[PATCH v2] security: Handle dentries without inode in security_path_post_mknod()

[Roberto Sassu]

From: Roberto Sassu <roberto.sassu at huawei.com>

Commit 08abce60d63fi ("security: Introduce path_post_mknod hook")
introduced security_path_post_mknod(), to replace the IMA-specific call to
ima_post_path_mknod().

For symmetry with security_path_mknod(), security_path_post_mknod() is
called after a successful mknod operation, for any file type, rather than
only for regular files at the time there was the IMA call.

However, as reported by VFS maintainers, successful mknod operation does
not mean that the dentry always has an inode attached to it (for example,
not for FIFOs on a SAMBA mount).

If that condition happens, the kernel crashes when
security_path_post_mknod() attempts to verify if the inode associated to
the dentry is private.

Add an extra check to first verify if there is an inode attached to the
dentry, before checking if the inode is private. Also add the same check to
the current users of the path_post_mknod hook, ima_post_path_mknod() and
evm_post_path_mknod().

Finally, use the proper helper, d_backing_inode(), to retrieve the inode
from the dentry in ima_post_path_mknod().

Reported-by: Steve French <smfrench at gmail.com>
Closes: https://lore.kernel.org/linux-kernel/CAH2r5msAVzxCUHHG8VKrMPUKQHmBpE6K9_vjhgDa1uAvwx4ppw@mail.gmail.com/
Fixes: 08abce60d63f ("security: Introduce path_post_mknod hook")
Signed-off-by: Roberto Sassu <roberto.sassu at huawei.com>
Acked-by: Mimi Zohar <zohar at linux.ibm.com>
Acked-by: Paul Moore <paul at paul-moore.com>
---
 security/integrity/evm/evm_main.c | 6 ++++--
 security/integrity/ima/ima_main.c | 5 +++--
 security/security.c               | 5 ++++-
 3 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index 81dbade5b9b3..ec1659273fcf 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -1037,11 +1037,13 @@ static void evm_file_release(struct file *file)
 static void evm_post_path_mknod(struct mnt_idmap *idmap, struct dentry *dentry)
 {
 	struct inode *inode = d_backing_inode(dentry);
-	struct evm_iint_cache *iint = evm_iint_inode(inode);
+	struct evm_iint_cache *iint;
 
-	if (!S_ISREG(inode->i_mode))
+	/* path_post_mknod hook might pass dentries without attached inode. */
+	if (!inode || !S_ISREG(inode->i_mode))
 		return;
 
+	iint = evm_iint_inode(inode);
 	if (iint)
 		iint->flags |= EVM_NEW_FILE;
 }
diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
index c84e8c55333d..afc883e60cf3 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -719,10 +719,11 @@ static void ima_post_create_tmpfile(struct mnt_idmap *idmap,
 static void ima_post_path_mknod(struct mnt_idmap *idmap, struct dentry *dentry)
 {
 	struct ima_iint_cache *iint;
-	struct inode *inode = dentry->d_inode;
+	struct inode *inode = d_backing_inode(dentry);
 	int must_appraise;
 
-	if (!ima_policy_flag || !S_ISREG(inode->i_mode))
+	/* path_post_mknod hook might pass dentries without attached inode. */
+	if (!ima_policy_flag || !inode || !S_ISREG(inode->i_mode))
 		return;
 
 	must_appraise = ima_must_appraise(idmap, inode, MAY_ACCESS,
diff --git a/security/security.c b/security/security.c
index 7e118858b545..391477687637 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1801,7 +1801,10 @@ EXPORT_SYMBOL(security_path_mknod);
  */
 void security_path_post_mknod(struct mnt_idmap *idmap, struct dentry *dentry)
 {
-	if (unlikely(IS_PRIVATE(d_backing_inode(dentry))))
+	struct inode *inode = d_backing_inode(dentry);
+
+	/* Not all dentries have an inode attached after mknod. */
+	if (inode && unlikely(IS_PRIVATE(inode)))
 		return;
 	call_void_hook(path_post_mknod, idmap, dentry);
 }
-- 
2.34.1