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Date: Wed, 16 Nov 2016 13:18:32 -0800
From: Mattias Nissler <mnissler@...omium.org>
To: linux-fsdevel@...r.kernel.org
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
linux-kernel@...r.kernel.org, Colin Walters <walters@...bum.org>,
"Austin S . Hemmelgarn" <ahferroin7@...il.com>
Subject: Re: [PATCH v2] Add a "nosymlinks" mount option.
I understand that silence suggests there's little interest, but here's
some new information I discovered today that may justify to reconsider
the patch:
The BSDs already have exactly what I propose, the mount option is
called "nosymfollow" there:
https://github.com/freebsd/freebsd/blob/a41f4cc9a57cd74604ae7b051eec2f48865f18d6/sys/kern/vfs_lookup.c#L939
There's also some evidence on the net that people have been using said
nosymfollow mount option to mitigate symlink attacks.
On Mon, Oct 24, 2016 at 7:09 AM, Mattias Nissler <mnissler@...omium.org> wrote:
> Friendly ping - does this version of the patch have any chance on
> getting included in mainline?
>
> On Wed, Oct 19, 2016 at 2:31 PM, Mattias Nissler <mnissler@...omium.org> wrote:
>> For mounts that have the new "nosymlinks" option, don't follow
>> symlinks when resolving paths. The new option is similar in spirit to
>> the existing "nodev", "noexec", and "nosuid" options.
>>
>> Note that symlinks may still be created on mounts where the
>> "nosymlinks" option is present. readlink() remains functional, so user
>> space code that is aware of symlinks can still choose to follow them
>> explicitly.
>>
>> Setting the "nosymlinks" mount option helps prevent privileged writers
>> from modifying files unintentionally in case there is an unexpected
>> link along the accessed path. The "nosymlinks" option is thus useful
>> as a defensive measure for systems that need to deal with untrusted
>> file systems in privileged contexts.
>>
>> Signed-off-by: Mattias Nissler <mnissler@...omium.org>
>> ---
>> fs/namei.c | 3 +++
>> fs/namespace.c | 9 ++++++---
>> fs/proc_namespace.c | 1 +
>> fs/statfs.c | 2 ++
>> include/linux/mount.h | 3 ++-
>> include/linux/statfs.h | 1 +
>> include/uapi/linux/fs.h | 1 +
>> 7 files changed, 16 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/namei.c b/fs/namei.c
>> index 5b4eed2..4cddcf3 100644
>> --- a/fs/namei.c
>> +++ b/fs/namei.c
>> @@ -1021,6 +1021,9 @@ const char *get_link(struct nameidata *nd)
>> touch_atime(&last->link);
>> }
>>
>> + if (nd->path.mnt->mnt_flags & MNT_NOSYMLINKS)
>> + return ERR_PTR(-EACCES);
>> +
>> error = security_inode_follow_link(dentry, inode,
>> nd->flags & LOOKUP_RCU);
>> if (unlikely(error))
>> diff --git a/fs/namespace.c b/fs/namespace.c
>> index e6c234b..deec84e 100644
>> --- a/fs/namespace.c
>> +++ b/fs/namespace.c
>> @@ -2732,6 +2732,8 @@ long do_mount(const char *dev_name, const char __user *dir_name,
>> mnt_flags &= ~(MNT_RELATIME | MNT_NOATIME);
>> if (flags & MS_RDONLY)
>> mnt_flags |= MNT_READONLY;
>> + if (flags & MS_NOSYMLINKS)
>> + mnt_flags |= MNT_NOSYMLINKS;
>>
>> /* The default atime for remount is preservation */
>> if ((flags & MS_REMOUNT) &&
>> @@ -2741,9 +2743,10 @@ long do_mount(const char *dev_name, const char __user *dir_name,
>> mnt_flags |= path.mnt->mnt_flags & MNT_ATIME_MASK;
>> }
>>
>> - flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE | MS_BORN |
>> - MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT |
>> - MS_STRICTATIME | MS_NOREMOTELOCK);
>> + flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_NOSYMLINKS |
>> + MS_ACTIVE | MS_BORN | MS_NOATIME | MS_NODIRATIME |
>> + MS_RELATIME | MS_KERNMOUNT | MS_STRICTATIME |
>> + MS_NOREMOTELOCK);
>>
>> if (flags & MS_REMOUNT)
>> retval = do_remount(&path, flags & ~MS_REMOUNT, mnt_flags,
>> diff --git a/fs/proc_namespace.c b/fs/proc_namespace.c
>> index 3f1190d..a1949d9 100644
>> --- a/fs/proc_namespace.c
>> +++ b/fs/proc_namespace.c
>> @@ -67,6 +67,7 @@ static void show_mnt_opts(struct seq_file *m, struct vfsmount *mnt)
>> { MNT_NOATIME, ",noatime" },
>> { MNT_NODIRATIME, ",nodiratime" },
>> { MNT_RELATIME, ",relatime" },
>> + { MNT_NOSYMLINKS, ",nosymlinks" },
>> { 0, NULL }
>> };
>> const struct proc_fs_info *fs_infop;
>> diff --git a/fs/statfs.c b/fs/statfs.c
>> index 083dc0a..7ff7c32 100644
>> --- a/fs/statfs.c
>> +++ b/fs/statfs.c
>> @@ -27,6 +27,8 @@ static int flags_by_mnt(int mnt_flags)
>> flags |= ST_NODIRATIME;
>> if (mnt_flags & MNT_RELATIME)
>> flags |= ST_RELATIME;
>> + if (mnt_flags & MNT_NOSYMLINKS)
>> + flags |= ST_NOSYMLINKS;
>> return flags;
>> }
>>
>> diff --git a/include/linux/mount.h b/include/linux/mount.h
>> index 1172cce..5e302f4 100644
>> --- a/include/linux/mount.h
>> +++ b/include/linux/mount.h
>> @@ -28,6 +28,7 @@ struct mnt_namespace;
>> #define MNT_NODIRATIME 0x10
>> #define MNT_RELATIME 0x20
>> #define MNT_READONLY 0x40 /* does the user want this to be r/o? */
>> +#define MNT_NOSYMLINKS 0x80
>>
>> #define MNT_SHRINKABLE 0x100
>> #define MNT_WRITE_HOLD 0x200
>> @@ -44,7 +45,7 @@ struct mnt_namespace;
>> #define MNT_SHARED_MASK (MNT_UNBINDABLE)
>> #define MNT_USER_SETTABLE_MASK (MNT_NOSUID | MNT_NODEV | MNT_NOEXEC \
>> | MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME \
>> - | MNT_READONLY)
>> + | MNT_READONLY | MNT_NOSYMLINKS)
>> #define MNT_ATIME_MASK (MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME )
>>
>> #define MNT_INTERNAL_FLAGS (MNT_SHARED | MNT_WRITE_HOLD | MNT_INTERNAL | \
>> diff --git a/include/linux/statfs.h b/include/linux/statfs.h
>> index 0166d32..994b059 100644
>> --- a/include/linux/statfs.h
>> +++ b/include/linux/statfs.h
>> @@ -39,5 +39,6 @@ struct kstatfs {
>> #define ST_NOATIME 0x0400 /* do not update access times */
>> #define ST_NODIRATIME 0x0800 /* do not update directory access times */
>> #define ST_RELATIME 0x1000 /* update atime relative to mtime/ctime */
>> +#define ST_NOSYMLINKS 0x2000 /* do not follow symbolic links */
>>
>> #endif
>> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
>> index acb2b61..06193d8 100644
>> --- a/include/uapi/linux/fs.h
>> +++ b/include/uapi/linux/fs.h
>> @@ -130,6 +130,7 @@ struct inodes_stat_t {
>> #define MS_I_VERSION (1<<23) /* Update inode I_version field */
>> #define MS_STRICTATIME (1<<24) /* Always perform atime updates */
>> #define MS_LAZYTIME (1<<25) /* Update the on-disk [acm]times lazily */
>> +#define MS_NOSYMLINKS (1<<26) /* Do not follow symbolic links */
>>
>> /* These sb flags are internal to the kernel */
>> #define MS_NOREMOTELOCK (1<<27)
>> --
>> 2.8.0.rc3.226.g39d4020
>>
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