Date: Sat, 7 Feb 2015 07:32:43 -0500 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: realpath() and setfsuid programs On Sat, Feb 07, 2015 at 07:26:03AM -0500, Rich Felker wrote: > On Sat, Feb 07, 2015 at 09:53:54AM +0200, Timo Teras wrote: > > Hi, > > > > It seems realpath() does not work in binaries using setfsuid(). (At > > least on grsec kernels, vanilla kernel might be affected too.) > > > > The problem is that realpath() opens the file, and then > > uses just readlink on /proc/self/fd/<fd> to read the canonicalized > > path. > > > > However, /proc/self/fd is not accessible if setfsuid() has been used to > > drop privileges. > > > > The problem I'm looking at in this case is fuse. fusermount, the > > suid wrapper to do user fuse mounts, seems to basically do: > > oldfsuid = setfsuid(getuid()) > > oldfsgid = setfsgid(getgid()) > > take realpath of mountpoint > > chdir("/") > > setfsuid(oldfsuid) > > setfsgid(oldfsgid) > > > > I believe they want to drop privileges so it works as also access check > > to the mount point directory. As realpath() in practice checks that the > > user has access to the entry too. > > Could you clarify what you think the security intent of this code is? > As far as I can tell it's nonsense. realpath is not usable for much of > anything security-related; in particular, it's non-atomic and subject > to all sorts of trickery involving renaming/moving directories during > its operation, even moreso when it's done component-by-component in > userspace. > > Why is the check not simply an ownership check for the mount point? I > suspect it has to do with the need to pass a pathname rather than fd > to mount, which is subject to renaming/moving races, but the realpath > call would be subject to the same and worse. Presumably the correct > way to do this is to open a fd to the mountpoint then pass > /proc/self/fd/%d to the mount function after checking ownership. Or of course just using chdir and checking ownership of ".". Rich
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