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Commit fd56e154 authored by Paolo Bonzini's avatar Paolo Bonzini
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KVM: emulator: fix execution close to the segment limit 



Emulation of code that is 14 bytes to the segment limit or closer
(e.g. RIP = 0xFFFFFFF2 after reset) is broken because we try to read as
many as 15 bytes from the beginning of the instruction, and __linearize
fails when the passed (address, size) pair reaches out of the segment.

To fix this, let __linearize return the maximum accessible size (clamped
to 2^32-1) for usage in __do_insn_fetch_bytes, and avoid the limit check
by passing zero for the desired size.

For expand-down segments, __linearize is performing a redundant check.
(u32)(addr.ea + size - 1) <= lim can only happen if addr.ea is close
to 4GB; in this case, addr.ea + size - 1 will also fail the check against
the upper bound of the segment (which is provided by the D/B bit).
After eliminating the redundant check, it is simple to compute
the *max_size for expand-down segments too.

Now that the limit check is done in __do_insn_fetch_bytes, we want
to inject a general protection fault there if size < op_size (like
__linearize would have done), instead of just aborting.

This fixes booting Tiano Core from emulated flash with EPT disabled.

Cc: stable@vger.kernel.org
Fixes: 719d5a9b
Reported-by: default avatarBorislav Petkov <bp@suse.de>
Tested-by: default avatarBorislav Petkov <bp@suse.de>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 3606189f

arch/x86/kvm/emulate.c

+33 −10
Original line number Diff line number Diff line
@@ -641,7 +641,8 @@ static bool insn_aligned(struct x86_emulate_ctxt *ctxt, unsigned size) 


static int __linearize(struct x86_emulate_ctxt *ctxt,

		     struct segmented_address addr,

		     unsigned size, bool write, bool fetch,

		     unsigned *max_size, unsigned size,

		     bool write, bool fetch,

		     ulong *linear)

{

	struct desc_struct desc;
@@ -652,10 +653,15 @@ static int __linearize(struct x86_emulate_ctxt *ctxt, 
	unsigned cpl;



	la = seg_base(ctxt, addr.seg) + addr.ea;

	*max_size = 0;

	switch (ctxt->mode) {

	case X86EMUL_MODE_PROT64:

		if (((signed long)la << 16) >> 16 != la)

			return emulate_gp(ctxt, 0);



		*max_size = min_t(u64, ~0u, (1ull << 48) - la);

		if (size > *max_size)

			goto bad;

		break;

	default:

		usable = ctxt->ops->get_segment(ctxt, &sel, &desc, NULL,
@@ -673,20 +679,25 @@ static int __linearize(struct x86_emulate_ctxt *ctxt, 
		if ((ctxt->mode == X86EMUL_MODE_REAL) && !fetch &&

		    (ctxt->d & NoBigReal)) {

			/* la is between zero and 0xffff */

			if (la > 0xffff || (u32)(la + size - 1) > 0xffff)

			if (la > 0xffff)

				goto bad;

			*max_size = 0x10000 - la;

		} else if ((desc.type & 8) || !(desc.type & 4)) {

			/* expand-up segment */

			if (addr.ea > lim || (u32)(addr.ea + size - 1) > lim)

			if (addr.ea > lim)

				goto bad;

			*max_size = min_t(u64, ~0u, (u64)lim + 1 - addr.ea);

		} else {

			/* expand-down segment */

			if (addr.ea <= lim || (u32)(addr.ea + size - 1) <= lim)

			if (addr.ea <= lim)

				goto bad;

			lim = desc.d ? 0xffffffff : 0xffff;

			if (addr.ea > lim || (u32)(addr.ea + size - 1) > lim)

			if (addr.ea > lim)

				goto bad;

			*max_size = min_t(u64, ~0u, (u64)lim + 1 - addr.ea);

		}

		if (size > *max_size)

			goto bad;

		cpl = ctxt->ops->cpl(ctxt);

		if (!(desc.type & 8)) {

			/* data segment */
@@ -721,7 +732,8 @@ static int linearize(struct x86_emulate_ctxt *ctxt, 
		     unsigned size, bool write,

		     ulong *linear)

{

	return __linearize(ctxt, addr, size, write, false, linear);

	unsigned max_size;

	return __linearize(ctxt, addr, &max_size, size, write, false, linear);

}
@@ -746,17 +758,27 @@ static int segmented_read_std(struct x86_emulate_ctxt *ctxt, 
static int __do_insn_fetch_bytes(struct x86_emulate_ctxt *ctxt, int op_size)

{

	int rc;

	unsigned size;

	unsigned size, max_size;

	unsigned long linear;

	int cur_size = ctxt->fetch.end - ctxt->fetch.data;

	struct segmented_address addr = { .seg = VCPU_SREG_CS,

					   .ea = ctxt->eip + cur_size };



	size = 15UL ^ cur_size;

	rc = __linearize(ctxt, addr, size, false, true, &linear);

	/*

	 * We do not know exactly how many bytes will be needed, and

	 * __linearize is expensive, so fetch as much as possible.  We

	 * just have to avoid going beyond the 15 byte limit, the end

	 * of the segment, or the end of the page.

	 *

	 * __linearize is called with size 0 so that it does not do any

	 * boundary check itself.  Instead, we use max_size to check

	 * against op_size.

	 */

	rc = __linearize(ctxt, addr, &max_size, 0, false, true, &linear);

	if (unlikely(rc != X86EMUL_CONTINUE))

		return rc;



	size = min_t(unsigned, 15UL ^ cur_size, max_size);

	size = min_t(unsigned, size, PAGE_SIZE - offset_in_page(linear));



	/*
@@ -766,7 +788,8 @@ static int __do_insn_fetch_bytes(struct x86_emulate_ctxt *ctxt, int op_size) 
	 * still, we must have hit the 15-byte boundary.

	 */

	if (unlikely(size < op_size))

		return X86EMUL_UNHANDLEABLE;

		return emulate_gp(ctxt, 0);



	rc = ctxt->ops->fetch(ctxt, linear, ctxt->fetch.end,

			      size, &ctxt->exception);

	if (unlikely(rc != X86EMUL_CONTINUE))