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Date: Tue, 15 May 2018 15:08:59 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Cc: linux-kernel@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] kernel: sys: fix potential Spectre v1
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On Mon, 14 May 2018 22:00:38 -0500 "Gustavo A. R. Silva" <gustavo@embeddedor.com> 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?

>  	task_unlock(current->group_leader);
> @@ -1470,6 +1474,7 @@ COMPAT_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);
>  	r = current->signal->rlim[resource];
>  	task_unlock(current->group_leader);