mnt: Only change user settable mount flags in remount (a6138db8) · Commits · 戴 / test

Kenton Varda <kenton@sandstorm.io> discovered that by remounting a read-only bind mount read-only in a user namespace the MNT_LOCK_READONLY bit would be cleared, allowing an unprivileged user to the remount a read-only mount read-write. Correct this by replacing the mask of mount flags to preserve with a mask of mount flags that may be changed, and preserve all others. This ensures that any future bugs with this mask and remount will fail in an easy to detect way where new mount flags simply won't change. Cc: stable@vger.kernel.org Acked-by:

Serge E. Hallyn <serge.hallyn@ubuntu.com> Signed-off-by:

"Eric W. Biederman" <ebiederm@xmission.com>

fs/namespace.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -1937,7 +1937,7 @@ static int do_remount(struct path *path, int flags, int mnt_flags,
		err = do_remount_sb(sb, flags, data, 0);
		if (!err) {
		lock_mount_hash();
		mnt_flags \|= mnt->mnt.mnt_flags & MNT_PROPAGATION_MASK;
		mnt_flags \|= mnt->mnt.mnt_flags & ~MNT_USER_SETTABLE_MASK;
		mnt->mnt.mnt_flags = mnt_flags;
		touch_mnt_namespace(mnt->mnt_ns);
		unlock_mount_hash();

include/linux/mount.h

+3 −1

Original line number	Diff line number	Diff line
		@@ -42,7 +42,9 @@ struct mnt_namespace;
		* flag, consider how it interacts with shared mounts.
		*/
		#define MNT_SHARED_MASK (MNT_UNBINDABLE)
		#define MNT_PROPAGATION_MASK (MNT_SHARED \| MNT_UNBINDABLE)
		#define MNT_USER_SETTABLE_MASK (MNT_NOSUID \| MNT_NODEV \| MNT_NOEXEC \
		\| MNT_NOATIME \| MNT_NODIRATIME \| MNT_RELATIME \
		\| MNT_READONLY)

		#define MNT_INTERNAL_FLAGS (MNT_SHARED \| MNT_WRITE_HOLD \| MNT_INTERNAL \| \
		MNT_DOOMED \| MNT_SYNC_UMOUNT \| MNT_MARKED)

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