linux-kernel - Re: [PATCH] kernel/sys.c: fix possible spectre-v1 in do

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Message-ID: <CAFbcbMCLwoBB8syLCSU8i0Hc7OMnHT4A+AzWdmF5g9BzbY7CXQ@mail.gmail.com>
Date:   Thu, 30 May 2019 13:45:16 +0800
From:   Dianzhang Chen <dianzhangchen0@...il.com>
To:     Cyrill Gorcunov <gorcunov@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kernel/sys.c: fix possible spectre-v1 in do_prlimit()

Though syscall `getrlimit` , it seems not works after check_prlimit_permission.

And the speculation windows are large, as said[1]:
>> 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().

[1] https://do-db2.lkml.org/lkml/2018/5/15/1056

On Wed, May 29, 2019 at 8:18 PM Cyrill Gorcunov <gorcunov@...il.com> wrote:
>
> On Wed, May 29, 2019 at 10:39:52AM +0800, Dianzhang Chen wrote:
> > Hi,
> >
> > Although when detect it is misprediction and drop the execution, but
> > it can not drop all the effects of speculative execution, like the
> > cache state. During the speculative execution, the:
> >
> >
> > rlim = tsk->signal->rlim + resource;    // use resource as index
> >
> > ...
> >
> >             *old_rlim = *rlim;
> >
> >
> > may read some secret data into cache.
> >
> > and then the attacker can use side-channel attack to find out what the
> > secret data is.
>
> This code works after check_prlimit_permission call, which means you already
> should have a permission granted. And you implies that misprediction gonna
> be that deep which involves a number of calls/read/writes/jumps/locks-rb-wb-flushes
> and a bunch or other instructions, moreover all conditions are "mispredicted".
> This is very bold and actually unproved claim!
>
> Note that I pointed the patch is fine in cleanup context but seriously I
> don't see how this all can be exploitable in sense of spectre.