[PATCH v2 0/3] KVM: PPC: Fixes for Book3S HV HPT locking and paired-single decoding

[PATCH v2 0/3] KVM: PPC: Fixes for Book3S HV HPT locking and paired-single decoding

[Amit Machhiwal]

This series addresses bug fixes across KVM PPC Book3S HV
locking/synchronization and paired-single instruction decoding.

Patches 1 & 2 fix synchronization and preemption issues introduced in
commit 6165d5dd99db ("KVM: PPC: Book3S HV: add virtual mode handlers for
HPT hcalls and page faults"):

- Patch 1 adds SRCU read lock protection when walking memslots during
  virtual-mode HPT hcalls to prevent use-after-free races with concurrent
  memslot updates.

- Patch 2 adds preempt_disable() around all virtual-mode HPTE bit-lock
  holders — both the guest vCPU paths (kvmppc_hpte_hv_fault() and
  kvmppc_pseries_do_hpt_hcall()) and the host-side paths
  (kvm_unmap_rmapp(), kvm_age_rmapp(), kvm_test_clear_dirty_npages(),
  resize_hpt_rehash_hpte()) — to prevent CPU stalls and deadlocks on
  preemption.

Patch 3 fixes paired-single D-form instruction emulation:

- Patch 3 fixes get_d_signext() to correctly extract the full 12-bit D
  displacement field and perform proper two's-complement sign extension.

Testing:
========
All test kernel builds were compiled with CONFIG_DEBUG_ATOMIC_SLEEP=y.
The following scenarios were verified:

1. Power9 PowerNV (L0) in Radix mode:
   - Booted L0 host with kernel containing all 3 patches.
   - Booted KVM guests in both Radix and Hash modes.
   - Ran kernel build workload inside guests — no errors observed.
   - Booted L1 guest with the same kernel — booted and ran cleanly.

2. Power9 PowerNV (L0) in Hash mode:
   - Booted L0 host with kernel containing all 3 patches.
   - Booted KVM guest in Hash mode.
   - Ran kernel build workload inside guest — no errors observed.
   - Booted L1 nested guest with the same kernel — booted and ran cleanly.

3. Power10 LPAR (L1):
   - Booted Power10 LPAR with the patched kernel.
   - Booted KVM guest and ran workloads — no errors observed.

Changes in v2:
==============
- v1: https://lore.kernel.org/all/20260928122837.8782-1-amachhiw@linux.ibm.com/
- Patch 2: Extended preempt_disable()/preempt_enable() coverage to also wrap
  the HPTE bit-lock hold windows in four host-side virtual-mode functions:
  kvm_unmap_rmapp(), kvm_age_rmapp(), kvm_test_clear_dirty_npages(), and
  resize_hpt_rehash_hpte(). These were identified as vulnerable by Sashiko AI
  review and confirmed correct by audit.
- Dropped Reviewed-by from Ritesh as the patch was materially extended.
- Patch 2: Added warning comment above kvmppc_pseries_do_hpt_hcall()
  documenting the preemption requirement.

Amit Machhiwal (3):
  KVM: PPC: Book3S HV: Add SRCU protection for virtual-mode HPT hcalls
  KVM: PPC: Book3S HV: Add preempt_disable() around virtual-mode HPTE
    bit-lock users
  KVM: PPC: Fix get_d_signext() 12-bit displacement for paired-single
    D-form

 arch/powerpc/kvm/book3s_64_mmu_hv.c      | 12 ++++
 arch/powerpc/kvm/book3s_hv.c             | 80 +++++++++++++-----------
 arch/powerpc/kvm/book3s_paired_singles.c |  7 +--
 3 files changed, 58 insertions(+), 41 deletions(-)


base-commit: 551c722f40809618230001baccf219193e22fc5a
-- 
2.54.0 (Apple Git-157)

[PATCH v2 1/3] KVM: PPC: Book3S HV: Add SRCU protection for virtual-mode HPT hcalls

[Amit Machhiwal]

The virtual-mode HPT hcall handlers (H_ENTER, H_REMOVE, H_BULK_REMOVE,
H_CLEAR_REF, H_CLEAR_MOD) call into book3s_hv_rm_mmu.c, which accesses
memslots via kvm_memslots_raw().  kvm_memslots_raw() uses
rcu_dereference_raw_check() to bypass SRCU lockdep annotation checking.
This is safe in real mode because the entire guest entry/exit is wrapped
in srcu_read_lock/unlock inside kvmppc_run_core() and
kvmhv_run_single_vcpu().

However, since commit 6165d5dd99db ("KVM: PPC: Book3S HV: add virtual
mode handlers for HPT hcalls and page faults"), these same handlers are
also executed in virtual mode via kvmppc_pseries_do_hcall(), which runs
after SRCU has already been released on guest exit.

A concurrent KVM_SET_USER_MEMORY_REGION deletion or move can therefore
call synchronize_srcu_expedited() — which does not wait for this thread —
and then kfree(slot) and vfree(slot->arch.rmap) while the hcall handler
still holds a raw pointer to the memslot.  lock_rmap() then writes to
freed vmalloc memory, leading to use-after-free and memory corruption.
Because kvm_memslots_raw() suppresses lockdep checks, this race is
entirely silent.

Fix this by factoring out the 7 HPT hcall handlers (H_REMOVE, H_ENTER,
H_READ, H_CLEAR_MOD, H_CLEAR_REF, H_PROTECT, H_BULK_REMOVE) into a
helper function, kvmppc_pseries_do_hpt_hcall(), and wrapping its call
site in srcu_read_lock(&kvm->srcu) / srcu_read_unlock(&kvm->srcu, idx).
Targeting only the HPT hcalls avoids wrapping non-HPT hcalls that sleep
(such as H_CONFER, H_REGISTER_VPA, H_PAGE_INIT) or handlers that already
acquire SRCU internally (such as H_RTAS).

Fixes: 6165d5dd99db ("KVM: PPC: Book3S HV: add virtual mode handlers for HPT hcalls and page faults")
Cc: stable@vger.kernel.org # v5.14+
Suggested-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
---
 arch/powerpc/kvm/book3s_hv.c | 70 ++++++++++++++++++------------------
 1 file changed, 35 insertions(+), 35 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index dbac3573b2c8..aa51968e206a 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -1159,6 +1159,38 @@ static long kvmppc_h_rpt_invalidate(struct kvm_vcpu *vcpu,
 	return H_SUCCESS;
 }
 
+static long kvmppc_pseries_do_hpt_hcall(struct kvm_vcpu *vcpu, unsigned long req)
+{
+	switch (req) {
+	case H_REMOVE:
+		return kvmppc_h_remove(vcpu, kvmppc_get_gpr(vcpu, 4),
+				       kvmppc_get_gpr(vcpu, 5),
+				       kvmppc_get_gpr(vcpu, 6));
+	case H_ENTER:
+		return kvmppc_h_enter(vcpu, kvmppc_get_gpr(vcpu, 4),
+				      kvmppc_get_gpr(vcpu, 5),
+				      kvmppc_get_gpr(vcpu, 6),
+				      kvmppc_get_gpr(vcpu, 7));
+	case H_READ:
+		return kvmppc_h_read(vcpu, kvmppc_get_gpr(vcpu, 4),
+				     kvmppc_get_gpr(vcpu, 5));
+	case H_CLEAR_MOD:
+		return kvmppc_h_clear_mod(vcpu, kvmppc_get_gpr(vcpu, 4),
+					  kvmppc_get_gpr(vcpu, 5));
+	case H_CLEAR_REF:
+		return kvmppc_h_clear_ref(vcpu, kvmppc_get_gpr(vcpu, 4),
+					  kvmppc_get_gpr(vcpu, 5));
+	case H_PROTECT:
+		return kvmppc_h_protect(vcpu, kvmppc_get_gpr(vcpu, 4),
+					kvmppc_get_gpr(vcpu, 5),
+					kvmppc_get_gpr(vcpu, 6));
+	case H_BULK_REMOVE:
+		return kvmppc_h_bulk_remove(vcpu);
+	}
+
+	return H_FUNCTION;
+}
+
 int kvmppc_pseries_do_hcall(struct kvm_vcpu *vcpu)
 {
 	struct kvm *kvm = vcpu->kvm;
@@ -1174,47 +1206,15 @@ int kvmppc_pseries_do_hcall(struct kvm_vcpu *vcpu)
 
 	switch (req) {
 	case H_REMOVE:
-		ret = kvmppc_h_remove(vcpu, kvmppc_get_gpr(vcpu, 4),
-					kvmppc_get_gpr(vcpu, 5),
-					kvmppc_get_gpr(vcpu, 6));
-		if (ret == H_TOO_HARD)
-			return RESUME_HOST;
-		break;
 	case H_ENTER:
-		ret = kvmppc_h_enter(vcpu, kvmppc_get_gpr(vcpu, 4),
-					kvmppc_get_gpr(vcpu, 5),
-					kvmppc_get_gpr(vcpu, 6),
-					kvmppc_get_gpr(vcpu, 7));
-		if (ret == H_TOO_HARD)
-			return RESUME_HOST;
-		break;
 	case H_READ:
-		ret = kvmppc_h_read(vcpu, kvmppc_get_gpr(vcpu, 4),
-					kvmppc_get_gpr(vcpu, 5));
-		if (ret == H_TOO_HARD)
-			return RESUME_HOST;
-		break;
 	case H_CLEAR_MOD:
-		ret = kvmppc_h_clear_mod(vcpu, kvmppc_get_gpr(vcpu, 4),
-					kvmppc_get_gpr(vcpu, 5));
-		if (ret == H_TOO_HARD)
-			return RESUME_HOST;
-		break;
 	case H_CLEAR_REF:
-		ret = kvmppc_h_clear_ref(vcpu, kvmppc_get_gpr(vcpu, 4),
-					kvmppc_get_gpr(vcpu, 5));
-		if (ret == H_TOO_HARD)
-			return RESUME_HOST;
-		break;
 	case H_PROTECT:
-		ret = kvmppc_h_protect(vcpu, kvmppc_get_gpr(vcpu, 4),
-					kvmppc_get_gpr(vcpu, 5),
-					kvmppc_get_gpr(vcpu, 6));
-		if (ret == H_TOO_HARD)
-			return RESUME_HOST;
-		break;
 	case H_BULK_REMOVE:
-		ret = kvmppc_h_bulk_remove(vcpu);
+		idx = srcu_read_lock(&kvm->srcu);
+		ret = kvmppc_pseries_do_hpt_hcall(vcpu, req);
+		srcu_read_unlock(&kvm->srcu, idx);
 		if (ret == H_TOO_HARD)
 			return RESUME_HOST;
 		break;
-- 
2.54.0 (Apple Git-157)

[PATCH v2 2/3] KVM: PPC: Book3S HV: Add preempt_disable() around virtual-mode HPTE bit-lock users

[Amit Machhiwal]

kvmppc_hv_find_lock_hpte() requires virtual-mode callers to run with
preemption disabled, because it can return with HPTE_V_HVLOCK still held
until the caller later unlocks the HPTE.  Existing virtual-mode callers
in book3s_64_mmu_hv.c already follow that rule, but several paths do
not.

kvmppc_handle_exit_hv() calls kvmppc_hpte_hv_fault() for hash-mode
data-side and instruction-side faults after guest exit with preemption
enabled.  kvmppc_pseries_do_hcall() executes virtual-mode HPT hcall
handlers via kvmppc_pseries_do_hpt_hcall() with preemption enabled; the
handlers for H_ENTER, H_REMOVE, H_READ, H_CLEAR_MOD, H_CLEAR_REF,
H_PROTECT, and H_BULK_REMOVE all spin on try_lock_hpte() or lock_rmap().
H_ENTER also reaches kvmppc_do_h_enter(), which uses arch_spin_lock() on
kvm->mmu_lock.  That raw lock choice is intentional because
kvmppc_do_h_enter() is also called from real-mode paths, so the correct
fix is to establish the proper preemption context at the virtual-mode
caller boundary.

On the host side, kvm_unmap_rmapp(), kvm_age_rmapp(),
kvm_test_clear_dirty_npages(), and resize_hpt_rehash_hpte() also acquire
HPTE_V_HVLOCK via try_lock_hpte() in process context with preemption
enabled, serving MMU notifier callbacks, dirty-log harvesting, and HPT
resize respectively.

If any of these threads is preempted while holding HPTE_V_HVLOCK, any
other thread on the same CPU spinning on the same bit-lock can never
make progress, as the lock owner cannot be rescheduled to release it.
This is particularly acute when the spinning thread has preemption
disabled: it will never yield, causing a permanent CPU hang.

Fix this by adding preempt_disable()/preempt_enable() pairs around the
two kvmppc_hpte_hv_fault() call sites in kvmppc_handle_exit_hv(), around
the kvmppc_pseries_do_hpt_hcall() invocation in kvmppc_pseries_do_hcall(),
and around the try_lock_hpte() hold windows in kvm_unmap_rmapp(),
kvm_age_rmapp(), kvm_test_clear_dirty_npages(), and resize_hpt_rehash_hpte().
On failed lock acquisition the guard is released before the cpu_relax()
spin so the lock owner can be scheduled.

Fixes: 6165d5dd99db ("KVM: PPC: Book3S HV: add virtual mode handlers for HPT hcalls and page faults")
Cc: stable@vger.kernel.org # v5.14+
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
---
Changes in v2:
- Extended preempt_disable()/preempt_enable() to also cover four
  host-side virtual-mode HPTE bit-lock users in book3s_64_mmu_hv.c.
- Added warning comment above kvmppc_pseries_do_hpt_hcall().
- Dropped Reviewed-by as the patch was materially extended.

 arch/powerpc/kvm/book3s_64_mmu_hv.c | 12 ++++++++++++
 arch/powerpc/kvm/book3s_hv.c        | 10 ++++++++++
 2 files changed, 22 insertions(+)

diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
index 2ccb3d138f46..59da958e09cb 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
@@ -823,7 +823,9 @@ static void kvm_unmap_rmapp(struct kvm *kvm, struct kvm_memory_slot *memslot,
 		 */
 		i = *rmapp & KVMPPC_RMAP_INDEX;
 		hptep = (__be64 *) (kvm->arch.hpt.virt + (i << 4));
+		preempt_disable();
 		if (!try_lock_hpte(hptep, HPTE_V_HVLOCK)) {
+			preempt_enable();
 			/* unlock rmap before spinning on the HPTE lock */
 			unlock_rmap(rmapp);
 			while (be64_to_cpu(hptep[0]) & HPTE_V_HVLOCK)
@@ -834,6 +836,7 @@ static void kvm_unmap_rmapp(struct kvm *kvm, struct kvm_memory_slot *memslot,
 		kvmppc_unmap_hpte(kvm, i, memslot, rmapp, gfn);
 		unlock_rmap(rmapp);
 		__unlock_hpte(hptep, be64_to_cpu(hptep[0]));
+		preempt_enable();
 	}
 }
 
@@ -909,7 +912,9 @@ static bool kvm_age_rmapp(struct kvm *kvm, struct kvm_memory_slot *memslot,
 		if (!(be64_to_cpu(hptep[1]) & HPTE_R_R))
 			continue;
 
+		preempt_disable();
 		if (!try_lock_hpte(hptep, HPTE_V_HVLOCK)) {
+			preempt_enable();
 			/* unlock rmap before spinning on the HPTE lock */
 			unlock_rmap(rmapp);
 			while (be64_to_cpu(hptep[0]) & HPTE_V_HVLOCK)
@@ -928,6 +933,7 @@ static bool kvm_age_rmapp(struct kvm *kvm, struct kvm_memory_slot *memslot,
 			ret = true;
 		}
 		__unlock_hpte(hptep, be64_to_cpu(hptep[0]));
+		preempt_enable();
 	} while ((i = j) != head);
 
 	unlock_rmap(rmapp);
@@ -1043,7 +1049,9 @@ static int kvm_test_clear_dirty_npages(struct kvm *kvm, unsigned long *rmapp)
 		    (!hpte_is_writable(hptep1) || vcpus_running(kvm)))
 			continue;
 
+		preempt_disable();
 		if (!try_lock_hpte(hptep, HPTE_V_HVLOCK)) {
+			preempt_enable();
 			/* unlock rmap before spinning on the HPTE lock */
 			unlock_rmap(rmapp);
 			while (hptep[0] & cpu_to_be64(HPTE_V_HVLOCK))
@@ -1054,6 +1062,7 @@ static int kvm_test_clear_dirty_npages(struct kvm *kvm, unsigned long *rmapp)
 		/* Now check and modify the HPTE */
 		if (!(hptep[0] & cpu_to_be64(HPTE_V_VALID))) {
 			__unlock_hpte(hptep, be64_to_cpu(hptep[0]));
+			preempt_enable();
 			continue;
 		}
 
@@ -1077,6 +1086,7 @@ static int kvm_test_clear_dirty_npages(struct kvm *kvm, unsigned long *rmapp)
 		v &= ~HPTE_V_ABSENT;
 		v |= HPTE_V_VALID;
 		__unlock_hpte(hptep, v);
+		preempt_enable();
 	} while ((i = j) != head);
 
 	unlock_rmap(rmapp);
@@ -1219,6 +1229,7 @@ static unsigned long resize_hpt_rehash_hpte(struct kvm_resize_hpt *resize,
 	if (!(vpte & HPTE_V_VALID) && !(vpte & HPTE_V_ABSENT))
 		return 0; /* nothing to do */
 
+	preempt_disable();
 	while (!try_lock_hpte(hptep, HPTE_V_HVLOCK))
 		cpu_relax();
 
@@ -1346,6 +1357,7 @@ static unsigned long resize_hpt_rehash_hpte(struct kvm_resize_hpt *resize,
 
 out:
 	unlock_hpte(hptep, vpte);
+	preempt_enable();
 	return ret;
 }
 
diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index aa51968e206a..0b7743bb89d9 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -1159,6 +1159,10 @@ static long kvmppc_h_rpt_invalidate(struct kvm_vcpu *vcpu,
 	return H_SUCCESS;
 }
 
+/*
+ * Must be called with preemption disabled. The HPT hcall handlers spin
+ * on HPTE bit-locks and cannot make any blocking/sleeping calls.
+ */
 static long kvmppc_pseries_do_hpt_hcall(struct kvm_vcpu *vcpu, unsigned long req)
 {
 	switch (req) {
@@ -1212,9 +1216,11 @@ int kvmppc_pseries_do_hcall(struct kvm_vcpu *vcpu)
 	case H_CLEAR_REF:
 	case H_PROTECT:
 	case H_BULK_REMOVE:
+		preempt_disable();
 		idx = srcu_read_lock(&kvm->srcu);
 		ret = kvmppc_pseries_do_hpt_hcall(vcpu, req);
 		srcu_read_unlock(&kvm->srcu, idx);
+		preempt_enable();
 		if (ret == H_TOO_HARD)
 			return RESUME_HOST;
 		break;
@@ -1834,8 +1840,10 @@ static int kvmppc_handle_exit_hv(struct kvm_vcpu *vcpu,
 		else
 			vsid = vcpu->arch.fault_gpa;
 
+		preempt_disable();
 		err = kvmppc_hpte_hv_fault(vcpu, vcpu->arch.fault_dar,
 				vsid, vcpu->arch.fault_dsisr, true);
+		preempt_enable();
 		if (err == 0) {
 			r = RESUME_GUEST;
 		} else if (err == -1 || err == -2) {
@@ -1881,8 +1889,10 @@ static int kvmppc_handle_exit_hv(struct kvm_vcpu *vcpu,
 		else
 			vsid = vcpu->arch.fault_gpa;
 
+		preempt_disable();
 		err = kvmppc_hpte_hv_fault(vcpu, vcpu->arch.fault_dar,
 				vsid, vcpu->arch.fault_dsisr, false);
+		preempt_enable();
 		if (err == 0) {
 			r = RESUME_GUEST;
 		} else if (err == -1) {
-- 
2.54.0 (Apple Git-157)

[PATCH v2 3/3] KVM: PPC: Fix get_d_signext() 12-bit displacement for paired-single D-form

[Amit Machhiwal]

get_d_signext() has two compounding bugs since its introduction in 2010:

1. The extraction mask 0x8FF silently drops bits 8-10 of the 12-bit D
   field (PPC ISA bits 21-23), corrupting any displacement that has any
   of those bits set.

2. The sign-magnitude idiom "-(d & 0x7ff)" is wrong for two's-complement:
   for D=0xFFC (encoding of -4) it returns -252 instead of -4.

Together these errors produce a wrong effective address for psq_l, psq_lu,
psq_st, and psq_stu whenever the displacement is negative or is a positive
value >= 0x100 with bits 8-10 set — essentially any real-world paired-
single stack-relative access.

The D field occupies the bottom 12 bits of the instruction word (confirmed
by the adjacent W and I extractions via inst_get_field(inst,16,16) and
inst_get_field(inst,17,19)). Replace the open-coded logic with the standard
sign_extend32(inst & 0xfff, 11), which correctly performs two's-complement
sign extension from 12 bits to 32 bits.

Fixes: 831317b605e7 ("KVM: PPC: Implement Paired Single emulation")
Cc: stable@vger.kernel.org
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
---
 arch/powerpc/kvm/book3s_paired_singles.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_paired_singles.c b/arch/powerpc/kvm/book3s_paired_singles.c
index bc39c76c9d9f..532f96293de0 100644
--- a/arch/powerpc/kvm/book3s_paired_singles.c
+++ b/arch/powerpc/kvm/book3s_paired_singles.c
@@ -479,12 +479,7 @@ static bool kvmppc_inst_is_paired_single(struct kvm_vcpu *vcpu, u32 inst)
 
 static int get_d_signext(u32 inst)
 {
-	int d = inst & 0x8ff;
-
-	if (d & 0x800)
-		return -(d & 0x7ff);
-
-	return (d & 0x7ff);
+	return sign_extend32(inst & 0xfff, 11);
 }
 
 static int kvmppc_ps_three_in(struct kvm_vcpu *vcpu, bool rc,
-- 
2.54.0 (Apple Git-157)