From dab741e0e02bd3c4f5e2e97be74b39df2523fc6e Mon Sep 17 00:00:00 2001 From: Mattias Nissler Date: Thu, 27 Aug 2020 11:09:46 -0600 Subject: Add a "nosymfollow" mount option. 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 Signed-off-by: Mattias Nissler Signed-off-by: Ross Zwisler Reviewed-by: Aleksa Sarai Signed-off-by: Al Viro --- fs/namespace.c | 2 ++ 1 file changed, 2 insertions(+) (limited to 'fs/namespace.c') diff --git a/fs/namespace.c b/fs/namespace.c index bae0e95b3713..6408788a649e 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -3160,6 +3160,8 @@ int path_mount(const char *dev_name, struct path *path, 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) && -- cgit v1.2.3