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

Greg Kroah-Hartman
2019-09-14T08:08:44Z

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.

thanks,

greg k-h