Commit f6122ed2 authored Jan 03, 2019 by Sahitya Tummala Committed by Christoph Hellwig May 28, 2019

configfs: Fix use-after-free when accessing sd->s_dentry

In the vfs_statx() context, during path lookup, the dentry gets
added to sd->s_dentry via configfs_attach_attr(). In the end,
vfs_statx() kills the dentry by calling path_put(), which invokes
configfs_d_iput(). Ideally, this dentry must be removed from
sd->s_dentry but it doesn't if the sd->s_count >= 3. As a result,
sd->s_dentry is holding reference to a stale dentry pointer whose
memory is already freed up. This results in use-after-free issue,
when this stale sd->s_dentry is accessed later in
configfs_readdir() path.

This issue can be easily reproduced, by running the LTP test case -
sh fs_racer_file_list.sh /config
(https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/fs/racer/fs_racer_file_list.sh

)

Fixes: 76ae281f ('configfs: fix race between dentry put and lookup')
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>

parent cd6c84d8

fs/configfs/dir.c

+6 −8

Original line number	Diff line number	Diff line
		@@ -58,15 +58,13 @@ static void configfs_d_iput(struct dentry * dentry,
		if (sd) {
		/* Coordinate with configfs_readdir */
		spin_lock(&configfs_dirent_lock);
		/* Coordinate with configfs_attach_attr where will increase
		* sd->s_count and update sd->s_dentry to new allocated one.
		* Only set sd->dentry to null when this dentry is the only
		* sd owner.
		* If not do so, configfs_d_iput may run just after
		* configfs_attach_attr and set sd->s_dentry to null
		* even it's still in use.
		/*
		* Set sd->s_dentry to null only when this dentry is the one
		* that is going to be killed. Otherwise configfs_d_iput may
		* run just after configfs_attach_attr and set sd->s_dentry to
		* NULL even it's still in use.
		*/
		if (atomic_read(&sd->s_count) <= 2)
		if (sd->s_dentry == dentry)
		sd->s_dentry = NULL;

		spin_unlock(&configfs_dirent_lock);

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