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

[Amit Machhiwal <amachhiw@linux.ibm.com>]

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)