[PATCH] fs: allow vfs code to open an O_PATH file with negative dentry

[Amir Goldstein <amir73il@gmail.com>]

The fields f_mapping, f_wb_err, f_sb_err are irrelevant for O_PATH file.
Skip setting them for O_PATH file, so that the O_PATH file could be
opened with a negative dentry.

This is not something that a user should be able to do, but vfs code,
such as ovl_tmpfile() can use this to open a backing O_PATH tmpfile
before instantiating the dentry.

Reported-by: syzbot+f34aab278bf5d664e2be@syzkaller.appspotmail.com
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---

Christian,

This patch fixes the syzbot report [1] that the
backing_file_user_path_file() patch [2] introduces.

This is not the only possible fix, but it is the cleanest one IMO.
There is a small risk in introducing a state of an O_PATH file with
NULL f_inode, but I (and the bots that I asked) did not find any
obvious risk in this state.

Note that specifically, the user path inode is accessed via d_inode()
and not via file_inode(), which makes this safe for file_user_inode()
callers.

BTW, I missed this regression with the original patch because I
only ran the quick overlayfs sanity test.

Now I ran a full quick fstest cycle and verified that the O_TMPFILE
test case is covered and that the bug is detected.

WDYT?

Thanks,
Amir.

[1] https://syzkaller.appspot.com/bug?extid=f34aab278bf5d664e2be
[2] https://lore.kernel.org/linux-fsdevel/20260318131258.1457101-1-amir73il@gmail.com/

 fs/open.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index 91f1139591abe..2004a8c0d9c97 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -893,9 +893,6 @@ static int do_dentry_open(struct file *f,
 
 	path_get(&f->f_path);
 	f->f_inode = inode;
-	f->f_mapping = inode->i_mapping;
-	f->f_wb_err = filemap_sample_wb_err(f->f_mapping);
-	f->f_sb_err = file_sample_sb_err(f);
 
 	if (unlikely(f->f_flags & O_PATH)) {
 		f->f_mode = FMODE_PATH | FMODE_OPENED;
@@ -904,6 +901,10 @@ static int do_dentry_open(struct file *f,
 		return 0;
 	}
 
+	f->f_mapping = inode->i_mapping;
+	f->f_wb_err = filemap_sample_wb_err(f->f_mapping);
+	f->f_sb_err = file_sample_sb_err(f);
+
 	if ((f->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ) {
 		i_readcount_inc(inode);
 	} else if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) {
-- 
2.53.0