linux-kernel - Re: [PATCH] fs: RESOLVE_CACHED final path component fix

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Thu, 9 Nov 2023 22:00:18 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Andreas Gruenbacher <agruenba@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Christian Brauner <brauner@...nel.org>,
        Abhi Das <adas@...hat.com>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: RESOLVE_CACHED final path component fix

On Thu, Nov 09, 2023 at 08:08:44PM +0100, Andreas Gruenbacher wrote:
> Jens,
> 
> since your commit 99668f618062, applications can request cached lookups
> with the RESOLVE_CACHED openat2() flag.  When adding support for that in
> gfs2, we found that this causes the ->permission inode operation to be
> called with the MAY_NOT_BLOCK flag set for directories along the path,
> which is good, but the ->permission check on the final path component is
> missing that flag.  The filesystem will then sleep when it needs to read
> in the ACL, for example.
> 
> This doesn't look like the intended RESOLVE_CACHED behavior.
> 
> The file permission checks in path_openat() happen as follows:
> 
> (1) link_path_walk() -> may_lookup() -> inode_permission() is called for
> each but the final path component. If the LOOKUP_RCU nameidata flag is
> set, may_lookup() passes the MAY_NOT_BLOCK flag on to
> inode_permission(), which passes it on to the permission inode
> operation.
> 
> (2) do_open() -> may_open() -> inode_permission() is called for the
> final path component. The MAY_* flags passed to inode_permission() are
> computed by build_open_flags(), outside of do_open(), and passed down
> from there. The MAY_NOT_BLOCK flag doesn't get set.
> 
> I think we can fix this in build_open_flags(), by setting the
> MAY_NOT_BLOCK flag when a RESOLVE_CACHED lookup is requested, right
> where RESOLVE_CACHED is mapped to LOOKUP_CACHED as well.

No.  This will expose ->permission() instances to previously impossible
cases of MAY_NOT_BLOCK lookups, and we already have enough trouble
in that area.  See RCU pathwalk patches I posted last cycle; I'm
planning to rebase what still needs to be rebased and feed the
fixes into mainline, but that won't happen until the end of this
week *AND* ->permission()-related part of code audit will need
to be repeated and extended.

Until then - no, with the side of fuck, no.