Quoting Eric W. Biederman (ebiederm@xxxxxxxxxxxx): > "Serge E. Hallyn" <serue@xxxxxxxxxx> writes: > > > Quoting H. Peter Anvin (hpa@xxxxxxxxx): > >> Miklos Szeredi wrote: > >> > > >> > Andrew, please skip this patch, for now. > >> > > >> > Serge found a problem with the fsuid approach: setfsuid(nonzero) will > >> > remove filesystem related capabilities. So even if root is trying to > >> > set the "user=UID" flag on a mount, access to the target (and in case > >> > of bind, the source) is checked with user privileges. > >> > > >> > Root should be able to set this flag on any mountpoint, _regardless_ > >> > of permissions. > >> > > >> > >> Right, if you're using fsuid != 0, you're not running as root > > > > Sure, but what I'm not clear on is why, if I've done a > > prctl(PR_SET_KEEPCAPS, 1) before the setfsuid, I still lose the > > CAP_FS_MASK perms. I see the special case handling in > > cap_task_post_setuid(). I'm sure there was a reason for it, but > > this is a piece of the capability implementation I don't understand > > right now. > > So we drop CAP_CHOWN, CAP_DAC_OVERRIDE, CAP_DAC_READ_SEARCH, > CAP_FOWNER, and CAP_FSETID > > Since we are checking CAP_SETUID or CAP_SYS_ADMIN how is that > a problem? > > Are there other permission checks that mount is doing that we > care about. Not mount itself, but in looking up /share/fa/root/home/fa, user fa doesn't have the rights to read /share, and by setting fsuid to fa and dropping CAP_DAC_READ_SEARCH the mount action fails. But the solution you outlined in your previous post would work around this perfectly. > >> (fsuid is > >> the equivalent to euid for the filesystem.) > > > > If it were really the equivalent then I could keep my capabilities :) > > after changing it. > > We drop all capabilities after we change the euid. Not if we've done prctl(PR_SET_KEEPCAPS, 1) > >> I fail to see how ruid should have *any* impact on mount(2). That seems > >> to be a design flaw. > > > > May be, but just using fsuid at this point stops me from enabling user > > mounts under /share if /share is chmod 000 (which it is). > > I'm dense today. If we can't work out the details we can always use a flag. > But what is the problem with fsuid? See above. > You are not trying to test this using a non-default security model are you? Nope, at the moment CONFIG_SECURITY=n so I'm running with capabilities only. thanks, -serge - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
