linux-kernel - Re: [PATCH] kernel/sysctl.c: fix out of bounds access in fs.file-max

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Message-ID: <20190403140203.qq37rgcikvoawb5f@brauner.io>
Date:   Wed, 3 Apr 2019 16:02:04 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Matteo Croce <mcroce@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>, willy@...radead.org,
        zev@...ilderbeest.net, akpm@...ux-foundation.org
Subject: Re: [PATCH] kernel/sysctl.c: fix out of bounds access in fs.file-max

On Thu, Mar 28, 2019 at 02:03:06PM +0100, Matteo Croce wrote:
> fs.file-max sysctl uses proc_doulongvec_minmax() as proc handler, which
> accesses *extra1 and *extra2 as unsigned long, but commit 32a5ad9c2285
> ("sysctl: handle overflow for file-max") assigns &zero, which is an int,
> to extra1, generating the following KASAN report.
> Fix this by changing 'zero' to long, which does not need to be duplicated
> like 'one' and 'one_ul' for two data types.

Yeah, maybe but it still feels cleaner and more obvious to just add:

static long long_zero;

given that most callers actually seem to want an (unsigned) int.

I don't have a strong opinion though so if others feel that it's just a
waste of space consider it acked.

> 
> ==================================================================
> BUG: KASAN: global-out-of-bounds in __do_proc_doulongvec_minmax+0x2f9/0x600
> Read of size 8 at addr ffffffff8233dc20 by task systemd/1
> 
> CPU: 0 PID: 1 Comm: systemd Not tainted 5.1.0-rc2-kvm+ #22
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS ?-20180724_192412-buildhw-07.phx2.fedoraproject.org-1.fc29 04/01/2014
> Call Trace:
>  print_address_description+0x67/0x23d
>  kasan_report.cold.3+0x1c/0x36
>  __do_proc_doulongvec_minmax+0x2f9/0x600
>  proc_doulongvec_minmax+0x3a/0x50
>  proc_sys_call_handler+0x11d/0x170
>  vfs_write+0xd7/0x200
>  ksys_write+0x93/0x110
>  do_syscall_64+0x57/0x140
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> RIP: 0033:0x7f67d33e8804
> Code: 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b3 0f 1f 80 00 00 00 00 48 8d 05 f9 5e 0d 00 8b 00 85 c0 75 13 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 54 c3 0f 1f 00 41 54 49 89 d4 55 48 89 f5 53
> RSP: 002b:00007fffd9992ed8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f67d33e8804
> RDX: 0000000000000015 RSI: 00005586ce2607b0 RDI: 0000000000000004
> RBP: 00007fffd9992f30 R08: 000000000000c0c0 R09: ffffffffffff0000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000004
> R13: 0000000000000015 R14: 00005586ce2607c4 R15: 00007fffd9992f70
> 
> The buggy address belongs to the variable:
>  0xffffffff8233dc20
> 
> Memory state around the buggy address:
>  ffffffff8233db00: 00 00 00 00 00 00 00 00 fa fa fa fa 04 fa fa fa
>  ffffffff8233db80: fa fa fa fa 04 fa fa fa fa fa fa fa 04 fa fa fa
> >ffffffff8233dc00: fa fa fa fa 04 fa fa fa fa fa fa fa 00 00 00 00
>                                ^
>  ffffffff8233dc80: fa fa fa fa fa fa fa fa 00 00 00 00 00 00 00 00
>  ffffffff8233dd00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> ==================================================================
> 
> Fixes: 32a5ad9c2285 ("sysctl: handle overflow for file-max")

Next time, please take the time to Cc the author of the Fixes patch as
well whose commit this is fixing right away.

> Signed-off-by: Matteo Croce <mcroce@...hat.com>
> ---
>  kernel/sysctl.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index e5da394d1ca3..3e959d67d619 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 long zero;

>  static int __maybe_unused one = 1;
>  static int __maybe_unused two = 2;
>  static int __maybe_unused four = 4;
> -- 
> 2.20.1
>