linux-kernel - [PATCH v4 2/5] fs/filesystems.c: downgrade user-reachable WARN_ONCE() to pr_warn

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Date:   Wed, 18 Mar 2020 16:05:12 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     linux-kernel@...r.kernel.org
Cc:     linux-fsdevel@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jeff Vander Stoep <jeffv@...gle.com>,
        Jessica Yu <jeyu@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        NeilBrown <neilb@...e.com>, stable@...r.kernel.org
Subject: [PATCH v4 2/5] fs/filesystems.c: downgrade user-reachable WARN_ONCE() to pr_warn_once()

From: Eric Biggers <ebiggers@...gle.com>

After request_module(), nothing is stopping the module from being
unloaded until someone takes a reference to it via try_get_module().

The WARN_ONCE() in get_fs_type() is thus user-reachable, via userspace
running 'rmmod' concurrently.

Since WARN_ONCE() is for kernel bugs only, not for user-reachable
situations, downgrade this warning to pr_warn_once().

Keep it printed once only, since the intent of this warning is to detect
a bug in modprobe at boot time.  Printing the warning more than once
wouldn't really provide any useful extra information.

Fixes: 41124db869b7 ("fs: warn in case userspace lied about modprobe return")
Acked-by: Luis Chamberlain <mcgrof@...nel.org>
Reviewed-by: Jessica Yu <jeyu@...nel.org>
Cc: <stable@...r.kernel.org> # v4.13+
Cc: Alexei Starovoitov <ast@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Jeff Vander Stoep <jeffv@...gle.com>
Cc: Kees Cook <keescook@...omium.org>
Cc: NeilBrown <neilb@...e.com>
Signed-off-by: Eric Biggers <ebiggers@...gle.com>
---
 fs/filesystems.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/filesystems.c b/fs/filesystems.c
index 77bf5f95362da..90b8d879fbaf3 100644
--- a/fs/filesystems.c
+++ b/fs/filesystems.c
@@ -272,7 +272,9 @@ struct file_system_type *get_fs_type(const char *name)
 	fs = __get_fs_type(name, len);
 	if (!fs && (request_module("fs-%.*s", len, name) == 0)) {
 		fs = __get_fs_type(name, len);
-		WARN_ONCE(!fs, "request_module fs-%.*s succeeded, but still no fs?\n", len, name);
+		if (!fs)
+			pr_warn_once("request_module fs-%.*s succeeded, but still no fs?\n",
+				     len, name);
 	}
 
 	if (dot && fs && !(fs->fs_flags & FS_HAS_SUBTYPE)) {
-- 
2.25.1