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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