unix: Preserve FD_RESOLVE_BENEATH when passing an fd

The FD_RESOLVE_BENEATH flag is supposed to be sticky.  It's set when you
receive an fd from a different jail and preserved by openat(<dfd>) etc..
However, if you send the fd to yourself, the flag is stripped since
SCM_RIGHTS message don't preserve file descriptor flags.

Fix this by preserving those flags and checking for UF_RESOLVE_BENEATH
in restrict_rights().

Approved by:	so
Security:	FreeBSD-SA-26:66.jail
Security:	CVE-2026-101306
Fixes:		350ba9672a ("unix: Set O_RESOLVE_BENEATH on fds transferred between jails")
Reviewed by:	kib
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D58317

(cherry picked from commit 586e2b3d89)
(cherry picked from commit 441885dcff)

sys/kern/uipc_usrreq.c

@@ -2433,15 +2433,25 @@ unp_freerights(struct filedescent **fdep, int fdcount)
 	free(fdep[0], M_FILECAPS);
 }
 
-static bool
-restrict_rights(struct file *fp, struct thread *td)
+/*
+ * Flags to set on the receiving side when externalizing a file descriptor.
+ * When transferring fds between jails, ensure that the receiver cannot use
+ * a dirfd to escape the jail chroot.
+ */
+static int
+externalize_fdflags(struct filedescent *fde, struct thread *td)
 {
 	struct prison *prison1, *prison2;
 
-	prison1 = fp->f_cred->cr_prison;
+	if ((fde->fde_flags & UF_RESOLVE_BENEATH) != 0)
+		return (O_RESOLVE_BENEATH);
+	prison1 = fde->fde_file->f_cred->cr_prison;
 	prison2 = td->td_ucred->cr_prison;
-	return (prison1 != prison2 && prison1->pr_root != prison2->pr_root &&
-	    prison2 != &prison0);
+	if (prison1 != prison2 && prison1->pr_root != prison2->pr_root &&
+	    prison2 != &prison0)
+		return (O_RESOLVE_BENEATH);
+	else
+		return (0);
 }
 
 static int
@@ -2506,9 +2516,9 @@ unp_externalize(struct mbuf *control, struct mbuf **controlp, int flags)
 				struct file *fp;
 
 				fp = fdep[i]->fde_file;
-				_finstall(fdesc, fp, *fdp, fdflags |
-				    (restrict_rights(fp, td) ?
-				    O_RESOLVE_BENEATH : 0), &fdep[i]->fde_caps);
+				_finstall(fdesc, fp, *fdp,
+				    fdflags | externalize_fdflags(fdep[i], td),
+				    &fdep[i]->fde_caps);
 				unp_externalize_fp(fp);
 			}
 
@@ -2744,6 +2754,7 @@ unp_internalize(struct mbuf **controlp, struct thread *td,
 				fdep[i]->fde_file = fde->fde_file;
 				filecaps_copy(&fde->fde_caps,
 				    &fdep[i]->fde_caps, true);
+				fdep[i]->fde_flags = fde->fde_flags;
 				unp_internalize_fp(fdep[i]->fde_file);
 			}
 			FILEDESC_SUNLOCK(fdesc);

tests/sys/kern/unix_passfd_test.c

@@ -1076,6 +1076,36 @@ ATF_TC_CLEANUP(cross_jail_dirfd, tc)
 		err(1, "jail_remove");
 }
 
+/*
+ * Verify that FD_RESOLVE_BENEATH is preserved when an fd is passed over a UNIX
+ * domain socket.
+ */
+ATF_TC_WITHOUT_HEAD(resolve_beneath_preserved);
+ATF_TC_BODY(resolve_beneath_preserved, tc)
+{
+	int fd[2], getfd, putfd, fdflags;
+
+	domainsocketpair(fd);
+	tempfile(&putfd);
+
+	fdflags = fcntl(putfd, F_GETFD);
+	ATF_REQUIRE(fdflags != -1);
+	ATF_REQUIRE(fcntl(putfd, F_SETFD, fdflags | FD_RESOLVE_BENEATH) != -1);
+	ATF_REQUIRE((fcntl(putfd, F_GETFD) & FD_RESOLVE_BENEATH) != 0);
+
+	sendfd(fd[0], putfd);
+	recvfd(fd[1], &getfd, 0);
+
+	fdflags = fcntl(getfd, F_GETFD);
+	ATF_REQUIRE(fdflags != -1);
+	ATF_REQUIRE_MSG((fdflags & FD_RESOLVE_BENEATH) != 0,
+	    "FD_RESOLVE_BENEATH was not preserved across SCM_RIGHTS transfer");
+
+	ATF_REQUIRE(close(putfd) == 0);
+	ATF_REQUIRE(close(getfd) == 0);
+	closesocketpair(fd);
+}
+
 ATF_TP_ADD_TCS(tp)
 {
 
@@ -1095,6 +1125,7 @@ ATF_TP_ADD_TCS(tp)
 	ATF_TP_ADD_TC(tp, copyout_rights_error);
 	ATF_TP_ADD_TC(tp, empty_rights_message);
 	ATF_TP_ADD_TC(tp, cross_jail_dirfd);
+	ATF_TP_ADD_TC(tp, resolve_beneath_preserved);
 
 	return (atf_no_error());
 }