[PATCH 1/1] x86/fpu: Fix copy_xstate_to_uabi() to copy init states correctly

[Chang S. Bae]

When an extended state component is present in fpstate, but in init state,
the function copies from init_fpstate via copy_feature().

But, dynamic states are not present in init_fpstate. Then accessing
init_fpstate for those will explode like this:

 BUG: kernel NULL pointer dereference, address: 0000000000000000
 ...
 RIP: 0010:memcpy_erms+0x6/0x10
  ? __copy_xstate_to_uabi_buf+0x381/0x870
  fpu_copy_guest_fpstate_to_uabi+0x28/0x80
  kvm_arch_vcpu_ioctl+0x14c/0x1460 [kvm]
  ? __this_cpu_preempt_check+0x13/0x20
  ? vmx_vcpu_put+0x2e/0x260 [kvm_intel]
  kvm_vcpu_ioctl+0xea/0x6b0 [kvm]
  ? kvm_vcpu_ioctl+0xea/0x6b0 [kvm]
  ? __fget_light+0xd4/0x130
  __x64_sys_ioctl+0xe3/0x910
  ? debug_smp_processor_id+0x17/0x20
  ? fpregs_assert_state_consistent+0x27/0x50
  do_syscall_64+0x3f/0x90
  entry_SYSCALL_64_after_hwframe+0x63/0xcd

Instead of referencing init_fpstate, simply zero out the userspace buffer
for the state component in an all-zeros init state.

Fixes: 2308ee57d93d ("x86/fpu/amx: Enable the AMX feature in 64-bit mode")
Reported-by: Yuan Yao <yuan.yao@xxxxxxxxx>
Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Tested-by: Yuan Yao <yuan.yao@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Link: https://lore.kernel.org/lkml/BYAPR11MB3717EDEF2351C958F2C86EED95259@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
---
 arch/x86/kernel/fpu/xstate.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index e77cabfa802f..efa9e3a269fc 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1141,10 +1141,14 @@ void __copy_xstate_to_uabi_buf(struct membuf to, struct fpstate *fpstate,
 			 */
 			pkru.pkru = pkru_val;
 			membuf_write(&to, &pkru, sizeof(pkru));
+		} else if (!(header.xfeatures & BIT_ULL(i))) {
+			/*
+			 * Every extended state component has an all zeros
+			 * init state.
+			 */
+			membuf_zero(&to, xstate_sizes[i]);
 		} else {
-			copy_feature(header.xfeatures & BIT_ULL(i), &to,
-				     __raw_xsave_addr(xsave, i),
-				     __raw_xsave_addr(xinit, i),
+			membuf_write(&to, __raw_xsave_addr(xsave, i),
 				     xstate_sizes[i]);
 		}
 		/*
-- 
2.17.1