[PATCH] exec: drop a racy path_noexec check

[Mateusz Guzik <mjguzik@gmail.com>]

Both i_mode and noexec checks wrapped in WARN_ON stem from an artifact
of the previous implementation. They used to legitimately check for the
condition, but that got moved up in two commits:
633fb6ac3980 ("exec: move S_ISREG() check earlier")
0fd338b2d2cd ("exec: move path_noexec() check earlier")

Instead of being removed said checks are WARN_ON'ed instead, which
has some debug value

However, the spurious path_noexec check is racy, resulting in unwarranted
warnings should someone race with setting the noexec flag.

One can note there is more to perm-checking whether execve is allowed
and none of the conditions are guaranteed to still hold after they were
tested for.

Additionally this does not validate whether the code path did any perm
checking to begin with -- it will pass if the inode happens to be
regular.

As such remove the racy check.

The S_ISREG thing is kept for the time being since it does not hurt.

Reword the commentary and do small tidy ups while here.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
---

On Mon, Aug 05, 2024 at 11:26:19AM +0200, Christian Brauner wrote:
> I think the immediate solution is to limit the scope of the
> WARN_ON_ONCE() to the ->i_mode check.
>

To my reading that path_noexec is still there only for debug, not
because of any security need.

To that end just I propose just whacking it.

 fs/exec.c | 31 ++++++++++++-------------------
 1 file changed, 12 insertions(+), 19 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index a126e3d1cacb..2938cbe38343 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -145,13 +145,10 @@ SYSCALL_DEFINE1(uselib, const char __user *, library)
 		goto out;
 
 	/*
-	 * may_open() has already checked for this, so it should be
-	 * impossible to trip now. But we need to be extra cautious
-	 * and check again at the very end too.
+	 * Check do_open_execat() for an explanation.
 	 */
 	error = -EACCES;
-	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
-			 path_noexec(&file->f_path)))
+	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
 		goto exit;
 
 	error = -ENOEXEC;
@@ -954,7 +951,6 @@ EXPORT_SYMBOL(transfer_args_to_stack);
 static struct file *do_open_execat(int fd, struct filename *name, int flags)
 {
 	struct file *file;
-	int err;
 	struct open_flags open_exec_flags = {
 		.open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
 		.acc_mode = MAY_EXEC,
@@ -971,24 +967,21 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
 
 	file = do_filp_open(fd, name, &open_exec_flags);
 	if (IS_ERR(file))
-		goto out;
+		return file;
 
 	/*
-	 * may_open() has already checked for this, so it should be
-	 * impossible to trip now. But we need to be extra cautious
-	 * and check again at the very end too.
+	 * Validate the type.
+	 *
+	 * In the past the regular type check was here. It moved to may_open() in
+	 * 633fb6ac3980 ("exec: move S_ISREG() check earlier"). Since then it is
+	 * an invariant that all non-regular files error out before we get here.
 	 */
-	err = -EACCES;
-	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
-			 path_noexec(&file->f_path)))
-		goto exit;
+	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode))) {
+		fput(file);
+		return ERR_PTR(-EACCES);
+	}
 
-out:
 	return file;
-
-exit:
-	fput(file);
-	return ERR_PTR(err);
 }
 
 /**
-- 
2.43.0