On Tue, Dec 04, 2018 at 10:15:49AM -0500, Vivek Goyal wrote: > On Tue, Dec 04, 2018 at 09:30:53AM -0500, Stephen Smalley wrote: > > On 12/4/18 8:32 AM, Miklos Szeredi wrote: > > > On Thu, Nov 29, 2018 at 10:16 PM Stephen Smalley <sds@xxxxxxxxxxxxx> wrote: > > > > > > > > On 11/29/18 4:03 PM, Stephen Smalley wrote: > > > > > On 11/29/18 2:47 PM, Miklos Szeredi wrote: > > > > > > On Thu, Nov 29, 2018 at 5:14 PM Stephen Smalley <sds@xxxxxxxxxxxxx> > > > > > > wrote: > > > > > > > > > > > > > Possibly I misunderstood you, but I don't think we want to copy-up on > > > > > > > permission denial, as that would still allow the mounter to read/write > > > > > > > special files or execute regular files to which it would normally be > > > > > > > denied access, because the copy would inherit the context specified by > > > > > > > the mounter in the context mount case. It still represents an > > > > > > > escalation of privilege for the mounter. In contrast, the copy-up on > > > > > > > write behavior does not allow the mounter to do anything it could not do > > > > > > > already (i.e. read from the lower, write to the upper). > > > > > > > > > > > > Let's get this straight: when file is copied up, it inherits label > > > > > > from context=, not from label of lower file? > > > > > > > > > > That's correct. The overlay inodes are all assigned the label from the > > > > > context= mount option, and so are any upper inodes created through the > > > > > overlay. At least that's my understanding of how it is supposed to > > > > > work. The original use case was for containers with the lower dir > > > > > labeled with a context that is read-only to the container context and > > > > > using a context that is writable by the container context for the > > > > > context= mount. > > > > > > > > > > > Next question: permission to change metadata is tied to permission to > > > > > > open? Is it possible that open is denied, but metadata can be > > > > > > changed? > > > > > > > > > > There is no metadata change occurring here. The overlay, upper, and > > > > > lower inodes all keep their labels intact for their lifetime (both > > > > > overlay and upper always have the context= label; upper has whatever its > > > > ^^lower^^ > > > > > > > > > original label was), unless explicitly relabeled by some process. And > > > > > when viewed through the overlay, the file always has the label specified > > > > > via context=, even before the copy-up. > > > > > > Okay. > > > > > > > > > > > > > > DAC model allows this: metadata change is tied to ownership, not mode > > > > > > bits. And different capability flag. > > > > > > > > > > > > If the same is true for MAC, then the pre-v4.20-rc1 is already > > > > > > susceptible to the privilege escalation you describe, right? > > > > > > > > > > Actually, I guess there wouldn't be a privilege escalation if you > > > > > checked the mounter's ability to create the new file upon copy-up, and > > > > > checked the mounter's access to the upper inode label upon the > > > > > subsequent read, write, or execute access. Then we'd typically block > > > > > the ability to create the device file and we'd block the ability to > > > > > execute files with the label from context=. > > > > > > > > > > But copy-up of special files seems undesirable for other reasons (e.g. > > > > > requiring mounters to be allowed to create device nodes just to permit > > > > > client's to read/write them, possible implications for nodev/noexec, > > > > > implications for socket and fifo files). > > > > > > I think you missed my point: opening a device file or executing an > > > executable wouldn't normally require copy-up. If > > > > > > - permission is granted on overlay to task, and > > > - permission is granted on lower layer to mounter, > > > > > > then copy-up wouldn't be performed. > > > > > > My proposed sequence would be > > > > > > a) check task's creds against overlay inode, fail -> return fail, otherwise: > > > b) check mounter's creds against lower inode, success -> return > > > success, otherwise: > > > c) copy up inode, fail -> return fail, otherwise > > > d) check mounter's creds against upper inode, return result. > > > > > > So, unlike write access to regular files, write access to special > > > files don't necessarily result in copy-up. > > > > > > You say this is an escalation of privilege, but I don't get it how. > > > As DWalsh points out downthread, if mounter cannot create device > > > files, then the copy-up will simply fail. If mounter can create > > > device files, then this is not an escalation of privilege for the > > > mounter. > > > > Yes, in that case there isn't an escalation of privilege for the mounter (I > > acknowledged that above). I'm still not sure copy-up of special files is a > > good idea though: > > > > - In the case of device files, there is the potential for mischief by the > > client task in misusing the mounter's privileges to gain access to otherwise > > unusable device node (nodev lower vs upper?), > > I was thinking about it as well. But client can always bypass permissions > of lower device inode by first removing device file and then by doing > a mknod. And that will be equivalent of copy up. IOW, IIUC, we do not deny > mknod to client and that always creates a way for it to write to device > file (and it does not matter what are permissions on lower?) Having said that, this still create little anomaly when mknod to client is not allowed on context label. So a device file, which is on lower and client can not open it for read/write on host, it can now be opened for read/write because mounter will allow access. So why it is different that regular copy up. Well, in regular copy up, we created a copy of the original object and allowed writing to that object (cp --preserve=all) model. But in case of device file, writes will go to same original object. (And not a separate copy). Thanks Vivek
