From 34c62ee90c69b07998629f6b5a06ab0120be681c Mon Sep 17 00:00:00 2001 From: Jann Horn Date: Fri, 13 Jul 2018 14:51:17 -0700 Subject: fusermount: prevent silent truncation of mount options Currently, in the kernel, copy_mount_options() copies in one page of userspace memory (or less if some of that memory area is not mapped). do_mount() then writes a null byte to the last byte of the copied page. This means that mount option strings longer than PAGE_SIZE-1 bytes get truncated silently. Therefore, this can happen: user@d9-ut:~$ _FUSE_COMMFD=10000 fusermount -o "$(perl -e 'print ","x4000')" mount sending file descriptor: Bad file descriptor user@d9-ut:~$ grep /mount /proc/mounts /dev/fuse /home/user/mount fuse rw,nosuid,nodev,relatime,user_id=1000,group_id=1000 0 0 user@d9-ut:~$ fusermount -u mount user@d9-ut:~$ _FUSE_COMMFD=10000 fusermount -o "$(perl -e 'print ","x4050')" mount sending file descriptor: Bad file descriptor user@d9-ut:~$ grep /mount /proc/mounts /dev/fuse /home/user/mount fuse rw,nosuid,nodev,relatime,user_id=1000,group_id=100 0 0 user@d9-ut:~$ fusermount -u mount user@d9-ut:~$ _FUSE_COMMFD=10000 fusermount -o "$(perl -e 'print ","x4051')" mount sending file descriptor: Bad file descriptor user@d9-ut:~$ grep /mount /proc/mounts /dev/fuse /home/user/mount fuse rw,nosuid,nodev,relatime,user_id=1000,group_id=10 0 0 user@d9-ut:~$ fusermount -u mount user@d9-ut:~$ _FUSE_COMMFD=10000 fusermount -o "$(perl -e 'print ","x4052')" mount sending file descriptor: Bad file descriptor user@d9-ut:~$ grep /mount /proc/mounts /dev/fuse /home/user/mount fuse rw,nosuid,nodev,relatime,user_id=1000,group_id=1 0 0 user@d9-ut:~$ fusermount -u mount I'm not aware of any context in which this is actually exploitable - you'd still need the UIDs to fit, and you can't do it if the three GIDs of the process don't match (in the case of a typical setgid binary), but it does look like something that should be fixed. I also plan to try to get this fixed on the kernel side. --- util/fusermount.c | 23 ++++++++++++++++++++--- 1 file changed, 20 insertions(+), 3 deletions(-) diff --git a/util/fusermount.c b/util/fusermount.c index a3cf513..0e1d34d 100644 --- a/util/fusermount.c +++ b/util/fusermount.c @@ -711,6 +711,23 @@ static int get_string_opt(const char *s, unsigned len, const char *opt, return 1; } +/* The kernel silently truncates the "data" argument to PAGE_SIZE-1 characters. + * This can be dangerous if it e.g. truncates the option "group_id=1000" to + * "group_id=1". + * This wrapper detects this case and bails out with an error. + */ +static int mount_notrunc(const char *source, const char *target, + const char *filesystemtype, unsigned long mountflags, + const char *data) { + if (strlen(data) > sysconf(_SC_PAGESIZE) - 1) { + fprintf(stderr, "%s: mount options too long\n", progname); + errno = EINVAL; + return -1; + } + return mount(source, target, filesystemtype, mountflags, data); +} + + static int do_mount(const char *mnt, char **typep, mode_t rootmode, int fd, const char *opts, const char *dev, char **sourcep, char **mnt_optsp) @@ -828,7 +845,7 @@ static int do_mount(const char *mnt, char **typep, mode_t rootmode, else strcpy(source, subtype ? subtype : dev); - res = mount(source, mnt, type, flags, optbuf); + res = mount_notrunc(source, mnt, type, flags, optbuf); if (res == -1 && errno == ENODEV && subtype) { /* Probably missing subtype support */ strcpy(type, blkdev ? "fuseblk" : "fuse"); @@ -839,13 +856,13 @@ static int do_mount(const char *mnt, char **typep, mode_t rootmode, strcpy(source, type); } - res = mount(source, mnt, type, flags, optbuf); + res = mount_notrunc(source, mnt, type, flags, optbuf); } if (res == -1 && errno == EINVAL) { /* It could be an old version not supporting group_id */ sprintf(d, "fd=%i,rootmode=%o,user_id=%u", fd, rootmode, getuid()); - res = mount(source, mnt, type, flags, optbuf); + res = mount_notrunc(source, mnt, type, flags, optbuf); } if (res == -1) { int errno_save = errno; -- cgit v1.2.3