Re: [PATCH] kernel: sys: fix potential Spectre v1

[Dan Williams]

On Tue, May 15, 2018 at 3:29 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> On Tue, 15 May 2018, Andrew Morton wrote:
>> On Mon, 14 May 2018 22:00:38 -0500 "Gustavo A. R. Silva" <gustavo@xxxxxxxxxxxxxx> wrote:
>>
>> > resource can be controlled by user-space, hence leading to a
>> > potential exploitation of the Spectre variant 1 vulnerability.
>> >
>> > This issue was detected with the help of Smatch:
>> >
>> > kernel/sys.c:1474 __do_compat_sys_old_getrlimit() warn: potential
>> > spectre issue 'get_current()->signal->rlim' (local cap)
>> > kernel/sys.c:1455 __do_sys_old_getrlimit() warn: potential spectre issue
>> > 'get_current()->signal->rlim' (local cap)
>> >
>> > Fix this by sanitizing *resource* before using it to index
>> > current->signal->rlim
>> >
>> > Notice that given that speculation windows are large, the policy is
>> > to kill the speculation on the first load and not worry if it can be
>> > completed with a dependent load/store [1].
>>
>> hm.  Not my area, but I'm always willing to learn ;)
>>
>> > --- a/kernel/sys.c
>> > +++ b/kernel/sys.c
>> > @@ -69,6 +69,9 @@
>> >  #include <asm/io.h>
>> >  #include <asm/unistd.h>
>> >
>> > +/* Hardening for Spectre-v1 */
>> > +#include <linux/nospec.h>
>> > +
>> >  #include "uid16.h"
>> >
>> >  #ifndef SET_UNALIGN_CTL
>> > @@ -1451,6 +1454,7 @@ SYSCALL_DEFINE2(old_getrlimit, unsigned int, resource,
>> >     if (resource >= RLIM_NLIMITS)
>> >             return -EINVAL;
>> >
>> > +   resource = array_index_nospec(resource, RLIM_NLIMITS);
>> >     task_lock(current->group_leader);
>> >     x = current->signal->rlim[resource];
>>
>> Can the speculation proceed past the task_lock()?  Or is the policy to
>> ignore such happy happenstances even if they are available?
>
> Locks are not in the way of speculation. Speculation has almost no limits
> except serializing instructions. At least they respect the magic AND
> limitation in array_index_nospec().

I'd say it another way, because they don't respect the magic AND, we
just make the result in the speculation path safe. So, it's controlled
speculation.