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: 719d5a9b2487e0562f178f61e323c3dc18a8b200
Reported-by: Borislav Petkov <bp@suse.de>
Tested-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 8aa6606..52a9627 100644 (file)
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -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))