Re: [PATCH v7] Add a "nosymfollow" mount option.

[Aleksa Sarai]

On 2020-08-11, Ross Zwisler <zwisler@xxxxxxxxxxxx> wrote:
> From: Mattias Nissler <mnissler@xxxxxxxxxxxx>
> 
> For mounts that have the new "nosymfollow" option, don't follow symlinks
> when resolving paths. The new option is similar in spirit to the
> existing "nodev", "noexec", and "nosuid" options, as well as to the
> LOOKUP_NO_SYMLINKS resolve flag in the openat2(2) syscall. Various BSD
> variants have been supporting the "nosymfollow" mount option for a long
> time with equivalent implementations.
> 
> Note that symlinks may still be created on file systems mounted with
> the "nosymfollow" option present. readlink() remains functional, so
> user space code that is aware of symlinks can still choose to follow
> them explicitly.
> 
> Setting the "nosymfollow" mount option helps prevent privileged
> writers from modifying files unintentionally in case there is an
> unexpected link along the accessed path. The "nosymfollow" option is
> thus useful as a defensive measure for systems that need to deal with
> untrusted file systems in privileged contexts.
> 
> More information on the history and motivation for this patch can be
> found here:
> 
> https://sites.google.com/a/chromium.org/dev/chromium-os/chromiumos-design-docs/hardening-against-malicious-stateful-data#TOC-Restricting-symlink-traversal

Looks good. Did you plan to add an in-tree test for this (you could
shove it in tools/testing/selftests/mount)?

Reviewed-by: Aleksa Sarai <cyphar@xxxxxxxxxx>

> Signed-off-by: Mattias Nissler <mnissler@xxxxxxxxxxxx>
> Signed-off-by: Ross Zwisler <zwisler@xxxxxxxxxx>
> ---
> Changes since v6 [1]:
>  * Rebased onto v5.8.
>  * Another round of testing including readlink(1), readlink(2),
>    realpath(1), realpath(3), statfs(2) and mount(2) to make sure
>    everything still works.
> 
> After this lands I will upstream changes to util-linux[2] and man-pages
> [3].
> 
> [1]: https://lkml.org/lkml/2020/3/4/770
> [2]: https://github.com/rzwisler/util-linux/commit/7f8771acd85edb70d97921c026c55e1e724d4e15
> [3]: https://github.com/rzwisler/man-pages/commit/b8fe8079f64b5068940c0144586e580399a71668
> ---
>  fs/namei.c                 | 3 ++-
>  fs/namespace.c             | 2 ++
>  fs/proc_namespace.c        | 1 +
>  fs/statfs.c                | 2 ++
>  include/linux/mount.h      | 3 ++-
>  include/linux/statfs.h     | 1 +
>  include/uapi/linux/mount.h | 1 +
>  7 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 72d4219c93acb..ed68478fb1fb6 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -1626,7 +1626,8 @@ static const char *pick_link(struct nameidata *nd, struct path *link,
>  			return ERR_PTR(error);
>  	}
>  
> -	if (unlikely(nd->flags & LOOKUP_NO_SYMLINKS))
> +	if (unlikely(nd->flags & LOOKUP_NO_SYMLINKS) ||
> +			unlikely(nd->path.mnt->mnt_flags & MNT_NOSYMFOLLOW))
>  		return ERR_PTR(-ELOOP);
>  
>  	if (!(nd->flags & LOOKUP_RCU)) {
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 4a0f600a33285..1cbbf5a9b954f 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -3167,6 +3167,8 @@ 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_NOSYMFOLLOW)
> +		mnt_flags |= MNT_NOSYMFOLLOW;
>  
>  	/* The default atime for remount is preservation */
>  	if ((flags & MS_REMOUNT) &&
> diff --git a/fs/proc_namespace.c b/fs/proc_namespace.c
> index 3059a9394c2d6..e59d4bb3a89e4 100644
> --- a/fs/proc_namespace.c
> +++ b/fs/proc_namespace.c
> @@ -70,6 +70,7 @@ static void show_mnt_opts(struct seq_file *m, struct vfsmount *mnt)
>  		{ MNT_NOATIME, ",noatime" },
>  		{ MNT_NODIRATIME, ",nodiratime" },
>  		{ MNT_RELATIME, ",relatime" },
> +		{ MNT_NOSYMFOLLOW, ",nosymfollow" },
>  		{ 0, NULL }
>  	};
>  	const struct proc_fs_opts *fs_infop;
> diff --git a/fs/statfs.c b/fs/statfs.c
> index 2616424012ea7..59f33752c1311 100644
> --- a/fs/statfs.c
> +++ b/fs/statfs.c
> @@ -29,6 +29,8 @@ static int flags_by_mnt(int mnt_flags)
>  		flags |= ST_NODIRATIME;
>  	if (mnt_flags & MNT_RELATIME)
>  		flags |= ST_RELATIME;
> +	if (mnt_flags & MNT_NOSYMFOLLOW)
> +		flags |= ST_NOSYMFOLLOW;
>  	return flags;
>  }
>  
> diff --git a/include/linux/mount.h b/include/linux/mount.h
> index de657bd211fa6..aaf343b38671c 100644
> --- a/include/linux/mount.h
> +++ b/include/linux/mount.h
> @@ -30,6 +30,7 @@ struct fs_context;
>  #define MNT_NODIRATIME	0x10
>  #define MNT_RELATIME	0x20
>  #define MNT_READONLY	0x40	/* does the user want this to be r/o? */
> +#define MNT_NOSYMFOLLOW	0x80
>  
>  #define MNT_SHRINKABLE	0x100
>  #define MNT_WRITE_HOLD	0x200
> @@ -46,7 +47,7 @@ struct fs_context;
>  #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_NOSYMFOLLOW)
>  #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 9bc69edb8f188..fac4356ea1bfc 100644
> --- a/include/linux/statfs.h
> +++ b/include/linux/statfs.h
> @@ -40,6 +40,7 @@ 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_NOSYMFOLLOW	0x2000	/* do not follow symlinks */
>  
>  struct dentry;
>  extern int vfs_get_fsid(struct dentry *dentry, __kernel_fsid_t *fsid);
> diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h
> index 96a0240f23fed..dd8306ea336c1 100644
> --- a/include/uapi/linux/mount.h
> +++ b/include/uapi/linux/mount.h
> @@ -16,6 +16,7 @@
>  #define MS_REMOUNT	32	/* Alter flags of a mounted FS */
>  #define MS_MANDLOCK	64	/* Allow mandatory locks on an FS */
>  #define MS_DIRSYNC	128	/* Directory modifications are synchronous */
> +#define MS_NOSYMFOLLOW	256	/* Do not follow symlinks */
>  #define MS_NOATIME	1024	/* Do not update access times. */
>  #define MS_NODIRATIME	2048	/* Do not update directory access times */
>  #define MS_BIND		4096
> -- 
> 2.28.0.236.gb10cc79966-goog
> 

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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