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KVM: x86: Introduce segmented_write_std

Introduces segemented_write_std.

Switches from emulated reads/writes to standard read/writes in fxsave,
fxrstor, sgdt, and sidt.  This fixes CVE-2017-2584, a longstanding
kernel memory leak.

Since commit 283c95d ("KVM: x86: emulate FXSAVE and FXRSTOR",
2016-11-09), which is luckily not yet in any final release, this would
also be an exploitable kernel memory *write*!

Reported-by: Dmitry Vyukov <>
Fixes: 9605157
Fixes: 283c95d
Suggested-by: Paolo Bonzini <>
Signed-off-by: Steve Rutherford <>
Signed-off-by: Paolo Bonzini <>
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stevenrutherford authored and bonzini committed Jan 12, 2017
1 parent cef84c3 commit 129a72a0d3c8e139a04512325384fe5ac119e74d
Showing with 18 additions and 4 deletions.
  1. +18 −4 arch/x86/kvm/emulate.c
@@ -818,6 +818,20 @@ static int segmented_read_std(struct x86_emulate_ctxt *ctxt,
return ctxt->ops->read_std(ctxt, linear, data, size, &ctxt->exception);

static int segmented_write_std(struct x86_emulate_ctxt *ctxt,
struct segmented_address addr,
void *data,
unsigned int size)
int rc;
ulong linear;

rc = linearize(ctxt, addr, size, true, &linear);
if (rc != X86EMUL_CONTINUE)
return rc;
return ctxt->ops->write_std(ctxt, linear, data, size, &ctxt->exception);

* Prefetch the remaining bytes of the instruction without crossing page
* boundary if they are not in fetch_cache yet.
@@ -3685,8 +3699,8 @@ static int emulate_store_desc_ptr(struct x86_emulate_ctxt *ctxt,
/* Disable writeback. */
ctxt->dst.type = OP_NONE;
return segmented_write(ctxt, ctxt->dst.addr.mem,
&desc_ptr, 2 + ctxt->op_bytes);
return segmented_write_std(ctxt, ctxt->dst.addr.mem,
&desc_ptr, 2 + ctxt->op_bytes);

static int em_sgdt(struct x86_emulate_ctxt *ctxt)
@@ -3932,7 +3946,7 @@ static int em_fxsave(struct x86_emulate_ctxt *ctxt)
size = offsetof(struct fxregs_state, xmm_space[0]);

return segmented_write(ctxt, ctxt->memop.addr.mem, &fx_state, size);
return segmented_write_std(ctxt, ctxt->memop.addr.mem, &fx_state, size);

static int fxrstor_fixup(struct x86_emulate_ctxt *ctxt,
@@ -3974,7 +3988,7 @@ static int em_fxrstor(struct x86_emulate_ctxt *ctxt)
if (rc != X86EMUL_CONTINUE)
return rc;

rc = segmented_read(ctxt, ctxt->memop.addr.mem, &fx_state, 512);
rc = segmented_read_std(ctxt, ctxt->memop.addr.mem, &fx_state, 512);
if (rc != X86EMUL_CONTINUE)
return rc;

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