linux-kernel - Re: [syzbot] [ext4?] WARNING in __fortify

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Date: Thu, 23 May 2024 09:04:56 -0400
From: "Theodore Ts'o" <tytso@....edu>
To: syzbot <syzbot+50835f73143cc2905b9e@...kaller.appspotmail.com>
Cc: adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        Justin Stitt <justinstitt@...gle.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [syzbot] [ext4?] WARNING in __fortify_report

On Wed, May 22, 2024 at 11:29:25PM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> dashboard link: https://syzkaller.appspot.com/bug?extid=50835f73143cc2905b9e

> ...
> strnlen: detected buffer overflow: 17 byte read of buffer size 16
> [<8080fe10>] (__fortify_report) from [<818e9a40>] (__fortify_panic+0x10/0x14 lib/string_helpers.c:1036)
> [<818e9a30>] (__fortify_panic) from [<8062a3b0>] (strnlen include/linux/fortify-string.h:221 [inline])
> [<818e9a30>] (__fortify_panic) from [<8062a3b0>] (sized_strscpy include/linux/fortify-string.h:295 [inline])
> [<818e9a30>] (__fortify_panic) from [<8062a3b0>] (ext4_ioctl_getlabel fs/ext4/ioctl.c:1154 [inline])

> [<818e9a30>] (__fortify_panic) from [<8062a3b0>] (ext4_fileattr_get+0x0/0x78 fs/ext4/ioctl.c:1609)
> [<8062829c>] (__ext4_ioctl) from [<8062aaac>] (ext4_ioctl+0x10/0x14 fs/ext4/ioctl.c:1626)
>  r10:836e6c00 r9:00000005 r8:845e7900 r7:00000000 r6:845e7900 r5:00000000

This is caused by commit 744a56389f73 ("ext4: replace deprecated
strncpy with alternatives") and it's unclear whether this is being
caused by a buggy implementation of strscpy_pad(), or a buggy fortify,
but a simple way to fix is to go back to the good-old strncpy(), which
is perfectly safe, and perfectly secure.

(And this is a great example of "security initiatives" being an
exercise in pain alocation tradeoffs between overworked maintainers
and security teams...  regardless of whether the bug is in fortify,
syzkaller, or an effort to completely convert away from strncpy()
because it makes security analysis easier.)

						- Ted