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 3E1B6140396; Thu, 11 Apr 2024 09:05:04 +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=1712826305; cv=none; b=DnlQmxIh1ELqNhvYfuDQGx/tbcYCIO/62N3D1L08ykvwCvsMoODDK+7mv1Nziz1DDzqPwKt3f92wzGJONfyVb8cGQbECYxpfGfvGNWlP72LrMLqh9cyHYD+mO8cRnH+tKMWn+4v4Rk4eXkFcSQlowdY8wCHatv1BDrdws3GtN94= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1712826305; c=relaxed/simple; bh=o+/0y3Ieo193BWmJYH5o8du85gicknBAkm/zQSbBIY0=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=DMY4Y1MCkXWoSX/liGxD0nBNEnqrKInP8RiVTLUmgS4DJDuhJsdE8ND5wo8BOiWbmKycoSgbL0Fpv3U5kn+Xu3q6t6V7omg4rQkBdvnoSMt6HDAjVAcMVy42x2iM+906YPqvaAc0IY/nOHhXSTLtL68Wyo4NfZyW33Ti5svdGQY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=T0De4/C7; 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="T0De4/C7" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6A143C433F1; Thu, 11 Apr 2024 09:05:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1712826304; bh=o+/0y3Ieo193BWmJYH5o8du85gicknBAkm/zQSbBIY0=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=T0De4/C7NxC5OdeB2sTjskbPNWWhQYBx5GyPYMEXilDrUsLdGguuBlEEB69qbIhYc pZ8Qc03Uc1uG0EbrSw3CASE+kcASpvWbKQ/IhHmHqAO5hZcZMcFsKN1+UAh27fMzA7 yc4TEQW3Otnplxqt1EUwLcS6nFEcva2IbtZcwiyJa79420u5eqJE8f1Yfhve2op0fb wK4+Uab/js5XcL0UUok3DThHW8ItWNVaX87IyEe4SHdvaUDKYSlV2MX0DjXBG5UXRi 32/VBO49xp5Jiri3T2g6rlhyYobGqMkQRfkrwChjpRt3fIxnivD8TSRZNNm9Jyx5kI yogJBhp0QpY6A== Date: Thu, 11 Apr 2024 11:04:59 +0200 From: Christian Brauner To: Linus Torvalds Cc: Alexander Viro , Jan Kara , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Andrew Lutomirski , Peter Anvin Subject: Re: [PATCH] vfs: relax linkat() AT_EMPTY_PATH - aka flink() - requirements Message-ID: <20240411-alben-kocht-219170e9dc99@brauner> References: <20240411001012.12513-1-torvalds@linux-foundation.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: On Wed, Apr 10, 2024 at 07:39:49PM -0700, Linus Torvalds wrote: > On Wed, 10 Apr 2024 at 17:10, Linus Torvalds > wrote: > > > > + if (flags & LOOKUP_DFD_MATCH_CREDS) { > > + if (f.file->f_cred != current_cred() && > > + !capable(CAP_DAC_READ_SEARCH)) { > > + fdput(f); > > + return ERR_PTR(-ENOENT); > > + } > > + } > > Side note: I suspect that this could possibly be relaxed further, by > making the rule be that if something has been explicitly opened to be > used as a path (ie O_PATH was used at open time), we can link to it > even across different credentials. I had a similar discussion a while back someone requested that we relax permissions so linkat can be used in containers. And I drafted the following patch back then: https://lore.kernel.org/all/20231113-undenkbar-gediegen-efde5f1c34bc@brauner IOW, I had intended to make this work with containers so that we check CAP_DAC_READ_SEARCH in the namespace of the opener of the file. My thinking had been that this can serve as a way to say "Hey, I could've opened this file in the openers namespace therefore let me make a path to it.". I didn't actually send it because I thought the original author would but imho, that would be a worthwhile addition to your patch if this makes sense... > > IOW, the above could perhaps even be > > + if (flags & LOOKUP_DFD_MATCH_CREDS) { > + if (!(f.file->f_mode & FMODE_PATH) && > + f.file->f_cred != current_cred() && > + !capable(CAP_DAC_READ_SEARCH)) { > + fdput(f); > + return ERR_PTR(-ENOENT); > + } > + } > > which would _allow_ people to pass in paths as file descriptors if > they actually wanted to. > > After all, the only thing you can do with an O_PATH file descriptor is > to use it as a path - there would be no other reason to use O_PATH in > the first place. So if you now pass it to somebody else, clearly you > are intentionally trying to make it available *as* a path. > > So you could imagine doing something like this: > > // Open path as root > int fd = open('filename", O_PATH); > > // drop privileges > // setresuid(..) or chmod() or enter new namespace or whatever > > linkat(fd, "", AT_FDCWD, "newname", AT_EMPTY_PATH); > > and it would open the path with one set of privileges, but then > intentionally go into a more restricted mode and create a link to the > source within that restricted environment. > > Sensible? Who knows. I'm just throwing this out as another "this may > be the solution to our historical flink() issues". > > Linus