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ext4: Fix function prototype mismatch for ext4_feat_ktype

PATCH Applied Kees Cook 3 participants 3 messages Jan 4, 2023 – Feb 19, 2023

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Kees Cook

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#0 lore raw

PATCH v2 1/1 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.

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 <[email protected]>
Cc Eric Biggers <[email protected]>
Cc [email protected]
Build-tested-by Gustavo A. R. Silva <[email protected]>
Reviewed-by Gustavo A. R. Silva <[email protected]>
Reviewed-by Nathan Chancellor <[email protected]>
Link https://lore.kernel.org/r/[email protected] open here
Signed-off-by Kees Cook <[email protected]>

---
v2: rename callback, improve commit log (ebiggers)
v1: https://lore.kernel.org/lkml/[email protected]

---
 fs/ext4/sysfs.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)
fs/ext4/sysfs.c +6 -1
index d233c24ea342..e2b8b3437c58 100644
@@ -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)
Signature · 1 line
2.34.1
Reviewed-by on v2 1/1
#1 lore raw
Quoted 35 lines · 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 <[email protected]>
Cc Eric Biggers <[email protected]>
Cc [email protected]
Build-tested-by Gustavo A. R. Silva <[email protected]>
Reviewed-by Gustavo A. R. Silva <[email protected]>
Reviewed-by Nathan Chancellor <[email protected]>
Link https://lore.kernel.org/r/[email protected] open here
Signed-off-by Kees Cook <[email protected]>
Quoted 7 lines

---
v2: rename callback, improve commit log (ebiggers)
v1: https://lore.kernel.org/lkml/[email protected]

---
 fs/ext4/sysfs.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)
Reviewed-by Eric Biggers <[email protected]>

- Eric

maintainer Applied
#2 lore raw
In reply to Kees Cook
Quoted 11 lines · 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.

[...]

Applied, thanks!

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

      commit: d99a55a94a0db8eda4a336a6f21730b844b8d2d2

Best regards,

Signature · 1 line
Theodore Ts'o <[email protected]>