xen: patch for XSAs: 206, 211, 212, 213, 214 and 215

XSA-206 Issue Description:

> xenstored supports transactions, such that if writes which would
> invalidate assumptions of a transaction occur, the entire transaction
> fails.  Typical response on a failed transaction is to simply retry
> the transaction until it succeeds.
>
> Unprivileged domains may issue writes to xenstore which conflict with
> transactions either of the toolstack or of backends such as the driver
> domain. Depending on the exact timing, repeated writes may cause
> transactions made by these entities to fail indefinitely.

More: https://xenbits.xen.org/xsa/advisory-206.html

XSA-211 Issue Description:

> When a graphics update command gets passed to the VGA emulator, there
> are 3 possible modes that can be used to update the display:
>
> * blank - Clears the display
> * text - Treats the display as showing text
> * graph - Treats the display as showing graphics
>
> After the display geometry gets changed (i.e., after the CIRRUS VGA
> emulation has resized the display), the VGA emulator will resize the
> console during the next update command. However, when a blank mode is
> also selected during an update, this resize doesn't happen. The resize
> will be properly handled during the next time a non-blank mode is
> selected during an update.
>
> However, other console components - such as the VNC emulation - will
> operate as though this resize had happened. When the display is
> resized to be larger than before, this can result in a heap overflow
> as console components will expect the display buffer to be larger than
> it is currently allocated.

More: https://xenbits.xen.org/xsa/advisory-211.html

XSA-212 Issue Description:

> The XSA-29 fix introduced an insufficient check on XENMEM_exchange
> input, allowing the caller to drive hypervisor memory accesses outside
> of the guest provided input/output arrays.

More: https://xenbits.xen.org/xsa/advisory-212.html

XSA-213 Issue Description:

> 64-bit PV guests typically use separate (root) page tables for their
> kernel and user modes.  Hypercalls are accessible to guest kernel
> context only, which certain hypercall handlers make assumptions on.
> The IRET hypercall (replacing the identically name CPU instruction)
> is used by guest kernels to transfer control from kernel mode to user
> mode.  If such an IRET hypercall is placed in the middle of a multicall
> batch, subsequent operations invoked by the same multicall batch may
> wrongly assume the guest to still be in kernel mode.  If one or more of
> these subsequent operations involve operations on page tables, they may
> be using the wrong root page table, confusing internal accounting.  As
> a result the guest may gain writable access to some of its page tables.

More: https://xenbits.xen.org/xsa/advisory-213.html

XSA-214 Issue Description:

> The GNTTABOP_transfer operation allows one guest to transfer a page to
> another guest.  The internal processing of this, however, does not
> include zapping the previous type of the page being transferred.  This
> makes it possible for a PV guest to transfer a page previously used as
> part of a segment descriptor table to another guest while retaining the
> "contains segment descriptors" property.
>
> If the destination guest is a PV one of different bitness, it may gain
> access to segment descriptors it is not normally allowed to have, like
> 64-bit code segments in a 32-bit PV guest.
>
> If the destination guest is a HVM one, that guest may freely alter the
> page contents and then hand the page back to the same or another PV
> guest.
>
> In either case, if the destination PV guest then inserts that page into
> one of its own descriptor tables, the page still having the designated
> type results in validation of its contents being skipped.

More: https://xenbits.xen.org/xsa/advisory-214.html

XSA-215 Issue Description:

> Under certain special conditions Xen reports an exception resulting
> from returning to guest mode not via ordinary exception entry points,
> but via a so call failsafe callback.  This callback, unlike exception
> handlers, takes 4 extra arguments on the stack (the saved data
> selectors DS, ES, FS, and GS).  Prior to placing exception or failsafe
> callback frames on the guest kernel stack, Xen checks the linear
> address range to not overlap with hypervisor space.  The range spanned
> by that check was mistakenly not covering these extra 4 slots.

More: https://xenbits.xen.org/xsa/advisory-215.html