Date: Mon, 19 Aug 2024 11:15:23 +0000 From: bugzilla-noreply@freebsd.org To: bugs@FreeBSD.org Subject: [Bug 280809] jail_attach(2) fails to document reason for EPERM Message-ID: <bug-280809-227-RvijC0B4mF@https.bugs.freebsd.org/bugzilla/> In-Reply-To: <bug-280809-227@https.bugs.freebsd.org/bugzilla/> References: <bug-280809-227@https.bugs.freebsd.org/bugzilla/>
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D280809 --- Comment #3 from crest@rlwinm.de --- There is the kern.chroot_allow_open_directories sysctl to allow chroot() wi= th directory file descriptors, but no such "backdoor" exists for jail_attach(), because jails are supposed to provide secure namespace isolation. Removing = this restriction would allow trivial jail escapes through fchdir() and openat(). Allowing jailed processes to safely share directory file descriptors either through inheritance or file descriptor passing could be very useful, but is totally unsafe right now. Sharing directory file descriptors through inheritance across jails is (and always has been) impossible because jail_attach() doesn't allow processes with having one or more such file descriptors to attach to a jail. Despite this it's possible to share file descriptors across jails via file descriptor passing over unix domain socke= ts. Passing directory file descriptors to a directory outside the receivers jail root into a jail is an instant file system escape because the jail root is enforced by comparing vnodes and if you `parent_fd =3D openat(dir_fd, "..", O_DIRECTORY)` you won't encounter the jail root vnode. By looping until "."= =3D=3D ".." the jailed process can find the hosts root filesystem. While the unix socket could be brought into a jail via jail_attach() the more likely real world situation would be intentionally sharing a nullfs mount across jails (e.g. a PostgreSQL jail <-> a FastCGI jail). One way I can think of to make it safe would be to add a O_RESOLVE_BENEATH = like write-once flag to file descriptors and allow only directory file descripto= rs with this flag to be shared across jails. It would have to be inherited too (e.g. openat(dir_fd, "<subdir>", O_DIRECTORY) on a sub-directory). If the f= lag could be automatically set by jail_attach() and file descriptor passing acr= oss jails would work without extra syscalls to set the new flag with the caveat that ".." inaccessible on such file descriptors and those derived from them. --=20 You are receiving this mail because: You are the assignee for the bug.=
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