[PATCH 4.19 208/219] KVM: arm64: Ensure params is initialised when looking up sys register

[Greg Kroah-Hartman]

From: Will Deacon <will@xxxxxxxxxx>

commit 1ce74e96c2407df2b5867e5d45a70aacb8923c14 upstream.

Commit 4b927b94d5df ("KVM: arm/arm64: vgic: Introduce find_reg_by_id()")
introduced 'find_reg_by_id()', which looks up a system register only if
the 'id' index parameter identifies a valid system register. As part of
the patch, existing callers of 'find_reg()' were ported over to the new
interface, but this breaks 'index_to_sys_reg_desc()' in the case that the
initial lookup in the vCPU target table fails because we will then call
into 'find_reg()' for the system register table with an uninitialised
'param' as the key to the lookup.

GCC 10 is bright enough to spot this (amongst a tonne of false positives,
but hey!):

  | arch/arm64/kvm/sys_regs.c: In function âindex_to_sys_reg_desc.part.0.israâ:
  | arch/arm64/kvm/sys_regs.c:983:33: warning: âparams.Op2â may be used uninitialized in this function [-Wmaybe-uninitialized]
  |   983 |   (u32)(x)->CRn, (u32)(x)->CRm, (u32)(x)->Op2);
  | [...]

Revert the hunk of 4b927b94d5df which breaks 'index_to_sys_reg_desc()' so
that the old behaviour of checking the index upfront is restored.

Fixes: 4b927b94d5df ("KVM: arm/arm64: vgic: Introduce find_reg_by_id()")
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20191212094049.12437-1-will@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 arch/arm64/kvm/sys_regs.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -2174,8 +2174,11 @@ static const struct sys_reg_desc *index_
 	if ((id & KVM_REG_ARM_COPROC_MASK) != KVM_REG_ARM64_SYSREG)
 		return NULL;
 
+	if (!index_to_params(id, &params))
+		return NULL;
+
 	table = get_target_table(vcpu->arch.target, true, &num);
-	r = find_reg_by_id(id, &params, table, num);
+	r = find_reg(&params, table, num);
 	if (!r)
 		r = find_reg(&params, sys_reg_descs, ARRAY_SIZE(sys_reg_descs));