Serge Hallyn <serge.hallyn@xxxxxxxxxx> writes:
> Quoting Eric W. Biederman (ebiederm@xxxxxxxxxxxx):
>>
>> Kenton Varda <kenton@xxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx
>> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
>
> Not exactly sure about the name. Actually seems like it should be
> caled MNT_USER_UNCLEARABLE_MASK or something, but
It is the set of mnt_flags that user space can cause to change.
So this patch inverts that mask and unconditionally preserves
everything else.
> Acked-by: Serge E. Hallyn <serge.hallyn@xxxxxxxxxx>
>
>> ---
>> fs/namespace.c | 2 +-
>> include/linux/mount.h | 4 +++-
>> 2 files changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/namespace.c b/fs/namespace.c
>> index 7187d01329c3..cb40449ea0df 100644
>> --- a/fs/namespace.c
>> +++ b/fs/namespace.c
>> @@ -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();
>> diff --git a/include/linux/mount.h b/include/linux/mount.h
>> index 839bac270904..b637a89e1fae 100644
>> --- a/include/linux/mount.h
>> +++ b/include/linux/mount.h
>> @@ -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)
>> --
>> 1.9.1
>>
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