详细分析：[Pandavirtualization: Exploiting the Xen hypervisor
](https://googleprojectzero.blogspot.tw/2017/04/pandavirtualization-exploiting-xen.html)
This bug report describes a vulnerability in memory_exchange() that
permits PV guest kernels to write to an arbitrary virtual address with
hypervisor privileges. The vulnerability was introduced through a
broken fix for CVE-2012-5513 / XSA-29.
The fix for CVE-2012-5513 / XSA-29 introduced the following check in
the memory_exchange() hypercall handler:
if ( !guest_handle_okay(exch.in.extent_start, exch.in.nr_extents) ||
!guest_handle_okay(exch.out.extent_start, exch.out.nr_extents) )
{
rc = -EFAULT;
goto fail_early;
}
guest_handle_okay() calls array_access_ok(), which calls access_ok(),
which is implemented as follows:
/*
* Valid if in +ve half of 48-bit address space, or above
* Xen-reserved area.
* This is also valid for range checks (addr, addr+size). As long
* as the start address is outside the Xen-reserved area then we
* will access a non-canonical address (and thus fault) before
* ever reaching VIRT_START.
*/
#define __addr_ok(addr) \
(((unsigned long)(addr) < (1UL<<47)) || \
((unsigned long)(addr) >= HYPERVISOR_VIRT_END))
#define access_ok(addr, size) \
(__addr_ok(addr) || is_compat_arg_xlat_range(addr, size))
As the comment states, access_ok() only checks the address, not the
size, if the address points to guest memory, based on the assumption
that any caller of access_ok() will access guest memory linearly,
starting at the supplied address. Callers that want to access a
subrange of the memory referenced by a guest handle are supposed to
use guest_handle_subrange_okay(), which takes an additional start
offset parameter, instead of guest_handle_okay().
memory_exchange() uses guest_handle_okay(), but only accesses the
guest memory arrays referenced by exch.in.extent_start and
exch.out.extent_start starting at exch.nr_exchanged, a 64-bit offset.
The intent behind exch.nr_exchanged is that guests always set it to 0
and nonzero values are only set when a hypercall has to be restarted
because of preemption, but this isn't enforced.
Therefore, by invoking this hypercall with crafted arguments, it is
possible to write to an arbitrary memory location that is encoded as
exch.out.extent_start + 8 * exch.nr_exchanged
where exch.out.extent_start points to guest memory and
exch.nr_exchanged is an attacker-chosen 64-bit value.
I have attached a proof of concept. This PoC demonstrates the issue by
overwriting the first 8 bytes of the IDT entry for #PF, causing the
next pagefault to doublefault. To run the PoC, unpack it in a normal
64-bit PV domain and run the following commands in the domain as root:
```
root@pv-guest:~# cd crashpoc
root@pv-guest:~/crashpoc# make -C /lib/modules/$(uname -r)/build M=$(pwd)
make: Entering directory '/usr/src/linux-headers-4.4.0-66-generic'
LD /root/crashpoc/built-in.o
CC [M] /root/crashpoc/module.o
nasm -f elf64 -o /root/crashpoc/native.o /root/crashpoc/native.asm
LD [M] /root/crashpoc/test.o
Building modules, stage 2.
MODPOST 1 modules
WARNING: could not find /root/crashpoc/.native.o.cmd for /root/crashpoc/native.o
CC /root/crashpoc/test.mod.o
LD [M] /root/crashpoc/test.ko
make: Leaving directory '/usr/src/linux-headers-4.4.0-66-generic'
root@pv-guest:~/crashpoc# insmod test.ko
root@pv-guest:~/crashpoc# rmmod test
```
The machine on which I tested the PoC was running Xen 4.6.0-1ubuntu4
(from Ubuntu 16.04.2). Executing the PoC caused the following console
output:
```
(XEN) *** DOUBLE FAULT ***
(XEN) ----[ Xen-4.6.0 x86_64 debug=n Tainted: C ]----
(XEN) CPU: 0
(XEN) RIP: e033:[<0000557b46f56860>] 0000557b46f56860
(XEN) RFLAGS: 0000000000010202 CONTEXT: hypervisor
(XEN) rax: 00007fffe9cfafd0 rbx: 00007fffe9cfd160 rcx: 0000557b47ebd040
(XEN) rdx: 0000000000000001 rsi: 0000000000000004 rdi: 0000557b47ec52e0
(XEN) rbp: 00007fffe9cfd158 rsp: 00007fffe9cfaf30 r8: 0000557b46f7df00
(XEN) r9: 0000557b46f7dec0 r10: 0000557b46f7df00 r11: 0000557b47ec5878
(XEN) r12: 0000557b47ebd040 r13: 00007fffe9cfb0c0 r14: 0000557b47ec52e0
(XEN) r15: 0000557b47ed5e70 cr0: 0000000080050033 cr4: 00000000001506a0
(XEN) cr3: 0000000098e2e000 cr2: 00007fffe9cfaf93
(XEN) ds: 0000 es: 0000 fs: 0000 gs: 0000 ss: e02b cs: e033
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 0:
(XEN) DOUBLE FAULT -- system shutdown
(XEN) ****************************************
(XEN)
(XEN) Reboot in five seconds...
```
I strongly recommend changing the semantics of access_ok() so that it
guarantees that any access to an address inside the specified range is
valid. Alternatively, add some prefix, e.g. "UNSAFE_", to the names of
access_ok() and appropriate wrappers to prevent people from using
these functions improperly. Currently, in my opinion, the function
name access_ok() is misleading.
I have not allocated a CVE number for this issue.
When disclosing this issue, please credit me as "Jann Horn of Google
Project Zero".
附件：[xen_memory_exchange_crashpoc.tar](https://bugs.chromium.org/p/project-zero/issues/attachment?aid=275151)
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