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Subject[PATCH] configfs: Fix use-after-free when accessing sd->s_dentry
Date
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)

Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
---
fs/configfs/dir.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c
index 39843fa..a5906c5 100644
--- a/fs/configfs/dir.c
+++ b/fs/configfs/dir.c
@@ -66,7 +66,7 @@ static void configfs_d_iput(struct dentry * dentry,
* 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);
--
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
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