Commit baba1b29 authored Oct 03, 2017 by Oleg Nesterov Committed by Linus Torvalds Oct 03, 2017

exec: binfmt_misc: don't nullify Node->dentry in kill_node()

kill_node() nullifies/checks Node->dentry to avoid double free.  This
complicates the next changes and this is very confusing:

 - we do not need to check dentry != NULL under entries_lock,
   kill_node() is always called under inode_lock(d_inode(root)) and we
   rely on this inode_lock() anyway, without this lock the
   MISC_FMT_OPEN_FILE cleanup could race with itself.

 - if kill_inode() was already called and ->dentry == NULL we should not
   even try to close e->interp_file.

We can change bm_entry_write() to simply check !list_empty(list) before
kill_node.  Again, we rely on inode_lock(), in particular it saves us
from the race with bm_status_write(), another caller of kill_node().

Link: http://lkml.kernel.org/r/20170922143641.GA17210@redhat.com


Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Ben Woodard <woodard@redhat.com>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Jim Foraker <foraker1@llnl.gov>
Cc: <tdhooge@llnl.gov>
Cc: Travis Gummels <tgummels@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent c2315c18

fs/binfmt_misc.c

+9 −13

Original line number	Diff line number	Diff line
		@@ -603,11 +603,7 @@ static void kill_node(Node *e)
		struct dentry *dentry;

		write_lock(&entries_lock);
		dentry = e->dentry;
		if (dentry) {
		list_del_init(&e->list);
		e->dentry = NULL;
		}
		write_unlock(&entries_lock);

		if ((e->flags & MISC_FMT_OPEN_FILE) && e->interp_file) {
		@@ -615,13 +611,12 @@ static void kill_node(Node *e)
		e->interp_file = NULL;
		}

		if (dentry) {
		dentry = e->dentry;
		drop_nlink(d_inode(dentry));
		d_drop(dentry);
		dput(dentry);
		simple_release_fs(&bm_mnt, &entry_count);
		}
		}

		/* /<entry> */

		@@ -665,6 +660,7 @@ static ssize_t bm_entry_write(struct file file, const char __user buffer,
		root = file_inode(file)->i_sb->s_root;
		inode_lock(d_inode(root));

		if (!list_empty(&e->list))
		kill_node(e);

		inode_unlock(d_inode(root));
		@@ -794,7 +790,7 @@ static ssize_t bm_status_write(struct file file, const char __user buffer,
		inode_lock(d_inode(root));

		while (!list_empty(&entries))
		kill_node(list_entry(entries.next, Node, list));
		kill_node(list_first_entry(&entries, Node, list));

		inode_unlock(d_inode(root));
		break;

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