[PATCH v2] ext4: Fix function prototype mismatch for ext4_feat_ktype

[PATCH v2] ext4: Fix function prototype mismatch for ext4_feat_ktype

[Kees Cook]

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.

Note that this code is only reached when ext4 is a loadable module and
it is being unloaded:

 CFI failure at kobject_put+0xbb/0x1b0 (target: kfree+0x0/0x180; expected type: 0x7c4aa698)
 ...
 RIP: 0010:kobject_put+0xbb/0x1b0
 ...
 Call Trace:
  <TASK>
  ext4_exit_sysfs+0x14/0x60 [ext4]
  cleanup_module+0x67/0xedb [ext4]

Fixes: b99fee58a20a ("ext4: create ext4_feat kobject dynamically")
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Eric Biggers <ebiggers@kernel.org>
Cc: stable@vger.kernel.org
Build-tested-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20230103234616.never.915-kees@kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
v2: rename callback, improve commit log (ebiggers)
v1: https://lore.kernel.org/lkml/20230103234616.never.915-kees@kernel.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..e2b8b3437c58 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_feat_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_feat_release,
 };
 
 void ext4_notify_error_sysfs(struct ext4_sb_info *sbi)
-- 
2.34.1

Re: [PATCH v2] ext4: Fix function prototype mismatch for ext4_feat_ktype

[Eric Biggers]

On Wed, Jan 04, 2023 at 01:09:12PM -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.
> 
> Note that this code is only reached when ext4 is a loadable module and
> it is being unloaded:
> 
>  CFI failure at kobject_put+0xbb/0x1b0 (target: kfree+0x0/0x180; expected type: 0x7c4aa698)
>  ...
>  RIP: 0010:kobject_put+0xbb/0x1b0
>  ...
>  Call Trace:
>   <TASK>
>   ext4_exit_sysfs+0x14/0x60 [ext4]
>   cleanup_module+0x67/0xedb [ext4]
> 
> Fixes: b99fee58a20a ("ext4: create ext4_feat kobject dynamically")
> Cc: Theodore Ts'o <tytso@mit.edu>
> Cc: Eric Biggers <ebiggers@kernel.org>
> Cc: stable@vger.kernel.org
> Build-tested-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> Reviewed-by: Nathan Chancellor <nathan@kernel.org>
> Link: https://lore.kernel.org/r/20230103234616.never.915-kees@kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> v2: rename callback, improve commit log (ebiggers)
> v1: https://lore.kernel.org/lkml/20230103234616.never.915-kees@kernel.org
> ---
>  fs/ext4/sysfs.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 

Reviewed-by: Eric Biggers <ebiggers@google.com>

- Eric

Re: [PATCH v2] ext4: Fix function prototype mismatch for ext4_feat_ktype

[Theodore Ts'o]

On Wed, 4 Jan 2023 13:09:12 -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.
> 
> [...]

Applied, thanks!

[1/1] ext4: Fix function prototype mismatch for ext4_feat_ktype
      commit: d99a55a94a0db8eda4a336a6f21730b844b8d2d2

Best regards,
-- 
Theodore Ts'o <tytso@mit.edu>