[RESEND PATCH v6 4/5] proc: Relax check of mount visibility

Alexey Gladkov
2021-07-16T10:46:53Z

Allow to mount procfs with subset=pid option even if the entire procfs
is not fully accessible to the user.

Signed-off-by: Alexey Gladkov <legion@kernel.org>
---
fs/namespace.c | 30 ++++++++++++++++++------------
fs/proc/root.c | 16 ++++++++++------
include/linux/fs.h | 1 +
3 files changed, 29 insertions(+), 18 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 9d33909d0f9e..f38570fdfc3f 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3951,7 +3951,8 @@ static bool mnt_already_visible(struct mnt_namespace *ns,
/* This mount is not fully visible if it's root directory
* is not the root directory of the filesystem.
*/
- if (mnt->mnt.mnt_root != mnt->mnt.mnt_sb->s_root)
+ if (!(sb->s_iflags & SB_I_DYNAMIC) &&
+ mnt->mnt.mnt_root != mnt->mnt.mnt_sb->s_root)
continue;

/* A local view of the mount flags */
@@ -3971,18 +3972,23 @@ static bool mnt_already_visible(struct mnt_namespace *ns,
((mnt_flags & MNT_ATIME_MASK) != (new_flags & MNT_ATIME_MASK)))
continue;

- /* This mount is not fully visible if there are any
- * locked child mounts that cover anything except for
- * empty directories.
+ /* If this filesystem is completely dynamic, then it
+ * makes no sense to check for any child mounts.
*/
- list_for_each_entry(child, &mnt->mnt_mounts, mnt_child) {
- struct inode *inode = child->mnt_mountpoint->d_inode;
- /* Only worry about locked mounts */
- if (!(child->mnt.mnt_flags & MNT_LOCKED))
- continue;
- /* Is the directory permanetly empty? */
- if (!is_empty_dir_inode(inode))
- goto next;
+ if (!(sb->s_iflags & SB_I_DYNAMIC)) {
+ /* This mount is not fully visible if there are any
+ * locked child mounts that cover anything except for
+ * empty directories.
+ */
+ list_for_each_entry(child, &mnt->mnt_mounts, mnt_child) {
+ struct inode *inode = child->mnt_mountpoint->d_inode;
+ /* Only worry about locked mounts */
+ if (!(child->mnt.mnt_flags & MNT_LOCKED))
+ continue;
+ /* Is the directory permanetly empty? */
+ if (!is_empty_dir_inode(inode))
+ goto next;
+ }
}
/* Preserve the locked attributes */
*new_mnt_flags |= mnt_flags & (MNT_LOCK_READONLY | \
diff --git a/fs/proc/root.c b/fs/proc/root.c
index 0d20bb67e79a..c739ed94246c 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -145,18 +145,21 @@ static int proc_parse_param(struct fs_context *fc, struct fs_parameter *param)
return 0;
}

-static int proc_apply_options(struct proc_fs_info *fs_info,
+static int proc_apply_options(struct super_block *s,
struct fs_context *fc,
struct user_namespace *user_ns)
{
struct proc_fs_context *ctx = fc->fs_private;
+ struct proc_fs_info *fs_info = proc_sb_info(s);

if (ctx->mask & (1 << Opt_gid))
fs_info->pid_gid = make_kgid(user_ns, ctx->gid);
if (ctx->mask & (1 << Opt_hidepid))
fs_info->hide_pid = ctx->hidepid;
if (ctx->mask & (1 << Opt_subset)) {
- if (ctx->pidonly != PROC_PIDONLY_ON && fs_info->pidonly == PROC_PIDONLY_ON)
+ if (ctx->pidonly == PROC_PIDONLY_ON)
+ s->s_iflags |= SB_I_DYNAMIC;
+ else if (fs_info->pidonly == PROC_PIDONLY_ON)
return invalf(fc, "proc: subset=pid cannot be unset\n");
fs_info->pidonly = ctx->pidonly;
}
@@ -176,9 +179,6 @@ static int proc_fill_super(struct super_block *s, struct fs_context *fc)

fs_info->pid_ns = get_pid_ns(ctx->pid_ns);
fs_info->mounter_cred = get_cred(fc->cred);
- ret = proc_apply_options(fs_info, fc, current_user_ns());
- if (ret)
- return ret;

/* User space would break if executables or devices appear on proc */
s->s_iflags |= SB_I_USERNS_VISIBLE | SB_I_NOEXEC | SB_I_NODEV;
@@ -190,6 +190,10 @@ static int proc_fill_super(struct super_block *s, struct fs_context *fc)
s->s_time_gran = 1;
s->s_fs_info = fs_info;

+ ret = proc_apply_options(s, fc, current_user_ns());
+ if (ret)
+ return ret;
+
/*
* procfs isn't actually a stacking filesystem; however, there is
* too much magic going on inside it to permit stacking things on
@@ -230,7 +234,7 @@ static int proc_reconfigure(struct fs_context *fc)
put_cred(fs_info->mounter_cred);
fs_info->mounter_cred = get_cred(fc->cred);

- return proc_apply_options(fs_info, fc, current_user_ns());
+ return proc_apply_options(sb, fc, current_user_ns());
}

static int proc_get_tree(struct fs_context *fc)
diff --git a/include/linux/fs.h b/include/linux/fs.h
index fd47deea7c17..2c9a47bad796 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1390,6 +1390,7 @@ extern int send_sigurg(struct fown_struct *fown);
#define SB_I_USERNS_VISIBLE 0x00000010 /* fstype already mounted */
#define SB_I_IMA_UNVERIFIABLE_SIGNATURE 0x00000020
#define SB_I_UNTRUSTED_MOUNTER 0x00000040
+#define SB_I_DYNAMIC 0x00000080

#define SB_I_SKIP_SYNC 0x00000100 /* Skip superblock at global sync */

--
2.29.3