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Subject[PATCH 1/1] x86/fpu: Fix copy_xstate_to_uabi() to copy init states correctly
Date
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@intel.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Tested-by: Yuan Yao <yuan.yao@intel.com>
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org
Link: https://lore.kernel.org/lkml/BYAPR11MB3717EDEF2351C958F2C86EED95259@BYAPR11MB3717.namprd11.prod.outlook.com/
---
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
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