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

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Date:   Thu, 28 Mar 2019 14:03:06 +0100
From:   Matteo Croce <mcroce@...hat.com>
To:     linux-fsdevel@...r.kernel.org
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>
Subject: [PATCH] kernel/sysctl.c: fix out of bounds access in fs.file-max

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.

==================================================================
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")
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