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Initialize for fuse

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zyppe 2024-02-10 22:19:31 +08:00
commit 2c967a8691
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fusermount-prevent-silent-truncation-of-mount-options.patch Normal file
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@ -0,0 +1,96 @@
From 34c62ee90c69b07998629f6b5a06ab0120be681c Mon Sep 17 00:00:00 2001
From: Jann Horn <jannh@google.com>
Date: Fri, 13 Jul 2018 14:51:17 -0700
Subject: [PATCH] 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(-)
--- a/util/fusermount.c
+++ b/util/fusermount.c
@@ -712,6 +712,23 @@ static int get_string_opt(const char *s,
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, off_t rootsize)
@@ -836,7 +853,7 @@ static int do_mount(const char *mnt, cha
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");
@@ -847,13 +864,13 @@ static int do_mount(const char *mnt, cha
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;