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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: 719d5a9b24
Reported-by: Borislav Petkov <bp@suse.de>
Tested-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2014-10-27 14:40:39 +01:00
parent 3606189fa3
commit fd56e1546a
1 changed files with 33 additions and 10 deletions
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43
arch/x86/kvm/emulate.c
View File

@ -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))