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Subject[PATCH] ovl: use a dedicated semaphore for dir upperfile caching
Date
The function ovl_dir_real_file() currently uses the semaphore of the
inode to synchronize write to the upperfile cache field.

However, this function will get called by ovl_ioctl_set_flags(), which
utilizes the inode semaphore too. In this case ovl_dir_real_file() will
try to claim a lock that is owned by a function in its call stack, which
won't get released before ovl_dir_real_file() returns.

Define a dedicated semaphore for the upperfile cache, so that the
deadlock won't happen.

Fixes: 61536bed2149 ("ovl: support [S|G]ETFLAGS and FS[S|G]ETXATTR ioctls for directories")
Cc: stable@vger.kernel.org # v5.10
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
---
fs/overlayfs/readdir.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index 01620ebae1bd..f10701aabb71 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -56,6 +56,7 @@ struct ovl_dir_file {
struct list_head *cursor;
struct file *realfile;
struct file *upperfile;
+ struct semaphore upperfile_sem;
};

static struct ovl_cache_entry *ovl_cache_entry_from_node(struct rb_node *n)
@@ -883,7 +884,7 @@ struct file *ovl_dir_real_file(const struct file *file, bool want_upper)
ovl_path_upper(dentry, &upperpath);
realfile = ovl_dir_open_realfile(file, &upperpath);

- inode_lock(inode);
+ down(&od->upperfile_sem);
if (!od->upperfile) {
if (IS_ERR(realfile)) {
inode_unlock(inode);
@@ -896,7 +897,7 @@ struct file *ovl_dir_real_file(const struct file *file, bool want_upper)
fput(realfile);
realfile = od->upperfile;
}
- inode_unlock(inode);
+ up(&od->upperfile_sem);
}
}

@@ -959,6 +960,7 @@ static int ovl_dir_open(struct inode *inode, struct file *file)
od->realfile = realfile;
od->is_real = ovl_dir_is_real(file->f_path.dentry);
od->is_upper = OVL_TYPE_UPPER(type);
+ sema_init(&od->upperfile_sem, 1);
file->private_data = od;

return 0;
--
2.28.0
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