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

Stefan Lippers-Hollmann
2019-09-14T07:09:18Z

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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.

Regards
Stefan Lippers-Hollmann

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