From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: From: "Frank Filz" To: "'Andy Lutomirski'" , "'Ben Hutchings'" Cc: "'Andy Lutomirski'" , , "'Konstantin Khlebnikov'" , "'Alexander Viro'" , "'Kees Cook'" , "'Willy Tarreau'" , , "'Andrew Morton'" , "'yalin wang'" , "'Linux Kernel Mailing List'" , "'Jan Kara'" , "'Linux FS Devel'" , "'stable'" References: <9318903980969a0e378dab2de4d803397adcd3cc.1485377903.git.luto@kernel.org> <1485380634.2998.161.camel@decadent.org.uk> In-Reply-To: Subject: RE: [PATCH 1/2] fs: Check f_cred instead of current's creds in should_remove_suid() Date: Wed, 25 Jan 2017 15:15:16 -0800 Message-ID: <014301d27760$e4d8b9a0$ae8a2ce0$@mindspring.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Content-Language: en-us Sender: owner-linux-mm@kvack.org List-ID: > On Wed, Jan 25, 2017 at 1:43 PM, Ben Hutchings > wrote: > > On Wed, 2017-01-25 at 13:06 -0800, Andy Lutomirski wrote: > >> If an unprivileged program opens a setgid file for write and passes > >> the fd to a privileged program and the privileged program writes to > >> it, we currently fail to clear the setgid bit. Fix it by checking > >> f_cred instead of current's creds whenever a struct file is involved. > > [...] > > > > What if, instead, a privileged program passes the fd to an un > > unprivileged program? It sounds like a bad idea to start with, but at > > least currently the unprivileged program is going to clear the setgid > > bit when it writes. This change would make that behaviour more > > dangerous. > > Hmm. Although, if a privileged program does something like: > > (sudo -u nobody echo blah) >setuid_program > > presumably it wanted to make the change. I'm not following all the intricacies here, though I need to... What about a privileged program that drops privilege for certain operations= ? Specifically the Ganesha user space NFS server runs as root, but sets fsuid= /fsgid for specific threads performing I/O operations on behalf of NFS clie= nts. I want to make sure setgid bit handling is proper for these cases. Ganesha does some permission checking, but this is one area I want to defer= to the underlying filesystem because it's not easy for Ganesha to get it = right. > > Perhaps there should be a capability check on both the current > > credentials and file credentials? (I realise that we've considered > > file credential checks to be sufficient elsewhere, but those cases > > involved virtual files with special semantics, where it's clearer that > > a privileged process should not pass them to an unprivileged process.) > > > > I could go either way. > > What I really want to do is to write a third patch that isn't for -stable= that just > removes the capable() check entirely. I'm reasonably confident it won't > break things for a silly reason: because it's capable() and not ns_capabl= e(), > anything it would break would also be broken in an unprivileged container= , > and I haven't seen any reports of package managers or similar breaking fo= r > this reason. Frank --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org