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Date: Sun, 19 Feb 2012 09:45:12 -0800
From: Kees Cook <keescook@...omium.org>
To: Ingo Molnar <mingo@...e.hu>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org,
Randy Dunlap <rdunlap@...otime.net>, Alexander Viro <viro@...iv.linux.org.uk>,
linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org,
kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH] fs: hardlink creation restrictions
On Sun, Feb 19, 2012 at 4:49 AM, Ingo Molnar <mingo@...e.hu> wrote:
>
> * Kees Cook <keescook@...omium.org> wrote:
>
>> This is the other half of link restrictions, now that symlink
>> restriction has landed in -mm.
>
> Nice features!
Thanks!
>> @@ -300,9 +302,29 @@ config PROTECTED_STICKY_SYMLINKS_ENABLED
>> via /proc/sys/kernel/protected_sticky_symlinks.
>>
>> config PROTECTED_STICKY_SYMLINKS_ENABLED_SYSCTL
>> - depends on PROTECTED_STICKY_SYMLINKS
>> + depends on PROTECTED_LINKS
>> int
>> default "1" if PROTECTED_STICKY_SYMLINKS_ENABLED
>> default "0"
>>
>> +config PROTECTED_NONACCESS_HARDLINKS_ENABLED
>> + depends on PROTECTED_LINKS
>> + bool "Disallow hardlink creation to non-accessible files"
>> + default y
>> + help
>> + Solve ToCToU hardlink race vulnerabilities by permitting hardlinks
>> + to be created only when to a regular file that is owned by the user,
>> + or is readable and writable by the user. Also blocks users from
>> + "pinning" vulnerable setuid/setgid programs from being upgraded by
>> + the administrator.
>> +
>> + When PROC_SYSCTL is enabled, this setting can also be controlled
>> + via /proc/sys/kernel/protected_nonaccess_hardlinks.
>
> I'd add a:
>
> See Documentation/sysctl/fs.txt for details.
>
> line as well.
Good call.
>> +config PROTECTED_NONACCESS_HARDLINKS_ENABLED_SYSCTL
>> + depends on PROTECTED_LINKS
>> + int
>> + default "1" if PROTECTED_NONACCESS_HARDLINKS_ENABLED
>> + default "0"
>
> Naming nit: how about dropping the _NONACCESS/_nonaccess names
> complete and turn it into protected_hardlinks? The longer
> variant is not any better descriptive, and needlessly longer.
>
> The PROTECTED_SYMLINKS/PROTECTED_HARDLINKS naming is thus also
> nicely symmetric.
Yeah, this really does look much better. I had opted for extreme
verbosity, but I would agree: it's not really called for here.
>> +#ifdef CONFIG_AUDIT
>> + if (error) {
>> + struct audit_buffer *ab;
>> +
>> + ab = audit_log_start(current->audit_context,
>> + GFP_KERNEL, AUDIT_AVC);
>> + audit_log_format(ab, "op=linkat action=denied");
>> + audit_log_format(ab, " pid=%d comm=", current->pid);
>> + audit_log_untrustedstring(ab, current->comm);
>> + audit_log_d_path(ab, " path=", old_path);
>> + audit_log_format(ab, " dev=");
>> + audit_log_untrustedstring(ab, inode->i_sb->s_id);
>> + audit_log_format(ab, " ino=%lu", inode->i_ino);
>> + audit_log_end(ab);
>> + }
>> +#endif
>
> Small detail: don't these audit methods map to nothing on
> !CONFIG_AUDIT, allowing the #ifdef to be dropped? (if not then
> it should really be so.)
Ah-ha; a more careful look at audit.h agrees. :) I'll adjust this as well.
> Acked-by: Ingo Molnar <mingo@...e.hu>
Thanks for the review,
-Kees
--
Kees Cook
ChromeOS Security
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