On Mon, 23 Nov 2009 14:05:24 -0800 ebiederm@xxxxxxxxxxxx (Eric W. Biederman) wrote: > Jeff Layton <jlayton@xxxxxxxxxx> writes: > > > There are a few situations where a lookup can end up returning a dentry > > without revalidating it, and without checking whether the calling > > process has permissions to access it. Two situations identified so far > > are: > > > > 1) LAST_BIND symlinks (such as those under /proc/<pid>) > > > > 2) file bind mounts > > > > This patchset is intended to fix this by forcing revalidation of the > > returned dentries at appropriate locations. > > > > In the case of LAST_BIND symlinks it also adds a check to verify that > > the target of the symlink is accessible by the current process by > > walking mounts and dentries back up to the root and checking permission > > on each inode. > > > > This set fixes the reproducers I have (including the reproducer that > > Pavel provided for the permissions bypass). It's still pretty rough > > though and I expect that it'll need revision. At this point, I'm mainly > > looking to get these questions answered: > > > > 1) what should we do if these dentries are found to be invalid? Is it ok > > to d_invalidate them? Or is that likely to break something (particularly > > in the case of file bind mounts)? > > The normal sequence in do_revalidate should be safe. In practice what we > should see is d_drop(). If we access the dentries via another path today > we already go through d_revalidate. It is only the reference count on > the dentry that keeps them alive and working. The cases I have looked > at for distributed filesystems have to call d_drop themselves so I don't > know if it would add anything if the vfs called d_revalidate. Especially > since FS_REVAL_DOT doesn't have that logic. > > > 2) I'm using FS_REVAL_DOT as an indicator of whether to force a > > d_revalidate. I think that it's appropriate to key off of that flag, but > > we may want to rename it (maybe FS_FORCE_D_REVAL ?). > > Perhaps FS_ALWAYS_REVAL. I don't think it makes much of > a difference either way. I expect a rename should come after we fix > nfsv4 so there is a chance at pushing the fixes back to stable. > > > 3) is check_path_accessible racy? It seems to work, but something > > doesn't seem quite right with this approach. Is this defeatable somehow? > > Could a rename of one of the intermediate path components cause > > problems? > > check_path_accessible seems pretty horrible. If a process is running > inside of a subdirectory it doesn't have permissions to access, say > a chroot, /proc/self/fd/XXX becomes completely unusable. > Hmm...I have this in there: + /* are we at global root or root of namespace? */ + if ((tdentry == root.dentry && vfsmnt == root.mnt) || + vfsmnt->mnt_parent == vfsmnt) + break; ...In the case of a chroot, wouldn't "current->fs->root" point to the root of the process' namespace? Or am I misunderstanding what current->fs actually represents? -- Jeff Layton <jlayton@xxxxxxxxxx> -- 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
