From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3D1A5313E38; Thu, 2 Apr 2026 14:05:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775138722; cv=none; b=lmcNu+rwdfarc08Q7xF9kcFmVFiufRDIcES6DcfGfUa71DjaVOBrIa+EL35ejz9bG9C30OT7seAdSgC0WLL0i9lhsHhXTPtEM8DQDmEzU4OXtWUUEfVRcL5TEZqT0dwjTiK3Ta8UeSzskA5uaYJNBa5s7ovpCsCrcRGaAPpNddY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775138722; c=relaxed/simple; bh=j1ENaQQxbZKRMMr9VxJbrcuncvknRfRVEvty55WyqWg=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=SjyiP8pR9UQchx/AFFw9E/uiJgCXuDw5zJfGbQ2XO/vrQfYkGpDebFA5UofbB+HvzOJynEVAGbeB4ZYnuNJifCG1TSi+ku2YQ49Jn5zNSEl4ZmNp6ev3X+xU3Z8kO0vmKu8sUM2yCqH/XH0xsVi8DoMelh+PlboR771yJKoT5Vg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=TWTO2q7M; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="TWTO2q7M" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 28293C116C6; Thu, 2 Apr 2026 14:05:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775138721; bh=j1ENaQQxbZKRMMr9VxJbrcuncvknRfRVEvty55WyqWg=; h=Date:Subject:From:To:Cc:References:In-Reply-To:From; b=TWTO2q7MvTsH3lDsiEwoizHDFAAXeg3iA4Pn/Kem3PcaBmNQr2mthKmLp1jyov+hS OD3DyFWKBDkC+8lSvbRrp1TT3vxpfNGcxMcLysiAJFzTFZZTnOQphjENI0EMa7QOzF OTItiQVKBgY7cT1lDxhYR17fKWUA3d9i65OOn4ae020LeGWVkjR3l/rQ7qTovS5iW8 qxeudpHJe9O3ID2Z4ZGw/a4tKPU7c6n39sLfDIT+zIQgQrbXmM06N4CCwgD9VzIY0X 1sXU+HibO/3lrZOesXxnGyY7JFwhfssCapT7SXQLmQMaFF+2TBImGz4ggUFTnURNLA aq6K5hmlcsOVA== Message-ID: <698daf83-1d5c-41c6-a36c-23a5b4633fbe@kernel.org> Date: Thu, 2 Apr 2026 16:05:17 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] prctl: require checkpoint_restore_ns_capable for PR_SET_MM_MAP From: "David Hildenbrand (Arm)" To: "Lorenzo Stoakes (Oracle)" , Qi Tang Cc: Andrew Morton , Cyrill Gorcunov , Oleg Nesterov , linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20260402111332.55957-1-tpluszz77@gmail.com> <686134c9-c2e3-444f-b83a-dd229c7b0102@lucifer.local> <389887c2-ddae-4456-b9d2-417aaaa2b340@kernel.org> Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/2/26 15:55, David Hildenbrand (Arm) wrote: > On 4/2/26 15:06, Lorenzo Stoakes (Oracle) wrote: >> On Thu, Apr 02, 2026 at 07:13:32PM +0800, Qi Tang wrote: >>> prctl_set_mm_map() allows modifying all mm_struct boundaries and >>> the saved auxv vector. The individual field path (PR_SET_MM_START_CODE >>> etc.) correctly requires CAP_SYS_RESOURCE, but the PR_SET_MM_MAP path >>> dispatches before this check and has no capability requirement of its >>> own when exe_fd is -1. >>> >>> This means any unprivileged user on a CONFIG_CHECKPOINT_RESTORE kernel >>> (nearly all distros) can rewrite mm boundaries including start_brk, brk, >>> arg_start/end, env_start/end and saved_auxv. Consequences include: >>> >>> - SELinux PROCESS__EXECHEAP bypass via start_brk manipulation >>> - procfs info disclosure by pointing arg/env ranges at other memory >>> - auxv poisoning (AT_SYSINFO_EHDR, AT_BASE, AT_ENTRY) >>> >>> The original commit f606b77f1a9e ("prctl: PR_SET_MM -- introduce >>> PR_SET_MM_MAP operation") states "we require the caller to be at least >>> user-namespace root user", but this was never enforced in the code. >>> >>> Add a checkpoint_restore_ns_capable() check at the top of >>> prctl_set_mm_map(), after the PR_SET_MM_MAP_SIZE early return. This >>> requires CAP_CHECKPOINT_RESTORE or CAP_SYS_ADMIN in the caller's >>> user namespace, matching the stated design intent and the existing >>> check for exe_fd changes. >>> >>> Fixes: f606b77f1a9e ("prctl: PR_SET_MM -- introduce PR_SET_MM_MAP operation") >> >> We've had a gaping security hole since 2014 and nobody noticed? I find it >> hard to believe. >> >>> Cc: stable@vger.kernel.org >>> Cc: Cyrill Gorcunov >>> Signed-off-by: Qi Tang >>> --- >>> kernel/sys.c | 3 +++ >>> 1 file changed, 3 insertions(+) >>> >>> diff --git a/kernel/sys.c b/kernel/sys.c >>> index c86eba9aa7e9..2b8c57f23a35 100644 >>> --- a/kernel/sys.c >>> +++ b/kernel/sys.c >>> @@ -2071,6 +2071,9 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data >>> return put_user((unsigned int)sizeof(prctl_map), >>> (unsigned int __user *)addr); >>> >>> + if (!checkpoint_restore_ns_capable(current_user_ns())) >>> + return -EPERM; >> >> Hmm there is already: >> >> if (prctl_map.exe_fd != (u32)-1) { >> /* >> * Check if the current user is checkpoint/restore capable. >> * At the time of this writing, it checks for CAP_SYS_ADMIN >> * or CAP_CHECKPOINT_RESTORE. >> * Note that a user with access to ptrace can masquerade an >> * arbitrary program as any executable, even setuid ones. >> * This may have implications in the tomoyo subsystem. >> */ >> if (!checkpoint_restore_ns_capable(current_user_ns())) >> return -EPERM; >> >> And you're proposing _adding_ this check on top of that? Seems super >> redundant. > > Yes, should be moved. > >> >> but also, this seems super-specific buuut... Then again #ifdef >> CONFIG_CHECKPOINT_RESTORE around this. Ugh. >> >> I _hate_ this inteface. HATE HATE HATE it. >> >> Anyway, does updating _your own_ auxv really require elevated permissions >> like this? >> >> I don't think so? Couldn't you go and manipulate that anyway without >> elevated anything? > > Hard to believe ... > > I was wondering whether this could break some users. At least CRIU doc > states: > > This option tells *criu* to accept the limitations when running > as non-root. Running as non-root requires *criu* at least to have > *CAP_SYS_ADMIN* or *CAP_CHECKPOINT_RESTORE*. For details about > running *criu* as non-root please consult the *NON-ROOT* section. Doing some digging, lxc seems to use that interface. https://github.com/lxc/lxc/blob/3ee89c5d95ee8f31bd81623fd73ad7beea4297f8/src/lxc/initutils.c#L311 I have no clue about capabilities there. -- Cheers, David