Re: [PATCH v12 05/12] namei: obey trailing magic-link DAC permissions

Jann Horn
2019-09-17T21:30:23Z

On Wed, Sep 4, 2019 at 10:21 PM Aleksa Sarai <cyphar@cyphar.com> wrote:
> The ability for userspace to "re-open" file descriptors through
> /proc/self/fd has been a very useful tool for all sorts of usecases
> (container runtimes are one common example). However, the current
> interface for doing this has resulted in some pretty subtle security
> holes. Userspace can re-open a file descriptor with more permissions
> than the original, which can result in cases such as /proc/$pid/exe
> being re-opened O_RDWR at a later date even though (by definition)
> /proc/$pid/exe cannot be opened for writing. When combined with O_PATH
> the results can get even more confusing.
[...]
> Instead we have to restrict it in such a way that it doesn't break
> (good) users but does block potential attackers. The solution applied in
> this patch is to restrict *re-opening* (not resolution through)
> magic-links by requiring that mode of the link be obeyed. Normal
> symlinks have modes of a+rwx but magic-links have other modes. These
> magic-link modes were historically ignored during path resolution, but
> they've now been re-purposed for more useful ends.

Thanks for dealing with this issue!

[...]
> diff --git a/fs/namei.c b/fs/namei.c
> index 209c51a5226c..54d57dad0f91 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -872,7 +872,7 @@ void nd_jump_link(struct path *path)
>
> nd->path = *path;
> nd->inode = nd->path.dentry->d_inode;
> - nd->flags |= LOOKUP_JUMPED;
> + nd->flags |= LOOKUP_JUMPED | LOOKUP_MAGICLINK_JUMPED;
> }
[...]
> +static int trailing_magiclink(struct nameidata *nd, int acc_mode,
> + fmode_t *opath_mask)
> +{
> + struct inode *inode = nd->link_inode;
> + fmode_t upgrade_mask = 0;
> +
> + /* Was the trailing_symlink() a magic-link? */
> + if (!(nd->flags & LOOKUP_MAGICLINK_JUMPED))
> + return 0;
> +
> + /*
> + * Figure out the upgrade-mask of the link_inode. Since these aren't
> + * strictly POSIX semantics we don't do an acl_permission_check() here,
> + * so we only care that at least one bit is set for each upgrade-mode.
> + */
> + if (inode->i_mode & S_IRUGO)
> + upgrade_mask |= FMODE_PATH_READ;
> + if (inode->i_mode & S_IWUGO)
> + upgrade_mask |= FMODE_PATH_WRITE;
> + /* Restrict the O_PATH upgrade-mask of the caller. */
> + if (opath_mask)
> + *opath_mask &= upgrade_mask;
> + return may_open_magiclink(upgrade_mask, acc_mode);
> }

This looks racy because entries in the file descriptor table can be
switched out as long as task->files->file_lock isn't held. Unless I'm
missing something, something like the following (untested) would
bypass this restriction:

int readonly_fd = ...; /* some read-only fd we want to reopen as writable */
int writable_fd = open("/dev/null", O_RDWR);
int flippy_fd = dup(writable_fd);
char flippy_fd_path[100];
sprintf(flippy_fd_path, "/proc/%d/fd/%d", getpid(), flippy_fd);
if (fork() == 0) {
while (1) {
int reopened_fd = open(flippy_fd_path, O_RDWR);
if (reopened_fd == -1) continue;
char reopened_fd_path[100];
sprintf(reopened_fd_path, "/proc/self/fd/%d", reopened_fd);
char reopened_fd_target[1000];
int target_len = readlink(reopened_fd_path, reopened_fd_target,
sizeof(reopened_fd_target)-1);
reopened_fd_target[target_len] = 0;
if (strcmp(reopened_fd_target, "/dev/null"))
printf("managed to reopen as writable\n");
close(reopened_fd);
}
} else {
while (1) {
dup2(readonly_fd, flippy_fd);
dup2(writable_fd, flippy_fd);
}
}

Perhaps you could change nd_jump_link() to "void nd_jump_link(struct
path *path, umode_t link_mode)", and let proc_pid_get_link() pass the
link_mode through from an out-argument of .proc_get_link()? Then
proc_fd_link() could grab the proper mode in a race-free manner. And
nd_jump_link() could stash the mode in the nameidata.

A sketch of how I imagine that:
===============================
diff --git a/fs/namei.c b/fs/namei.c
index 6b936038319b..14c6790203c7 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -506,6 +506,7 @@ struct nameidata {
struct inode *link_inode;
unsigned root_seq;
int dfd;
+ umode_t last_link_mode;
} __randomize_layout;

static void set_nameidata(struct nameidata *p, int dfd, struct filename *name)
@@ -890,7 +891,7 @@ static int nd_jump_root(struct nameidata *nd)
* Helper to directly jump to a known parsed path from ->get_link,
* caller must have taken a reference to path beforehand.
*/
-void nd_jump_link(struct path *path)
+void nd_jump_link(struct path *path, umode_t link_mode)
{
struct nameidata *nd = current->nameidata;
path_put(&nd->path);
@@ -898,6 +899,7 @@ void nd_jump_link(struct path *path)
nd->path = *path;
nd->inode = nd->path.dentry->d_inode;
nd->flags |= LOOKUP_JUMPED | LOOKUP_MAGICLINK_JUMPED;
+ nd->last_link_mode = link_mode;
}

static inline void put_link(struct nameidata *nd)
@@ -3654,9 +3656,9 @@ static int trailing_magiclink(struct nameidata
*nd, int acc_mode,
* strictly POSIX semantics we don't do an acl_permission_check() here,
* so we only care that at least one bit is set for each upgrade-mode.
*/
- if (inode->i_mode & S_IRUGO)
+ if (nd->last_link_mode & S_IRUGO)
upgrade_mask |= FMODE_PATH_READ;
- if (inode->i_mode & S_IWUGO)
+ if (nd->last_link_mode & S_IWUGO)
upgrade_mask |= FMODE_PATH_WRITE;
/* Restrict the O_PATH upgrade-mask of the caller. */
if (opath_mask)
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 297242174402..af0218447571 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1614,6 +1614,7 @@ static const char *proc_pid_get_link(struct
dentry *dentry,
{
struct path path;
int error = -EACCES;
+ umode_t link_mode;

if (!dentry)
return ERR_PTR(-ECHILD);
@@ -1622,11 +1623,11 @@ static const char *proc_pid_get_link(struct
dentry *dentry,
if (!proc_fd_access_allowed(inode))
goto out;

- error = PROC_I(inode)->op.proc_get_link(dentry, &path);
+ error = PROC_I(inode)->op.proc_get_link(dentry, &path, &link_mode);
if (error)
goto out;

- nd_jump_link(&path);
+ nd_jump_link(&path, link_mode);
return NULL;
out:
return ERR_PTR(error);
diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index 9b7d8becb002..9c1d247177b1 100644
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -163,7 +163,8 @@ static const struct dentry_operations
tid_fd_dentry_operations = {
.d_delete = pid_delete_dentry,
};

-static int proc_fd_link(struct dentry *dentry, struct path *path)
+static int proc_fd_link(struct dentry *dentry, struct path *path,
+ umode_t *link_mode)
{
struct files_struct *files = NULL;
struct task_struct *task;
@@ -184,6 +185,7 @@ static int proc_fd_link(struct dentry *dentry,
struct path *path)
if (fd_file) {
*path = fd_file->f_path;
path_get(&fd_file->f_path);
+ *link_mode = /* something based on fd_file->f_mode */;
ret = 0;
}
spin_unlock(&files->file_lock);
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index cd0c8d5ce9a1..a090fff984ed 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -74,7 +74,7 @@ extern struct kmem_cache *proc_dir_entry_cache;
void pde_free(struct proc_dir_entry *pde);

union proc_op {
- int (*proc_get_link)(struct dentry *, struct path *);
+ int (*proc_get_link)(struct dentry *, struct path *, umode_t *);
int (*proc_show)(struct seq_file *m,
struct pid_namespace *ns, struct pid *pid,
struct task_struct *task);
===============================