linux-kernel - [PATCH] sysctl: redefine zero as a unsigned long

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Message-ID: <20190405065217.125140-1-houtao1@huawei.com>
Date:   Fri, 5 Apr 2019 14:52:17 +0800
From:   Hou Tao <houtao1@...wei.com>
To:     <mcgrof@...nel.org>, <keescook@...omium.org>
CC:     <linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>
Subject: [PATCH] sysctl: redefine zero as a unsigned long

We have got KASAN splat when tried to set /proc/sys/fs/file-max:

  BUG: KASAN: global-out-of-bounds in __do_proc_doulongvec_minmax+0x3e4/0x8f0
  Read of size 8 at addr ffff20000f9b2980 by task file-max.sh/36819

  Call trace:
   dump_backtrace+0x0/0x3f8
   show_stack+0x3c/0x60
   dump_stack+0x150/0x1a8
   print_address_description+0x2b8/0x5a0
   kasan_report+0x278/0x648
   __asan_load8+0x124/0x170
   __do_proc_doulongvec_minmax+0x3e4/0x8f0
   proc_doulongvec_minmax+0x80/0xa0
   proc_sys_call_handler+0x188/0x2a0
   proc_sys_write+0x5c/0x80
   __vfs_write+0x118/0x578
   vfs_write+0x184/0x418
   ksys_write+0xfc/0x1e8
   __arm64_sys_write+0x88/0xa8
   el0_svc_common+0x1a4/0x500
   el0_svc_handler+0xb8/0x180
   el0_svc+0x8/0xc

  The buggy address belongs to the variable:
   zero+0x0/0x40

The cause is that proc_doulongvec_minmax() is trying to cast an int
pointer (namely &zero) to a unsigned long pointer, and dereferencing it.

Although the warning seems does no harm, because zero will be placed
in a .bss section, but it's better to kill the KASAN warning by
redefining zero as a unsigned long, so it's OK whenever it is accessed
as an int or a a unsigned long.

An alternative fix seems to be set the minimal value of file-max to be 1,
so one_ul can be used instead, but I'm not sure whether or not a file-max
with a value of zero has special purpose (e.g., prohibit the file-related
activities of all no-privileged users).

Signed-off-by: Hou Tao <houtao1@...wei.com>
---
 kernel/sysctl.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index e5da394d1ca3..03846e015013 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -124,7 +124,7 @@ static int sixty = 60;

 static int __maybe_unused neg_one = -1;

-static int zero;
+static unsigned long zero;
 static int __maybe_unused one = 1;
 static int __maybe_unused two = 2;
 static int __maybe_unused four = 4;
-- 
2.16.2.dirty