linux-kernel - [PATCH 3.16 358/366] kernel/sys.c: fix potential Spectre v1 issue

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Message-ID: <lsq.1539530741.914684577@decadent.org.uk>
Date:   Sun, 14 Oct 2018 16:25:41 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Peter Zijlstra" <peterz@...radead.org>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Alexei Starovoitov" <ast@...nel.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Dan Williams" <dan.j.williams@...el.com>
Subject: [PATCH 3.16 358/366] kernel/sys.c: fix potential Spectre v1 issue

3.16.60-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>

commit 23d6aef74da86a33fa6bb75f79565e0a16ee97c2 upstream.

`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].

[1] https://marc.info/?l=linux-kernel&m=152449131114778&w=2

Link: http://lkml.kernel.org/r/20180515030038.GA11822@embeddedor.com
Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
Reviewed-by: Andrew Morton <akpm@...ux-foundation.org>
Cc: Alexei Starovoitov <ast@...nel.org>
Cc: Dan Williams <dan.j.williams@...el.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
[bwh: Backported to 3.16:
 - Drop changes to compat implementation, which is a wrapper for the
   regular implementation here
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -63,6 +63,9 @@
 #include <asm/io.h>
 #include <asm/unistd.h>
 
+/* Hardening for Spectre-v1 */
+#include <linux/nospec.h>
+
 #ifndef SET_UNALIGN_CTL
 # define SET_UNALIGN_CTL(a,b)	(-EINVAL)
 #endif
@@ -1294,6 +1297,7 @@ SYSCALL_DEFINE2(old_getrlimit, unsigned
 	if (resource >= RLIM_NLIMITS)
 		return -EINVAL;
 
+	resource = array_index_nospec(resource, RLIM_NLIMITS);
 	task_lock(current->group_leader);
 	x = current->signal->rlim[resource];
 	task_unlock(current->group_leader);