linux-hardening - [PATCH] ext4: Fix function prototype mismatch for ext4_feat

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Date:   Tue,  3 Jan 2023 15:46:20 -0800
From:   Kees Cook <keescook@...omium.org>
To:     tytso@....edu
Cc:     Kees Cook <keescook@...omium.org>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        linux-ext4@...r.kernel.org, llvm@...ts.linux.dev,
        linux-hardening@...r.kernel.org
Subject: [PATCH] ext4: Fix function prototype mismatch for ext4_feat_ktype

With clang's kernel control flow integrity (kCFI, CONFIG_CFI_CLANG),
indirect call targets are validated against the expected function
pointer prototype to make sure the call target is valid to help mitigate
ROP attacks. If they are not identical, there is a failure at run time,
which manifests as either a kernel panic or thread getting killed.

ext4_feat_ktype was setting the "release" handler to "kfree", which
doesn't have a matching function prototype. Add a simple wrapper
with the correct prototype.

This was found as a result of Clang's new -Wcast-function-type-strict
flag, which is more sensitive than the simpler -Wcast-function-type,
which only checks for type width mismatches.

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 fs/ext4/sysfs.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/sysfs.c b/fs/ext4/sysfs.c
index d233c24ea342..83cf8b5afb54 100644
--- a/fs/ext4/sysfs.c
+++ b/fs/ext4/sysfs.c
@@ -491,6 +491,11 @@ static void ext4_sb_release(struct kobject *kobj)
 	complete(&sbi->s_kobj_unregister);
 }

+static void ext4_kobject_release(struct kobject *kobj)
+{
+	kfree(kobj);
+}
+
 static const struct sysfs_ops ext4_attr_ops = {
 	.show	= ext4_attr_show,
 	.store	= ext4_attr_store,
@@ -505,7 +510,7 @@ static struct kobj_type ext4_sb_ktype = {
 static struct kobj_type ext4_feat_ktype = {
 	.default_groups = ext4_feat_groups,
 	.sysfs_ops	= &ext4_attr_ops,
-	.release	= (void (*)(struct kobject *))kfree,
+	.release	= ext4_kobject_release,
 };

 void ext4_notify_error_sysfs(struct ext4_sb_info *sbi)
-- 
2.34.1