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From 852a0a22d674b0594aecf0912a0885d197f34978 Mon Sep 17 00:00:00 2001
From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Date: Tue, 5 May 2020 16:35:57 +0100
Subject: [PATCH 6/9] virtiofsd: jail lo->proc_self_fd

RH-Author: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-id: <20200505163600.22956-5-dgilbert@redhat.com>
Patchwork-id: 96275
O-Subject: [RHEL-AV-8.2.1 qemu-kvm PATCH 4/7] virtiofsd: jail lo->proc_self_fd
Bugzilla: 1817445
RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
RH-Acked-by: Max Reitz <mreitz@redhat.com>
RH-Acked-by: Michael S. Tsirkin <mst@redhat.com>

From: Miklos Szeredi <mszeredi@redhat.com>

While it's not possible to escape the proc filesystem through
lo->proc_self_fd, it is possible to escape to the root of the proc
filesystem itself through "../..".

Use a temporary mount for opening lo->proc_self_fd, that has it's root at
/proc/self/fd/, preventing access to the ancestor directories.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Message-Id: <20200429124733.22488-1-mszeredi@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
(cherry picked from commit 397ae982f4df46e7d4b2625c431062c9146f3b83)
Signed-off-by: Danilo C. L. de Paula <ddepaula@redhat.com>
---
 tools/virtiofsd/passthrough_ll.c | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
index 184ad0f..73d8405 100644
--- a/tools/virtiofsd/passthrough_ll.c
+++ b/tools/virtiofsd/passthrough_ll.c
@@ -2540,6 +2540,8 @@ static void print_capabilities(void)
 static void setup_namespaces(struct lo_data *lo, struct fuse_session *se)
 {
     pid_t child;
+    char template[] = "virtiofsd-XXXXXX";
+    char *tmpdir;
 
     /*
      * Create a new pid namespace for *child* processes.  We'll have to
@@ -2601,12 +2603,33 @@ static void setup_namespaces(struct lo_data *lo, struct fuse_session *se)
         exit(1);
     }
 
+    tmpdir = mkdtemp(template);
+    if (!tmpdir) {
+        fuse_log(FUSE_LOG_ERR, "tmpdir(%s): %m\n", template);
+        exit(1);
+    }
+
+    if (mount("/proc/self/fd", tmpdir, NULL, MS_BIND, NULL) < 0) {
+        fuse_log(FUSE_LOG_ERR, "mount(/proc/self/fd, %s, MS_BIND): %m\n",
+                 tmpdir);
+        exit(1);
+    }
+
     /* Now we can get our /proc/self/fd directory file descriptor */
-    lo->proc_self_fd = open("/proc/self/fd", O_PATH);
+    lo->proc_self_fd = open(tmpdir, O_PATH);
     if (lo->proc_self_fd == -1) {
-        fuse_log(FUSE_LOG_ERR, "open(/proc/self/fd, O_PATH): %m\n");
+        fuse_log(FUSE_LOG_ERR, "open(%s, O_PATH): %m\n", tmpdir);
         exit(1);
     }
+
+    if (umount2(tmpdir, MNT_DETACH) < 0) {
+        fuse_log(FUSE_LOG_ERR, "umount2(%s, MNT_DETACH): %m\n", tmpdir);
+        exit(1);
+    }
+
+    if (rmdir(tmpdir) < 0) {
+        fuse_log(FUSE_LOG_ERR, "rmdir(%s): %m\n", tmpdir);
+    }
 }
 
 /*
-- 
1.8.3.1