118 строки
4.3 KiB
Diff
118 строки
4.3 KiB
Diff
From d4865c4cdd0bb67587ef339af817902db9cbfcd0 Mon Sep 17 00:00:00 2001
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From: Jann Horn <jannh@google.com>
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Date: Sat, 14 Jul 2018 13:37:41 +0200
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Subject: [PATCH] fusermount: whitelist known-good filesystems for mountpoints
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Before:
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$ _FUSE_COMMFD=1 priv_strace -s8000 -e trace=mount util/fusermount3 /proc/self/fd
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mount("/dev/fuse", ".", "fuse", MS_NOSUID|MS_NODEV, "fd=3,rootmode=40000,user_id=379777,group_id=5001") = 0
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sending file descriptor: Socket operation on non-socket
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+++ exited with 1 +++
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After:
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$ _FUSE_COMMFD=1 priv_strace -s8000 -e trace=mount util/fusermount3 /proc/self/fd
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util/fusermount3: mounting over filesystem type 0x009fa0 is forbidden
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+++ exited with 1 +++
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This patch could potentially have security
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impact on some systems that are configured with allow_other;
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see https://launchpad.net/bugs/1530566 for an example of how a similar
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issue in the ecryptfs mount helper was exploitable. However, the FUSE
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mount helper performs slightly different security checks, so that exact
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attack doesn't work with fusermount; I don't know of any specific attack
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you could perform using this, apart from faking the SELinux context of your
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process when someone's looking at a process listing. Potential targets for
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overwrite are (looking on a system with a 4.9 kernel):
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writable only for the current process:
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/proc/self/{fd,map_files}
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(Yes, "ls -l" claims that you don't have write access, but that's not true;
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"find -writable" will show you what access you really have.)
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writable also for other owned processes:
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/proc/$pid/{sched,autogroup,comm,mem,clear_refs,attr/*,oom_adj,
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oom_score_adj,loginuid,coredump_filter,uid_map,gid_map,projid_map,
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setgroups,timerslack_ns}
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---
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util/fusermount.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++-
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1 file changed, 49 insertions(+), 1 deletion(-)
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diff --git a/util/fusermount.c b/util/fusermount.c
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index 27924073..c63c50ef 100644
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--- a/util/fusermount.c
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+++ b/util/fusermount.c
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@@ -30,6 +30,7 @@
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#include <sys/utsname.h>
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#include <sched.h>
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#include <stdbool.h>
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+#include <sys/vfs.h>
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#define FUSE_COMMFD_ENV "_FUSE_COMMFD"
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@@ -915,6 +916,8 @@ static int check_perm(const char **mntp, struct stat *stbuf, int *mountpoint_fd)
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int res;
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const char *mnt = *mntp;
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const char *origmnt = mnt;
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+ struct statfs fs_buf;
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+ size_t i;
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res = lstat(mnt, stbuf);
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if (res == -1) {
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@@ -987,8 +990,53 @@ static int check_perm(const char **mntp, struct stat *stbuf, int *mountpoint_fd)
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return -1;
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}
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+ /* Do not permit mounting over anything in procfs - it has a couple
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+ * places to which we have "write access" without being supposed to be
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+ * able to just put anything we want there.
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+ * Luckily, without allow_other, we can't get other users to actually
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+ * use any fake information we try to put there anyway.
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+ * Use a whitelist to be safe. */
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+ if (statfs(*mntp, &fs_buf)) {
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+ fprintf(stderr, "%s: failed to access mountpoint %s: %s\n",
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+ progname, mnt, strerror(errno));
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+ return -1;
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+ }
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- return 0;
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+ /* Use the same list of permitted filesystems for the mount target as
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+ * the ecryptfs mount helper
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+ * (https://bazaar.launchpad.net/~ecryptfs/ecryptfs/trunk/view/head:/src/utils/mount.ecryptfs_private.c#L225). */
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+ typeof(fs_buf.f_type) f_type_whitelist[] = {
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+ 0x61756673 /* AUFS_SUPER_MAGIC */,
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+ 0x9123683E /* BTRFS_SUPER_MAGIC */,
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+ 0x00C36400 /* CEPH_SUPER_MAGIC */,
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+ 0xFF534D42 /* CIFS_MAGIC_NUMBER */,
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+ 0x0000F15F /* ECRYPTFS_SUPER_MAGIC */,
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+ 0x0000EF53 /* EXT[234]_SUPER_MAGIC */,
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+ 0xF2F52010 /* F2FS_SUPER_MAGIC */,
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+ 0x65735546 /* FUSE_SUPER_MAGIC */,
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+ 0x01161970 /* GFS2_MAGIC */,
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+ 0x3153464A /* JFS_SUPER_MAGIC */,
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+ 0x000072B6 /* JFFS2_SUPER_MAGIC */,
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+ 0x0000564C /* NCP_SUPER_MAGIC */,
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+ 0x00006969 /* NFS_SUPER_MAGIC */,
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+ 0x00003434 /* NILFS_SUPER_MAGIC */,
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+ 0x5346544E /* NTFS_SB_MAGIC */,
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+ 0x794C7630 /* OVERLAYFS_SUPER_MAGIC */,
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+ 0x52654973 /* REISERFS_SUPER_MAGIC */,
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+ 0x73717368 /* SQUASHFS_MAGIC */,
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+ 0x01021994 /* TMPFS_MAGIC */,
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+ 0x24051905 /* UBIFS_SUPER_MAGIC */,
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+ 0x58465342 /* XFS_SB_MAGIC */,
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+ 0x2FC12FC1 /* ZFS_SUPER_MAGIC */,
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+ };
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+ for (i = 0; i < sizeof(f_type_whitelist)/sizeof(f_type_whitelist[0]); i++) {
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+ if (f_type_whitelist[i] == fs_buf.f_type)
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+ return 0;
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+ }
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+
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+ fprintf(stderr, "%s: mounting over filesystem type %#010lx is forbidden\n",
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+ progname, (unsigned long)fs_buf.f_type);
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+ return -1;
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}
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static int try_open(const char *dev, char **devp, int silent)
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