I have a requirement where I want to make sure that mount() fails if mount point is a symlink. Hence introducing a new mount flag MS_NOSYMLINK. Following is little more info on what I am trying to do. I am trying to write patches for signed /sbin/kexec. That is /sbin/kexec binary will be signed and in secureboot environment kernel will verify signature of /sbin/kexec and upon successful verfication, /sbin/kexec will be trusted and allowed to load new kernel. /sbin/kexec gathers bunch of data from /sys and /proc. Given the fact that only /sbin/kexec is trusted and not other root processes, one need to make sure that a root process can not alter /sys or /proc to fool /sbin/kexec. So requirement is that /sbin/kexec needs to make sure that it is looking at /proc and /sys as exported by kernel (and not an artificial view possibly created by a root process). Eric Biederman suggested that use per process mount name space functionality. /sbin/kexec runs as root. So create separate mount namespace. Make it recursively private to disable any event propogation. Unmount existing /proc and /sys and remount them. Actual code of what I am trying to do in kexec-tools is posted here. https://lists.fedoraproject.org/pipermail/kernel/2013-September/004463.html Al Viro mentioned that one needs to make sure /proc and /sys are not symlinks. Otherwise after remounting, root could remove symlinks and create /proc and /sys with its own files. And there comes the need to make sure mount point is not a symlink and hence this patch. I did basic testing by doing following and it seems to work. syscall(__NR_mount, "none", <mount-point>, "proc", 1<<25,""); Signed-off-by: Vivek Goyal <vgoyal at redhat.com> --- fs/namespace.c | 6 +++++- include/uapi/linux/fs.h | 1 + 2 files changed, 6 insertions(+), 1 deletion(-) diff --git a/fs/namespace.c b/fs/namespace.c index 7b1ca9b..d19627e 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -2278,7 +2278,11 @@ long do_mount(const char *dev_name, const char *dir_name, ((char *)data_page)[PAGE_SIZE - 1] = 0; /* ... and get the mountpoint */ - retval = kern_path(dir_name, LOOKUP_FOLLOW, &path); + if (flags & MS_NOSYMLINK) + retval = kern_path(dir_name, 0, &path); + else + retval = kern_path(dir_name, LOOKUP_FOLLOW, &path); + if (retval) return retval; diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h index a4ed56c..584f083 100644 --- a/include/uapi/linux/fs.h +++ b/include/uapi/linux/fs.h @@ -86,6 +86,7 @@ struct inodes_stat_t { #define MS_KERNMOUNT (1<<22) /* this is a kern_mount call */ #define MS_I_VERSION (1<<23) /* Update inode I_version field */ #define MS_STRICTATIME (1<<24) /* Always perform atime updates */ +#define MS_NOSYMLINK (1<<25) /* Do not follow symlink at the end */ /* These sb flags are internal to the kernel */ #define MS_NOSEC (1<<28) -- 1.8.3.1
