[PATCH 6.1 294/303] binfmt_misc: use exe_file_deny_write_access() for the interpreter clone

6.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Christian Brauner <[email protected]>

commit fa5990ca8fd917003e526036bcc50413edb9722c upstream.

For MISC_FMT_OPEN_FILE entries load_misc_binary() clones the
registered interpreter file and denies write access to the clone via
plain deny_write_access(). The clone is installed as
bprm->interpreter and later released by the exec machinery through
exe_file_allow_write_access() which skips the i_writecount increment
for files with FMODE_FSNOTIFY_HSM set.

The deny and allow side can therefore come to different conclusions
when pre-content watches are in play: if a pre-content watch is added
to the interpreter after registration every subsequent exec through
that entry takes a write denial on the clone that is never paired
with a write allowance, driving the interpreter inode's i_writecount
further down with each exec and leaving the interpreter unwritable
even after the entry and all its users are gone.

Take the write denial via exe_file_deny_write_access() so both sides
of the pairing base their decision on the same file mode, and
propagate failure instead of silently ignoring it: an interpreter
that is concurrently open for writing now fails the exec with
ETXTBSY, exactly like an interpreter freshly opened via open_exec()
would.

Link: https://patch.msgid.link/[email protected]
Fixes: 0357ef03c94e ("fs: don't block write during exec on pre-content watched files")
Cc: [email protected]
Signed-off-by: Christian Brauner (Amutable) <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
 fs/binfmt_misc.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -220,8 +220,14 @@ static int load_misc_binary(struct linux
 
 	if (fmt->flags & MISC_FMT_OPEN_FILE) {
 		interp_file = file_clone_open(fmt->interp_file);
-		if (!IS_ERR(interp_file))
-			deny_write_access(interp_file);
+		if (!IS_ERR(interp_file)) {
+			int err = exe_file_deny_write_access(interp_file);
+
+			if (err) {
+				fput(interp_file);
+				interp_file = ERR_PTR(err);
+			}
+		}
 	} else {
 		interp_file = open_exec(fmt->interpreter);
 	}