,r0
+ li r4,1
+ slbmte r0,r4
+ li r4,2
+ slbmte r0,r4
+ li r4,3
+ slbmte r0,r4
li r0, KVM_GUEST_MODE_NONE
stb r0, HSTATE_IN_GUEST(r13)
@@ -1469,7 +1483,7 @@ guest_exit_cont: /* r9 = vcpu, r12 = trap, r13 =
paca */
lbz r0, KVM_RADIX(r5)
li r5, 0
cmpwi r0, 0
- bne 3f /* for radix, save 0 entries */
+ bne 0f /* for radix, save 0 entries */
lwz r0,VCPU_SLB_NR(r9) /* number of entries in SLB */
mtctr r0
li r6,0
@@ -1490,12 +1504,9 @@ guest_exit_cont: /* r9 = vcpu, r12 = trap, r13 =
paca */
slbmte r0,r0
slbia
ptesync
-3: stw r5,VCPU_SLB_MAX(r9)
+ stw r5,VCPU_SLB_MAX(r9)
/* load host SLB entries */
-BEGIN_MMU_FTR_SECTION
- b 0f
-END_MMU_FTR_SECTION_IFSET(MMU_FTR_TYPE_RADIX)
ld r8,PACA_SLBSHADOWPTR(r13)
.rept SLB_NUM_BOLTED
@@ -1508,7 +1519,17 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_TYPE_RADIX)
slbmte r6,r5
1: addi r8,r8,16
.endr
-0:
+ b guest_bypass
+
+0: /* Sanitise radix guest SLB, see guest_exit_short_path comment. */
+ li r0,0
+ slbmte r0,r0
+ li r4,1
+ slbmte r0,r4
+ li r4,2
+ slbmte r0,r4
+ li r4,3
+ slbmte r0,r4
guest_bypass:
stw r12, STACK_SLOT_TRAP(r1)
@@ -3302,12 +3323,14 @@ END_FTR_SECTION_IFCLR(CPU_FTR_ARCH_300)
mtspr SPRN_CIABR, r0
mtspr SPRN_DAWRX0, r0
+ /* Clear hash and radix guest SLB, see guest_exit_short_path comment. */
+ slbmte r0, r0
+ slbia
+
BEGIN_MMU_FTR_SECTION
b 4f
END_MMU_FTR_SECTION_IFSET(MMU_FTR_TYPE_RADIX)
- slbmte r0, r0
- slbia
ptesync
ld r8, PACA_SLBSHADOWPTR(r13)
.rept SLB_NUM_BOLTED
--
2.23.0
