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

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Date:   Wed, 4 Jan 2023 00:23:44 +0000
From:   Eric Biggers <ebiggers@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     tytso@....edu, 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: Re: [PATCH] ext4: Fix function prototype mismatch for ext4_feat_ktype

On Tue, Jan 03, 2023 at 03:46:20PM -0800, Kees Cook wrote:
> 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,

For consistency, maybe call this ext4_feat_release?  So ext4_sb_ktype would have
ext4_sb_release, and ext4_feat_ktype would have ext4_feat_release.

I'm also surprised that this wasn't found earlier.  Is it possible that CFI does
not actually distinguish between the two function prototypes here?

- Eric