The IBM Tivoli Provisioning Manager for OS Deployment (also known as
TPMfOSD, Rembo-ia32, or Rembo Auto-Deploy) has a serious bug in some
older versions (observed with v5.1.1.0, apparently fixed by v7.1.1.0)
which can lead to arbitrary data corruption.

As mentioned in commit 87723a0 ("[libflat] Test A20 gate without
switching to flat real mode"), Tivoli's NBP sets up a VMM and makes
calls to the PXE stack in VM86 mode.  This appears to be some kind of
attempt to run PXE API calls inside a sandbox.  The VMM is fairly
sophisticated: for example, it handles our attempts to switch into
protected mode and patches our GDT so that our protected-mode code
runs in ring 1 instead of ring 0.  However, it neglects to apply any
memory protections.  In particular, it does not enable paging and
leaves us with 4GB segment limits.  We can therefore trivially break
out of the sandbox by simply overwriting the GDT (or by modifying any
of Tivoli's VMM code or data structures).

When we attempt to execute privileged instructions (such as "lidt"),
the CPU raises an exception and control is passed to the Tivoli VMM.
This may result in a call to Tivoli's memcpy() function.

Tivoli's memcpy() function includes optimisations which use the SSE
registers %xmm0-%xmm3 to speed up aligned memory copies.

Unfortunately, the Tivoli VMM's exception handler does not save or
restore %xmm0-%xmm3.  The net effect of this bug in the Tivoli VMM is
that any privileged instruction (such as "lidt") issued by iPXE may
result in unexpected corruption of the %xmm0-%xmm3 registers.

Even more unfortunately, this problem affects the code path taken in
response to a hardware interrupt from the NIC, since that code path
will call PXENV_UNDI_ISR.  The net effect therefore becomes that any
NIC hardware interrupt (e.g. due to a received packet) may result in
unexpected corruption of the %xmm0-%xmm3 registers.

If a packet arrives while Tivoli is in the middle of using its
memcpy() function, then the unexpected corruption of the %xmm0-%xmm3
registers will result in unexpected corruption in the destination
buffer.  The net effect therefore becomes that any received packet may
result in a 16-byte block of corruption somewhere in any data that
Tivoli copied using its memcpy() function.

We can work around this bug in the Tivoli VMM by saving and restoring
the %xmm0-%xmm3 registers across calls to virt_call().  To work around
the problem, we need to save registers before attempting to execute
any privileged instructions, and ensure that we attempt no further
privileged instructions after restoring the registers.

This is less simple than it may sound.  We can use the "movups"
instruction to save and restore individual registers, but this will
itself generate an undefined opcode exception if SSE is not currently
enabled according to the flags in %cr0 and %cr4.  We can't access %cr0
or %cr4 before attempting the "movups" instruction, because access a
control register is itself a privileged instruction (which may
therefore trigger corruption of the registers that we're trying to
save).

The best solution seems to be to use the "fxsave" and "fxrstor"
instructions.  If SSE is not enabled, then these instructions may fail
to save and restore the SSE register contents, but will not generate
an undefined opcode exception.  (If SSE is not enabled, then we don't
really care about preserving the SSE register contents anyway.)

The use of "fxsave" and "fxrstor" introduces an implicit assumption
that the CPU supports SSE instructions (even though we make no
assumption about whether or not SSE is currently enabled).  SSE was
introduced in 1999 with the Pentium III (and added by AMD in 2001),
and is an architectural requirement for x86_64.  Experimentation with
current versions of gcc suggest that it may generate SSE instructions
even when using "-m32", unless an explicit "-march=i386" or "-mno-sse"
is used to inhibit this.  It therefore seems reasonable to assume that
SSE will be supported on any hardware that might realistically be used
with new iPXE builds.

As a side benefit of this change, the MMX register %mm0 will now be
preserved across virt_call() even in an i386 build of iPXE using a
driver that requires readq()/writeq(), and the SSE registers
%xmm0-%xmm5 will now be preserved across virt_call() even in an x86_64
build of iPXE using the Hyper-V netvsc driver.

Experimentation suggests that this change adds around 10% to the
number of cycles required for a do-nothing virt_call(), most of which
are due to the extra bytes copied using "rep movsb".  Since the number
of bytes copied is a compile-time constant local to librm.S, we could
potentially reduce this impact by ensuring that we always copy a whole
number of dwords and so can use "rep movsl" instead of "rep movsb".

Signed-off-by: Michael Brown <mcb30@ipxe.org>