Re: [PATCH 5.2 36/37] vhost: block speculation of translated descriptors

Thomas Backlund
2019-09-15T09:35:05Z

Den 14-09-2019 kl. 11:08, skrev Greg Kroah-Hartman:
> On Sat, Sep 14, 2019 at 09:15:48AM +0200, Stefan Lippers-Hollmann wrote:
>> Hi
>>
>> On 2019-09-14, Greg Kroah-Hartman wrote:
>>> On Sat, Sep 14, 2019 at 02:54:11AM +0200, Stefan Lippers-Hollmann wrote:
>>>> On 2019-09-13, Greg Kroah-Hartman wrote:
>>>>> From: Michael S. Tsirkin <mst@redhat.com>
>>>>>
>>>>> commit a89db445fbd7f1f8457b03759aa7343fa530ef6b upstream.
>>>>>
>>>>> iovec addresses coming from vhost are assumed to be
>>>>> pre-validated, but in fact can be speculated to a value
>>>>> out of range.
>>>>>
>>>>> Userspace address are later validated with array_index_nospec so we can
>>>>> be sure kernel info does not leak through these addresses, but vhost
>>>>> must also not leak userspace info outside the allowed memory table to
>>>>> guests.
>>>>>
>>>>> Following the defence in depth principle, make sure
>>>>> the address is not validated out of node range.
>> [...]
>>> Do you have the same problem with Linus's tree right now?
>>
>> Actually, yes I do (I had not tested i386 for 5.3~ within the last ~2
>> weeks, only amd64). Very similar kernel config, same compiler versions
>> but built in a slightly different environment (built directly on the bare
>> iron, in a amd64 multilib userspace, rather than a pure-i386 chroot on an
>> amd64 kernel).
>>
>> $ git describe
>> v5.3-rc8-36-ga7f89616b737
>>
>> $ LANG= make ARCH=x86 -j1 bzImage modules
>> CALL scripts/checksyscalls.sh
>> CALL scripts/atomic/check-atomics.sh
>> CHK include/generated/compile.h
>> CHK kernel/kheaders_data.tar.xz
>> CC [M] drivers/vhost/vhost.o
>> In file included from ./include/linux/export.h:45,
>> from ./include/linux/linkage.h:7,
>> from ./include/linux/kernel.h:8,
>> from ./include/linux/list.h:9,
>> from ./include/linux/wait.h:7,
>> from ./include/linux/eventfd.h:13,
>> from drivers/vhost/vhost.c:13:
>> drivers/vhost/vhost.c: In function 'translate_desc':
>> ./include/linux/compiler.h:350:38: error: call to '__compiletime_assert_2076' declared with attribute error: BUILD_BUG_ON failed: sizeof(_s) > sizeof(long)
>> 350 | _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
>> | ^
>> ./include/linux/compiler.h:331:4: note: in definition of macro '__compiletime_assert'
>> 331 | prefix ## suffix(); \
>> | ^~~~~~
>> ./include/linux/compiler.h:350:2: note: in expansion of macro '_compiletime_assert'
>> 350 | _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
>> | ^~~~~~~~~~~~~~~~~~~
>> ./include/linux/build_bug.h:39:37: note: in expansion of macro 'compiletime_assert'
>> 39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
>> | ^~~~~~~~~~~~~~~~~~
>> ./include/linux/build_bug.h:50:2: note: in expansion of macro 'BUILD_BUG_ON_MSG'
>> 50 | BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
>> | ^~~~~~~~~~~~~~~~
>> ./include/linux/nospec.h:56:2: note: in expansion of macro 'BUILD_BUG_ON'
>> 56 | BUILD_BUG_ON(sizeof(_s) > sizeof(long)); \
>> | ^~~~~~~~~~~~
>> drivers/vhost/vhost.c:2076:5: note: in expansion of macro 'array_index_nospec'
>> 2076 | array_index_nospec((unsigned long)(addr - node->start),
>> | ^~~~~~~~~~~~~~~~~~
>> make[2]: *** [scripts/Makefile.build:281: drivers/vhost/vhost.o] Error 1
>> make[1]: *** [scripts/Makefile.build:497: drivers/vhost] Error 2
>> make: *** [Makefile:1083: drivers] Error 2
>>
>> $ git revert a89db445fbd7f1f8457b03759aa7343fa530ef6b
>>
>> $ LANG= make ARCH=x86 -j16 bzImage modules
>> CALL scripts/atomic/check-atomics.sh
>> CALL scripts/checksyscalls.sh
>> CHK include/generated/compile.h
>> CHK kernel/kheaders_data.tar.xz
>> Building modules, stage 2.
>> Kernel: arch/x86/boot/bzImage is ready (#1)
>> MODPOST 3464 modules
>>
>> $ echo $?
>> 0
>>
>> $ find . -name vhost\\.ko
>> ./drivers/vhost/vhost.ko
>>
>> I've attached the affected kernel config for v5.3~/ i386.
>
> Ok, good, we will be "bug compatible" at the very least now :)
>
> When this gets fixed in Linus's tree we can backport the fix here as
> well. The number of users of that compiler version/configuration is
> probably pretty low at the moment to want to hold off on this fix.
>

This is now reverted upstream:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0d4a3f2abbef73b9e5bb5f12213c275565473588

--
Thomas