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Message-ID: <20250619073136.506022-2-shivankg@amd.com>
Date: Thu, 19 Jun 2025 07:31:37 +0000
From: Shivank Garg <shivankg@....com>
To: <david@...hat.com>, <akpm@...ux-foundation.org>, <brauner@...nel.org>,
<paul@...l-moore.com>, <rppt@...nel.org>, <viro@...iv.linux.org.uk>
CC: <seanjc@...gle.com>, <vbabka@...e.cz>, <willy@...radead.org>,
<pbonzini@...hat.com>, <tabba@...gle.com>, <afranji@...gle.com>,
<ackerleytng@...gle.com>, <shivankg@....com>, <jack@...e.cz>,
<hch@...radead.org>, <cgzones@...glemail.com>, <ira.weiny@...el.com>,
<roypat@...zon.co.uk>, <linux-fsdevel@...r.kernel.org>, <linux-mm@...ck.org>,
<linux-kernel@...r.kernel.org>, <linux-security-module@...r.kernel.org>
Subject: [PATCH] fs: export anon_inode_make_secure_inode() and fix secretmem LSM bypass
Export anon_inode_make_secure_inode() to allow KVM guest_memfd to create
anonymous inodes with proper security context. This replaces the current
pattern of calling alloc_anon_inode() followed by
inode_init_security_anon() for creating security context manually.
This change also fixes a security regression in secretmem where the
S_PRIVATE flag was not cleared after alloc_anon_inode(), causing
LSM/SELinux checks to be bypassed for secretmem file descriptors.
As guest_memfd currently resides in the KVM module, we need to export this
symbol for use outside the core kernel. In the future, guest_memfd might be
moved to core-mm, at which point the symbols no longer would have to be
exported. When/if that happens is still unclear.
Fixes: 2bfe15c52612 ("mm: create security context for memfd_secret inodes")
Suggested-by: David Hildenbrand <david@...hat.com>
Suggested-by: Mike Rapoport <rppt@...nel.org>
Signed-off-by: Shivank Garg <shivankg@....com>
---
The handling of the S_PRIVATE flag for these inodes was discussed
extensively ([1], [2], [3]).
As per discussion [3] with Mike and Paul, KVM guest_memfd and secretmem
results in user-visible file descriptors, so they should be subject to
LSM/SELinux security policies rather than bypassing them with S_PRIVATE.
[1] https://lore.kernel.org/all/b9e5fa41-62fd-4b3d-bb2d-24ae9d3c33da@redhat.com
[2] https://lore.kernel.org/all/cover.1748890962.git.ackerleytng@google.com
[3] https://lore.kernel.org/all/aFOh8N_rRdSi_Fbc@kernel.org
fs/anon_inodes.c | 23 ++++++++++++++++++-----
include/linux/fs.h | 2 ++
mm/secretmem.c | 9 +--------
3 files changed, 21 insertions(+), 13 deletions(-)
diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
index e51e7d88980a..ca6cfb1cd496 100644
--- a/fs/anon_inodes.c
+++ b/fs/anon_inodes.c
@@ -98,14 +98,25 @@ static struct file_system_type anon_inode_fs_type = {
.kill_sb = kill_anon_super,
};
-static struct inode *anon_inode_make_secure_inode(
- const char *name,
- const struct inode *context_inode)
+/**
+ * anon_inode_make_secure_inode - allocate an anonymous inode with security context
+ * @sb: [in] Superblock to allocate from
+ * @name: [in] Name of the class of the newfile (e.g., "secretmem")
+ * @context_inode:
+ * [in] Optional parent inode for security inheritance
+ *
+ * The function ensures proper security initialization through the LSM hook
+ * security_inode_init_security_anon().
+ *
+ * Return: Pointer to new inode on success, ERR_PTR on failure.
+ */
+struct inode *anon_inode_make_secure_inode(struct super_block *sb, const char *name,
+ const struct inode *context_inode)
{
struct inode *inode;
int error;
- inode = alloc_anon_inode(anon_inode_mnt->mnt_sb);
+ inode = alloc_anon_inode(sb);
if (IS_ERR(inode))
return inode;
inode->i_flags &= ~S_PRIVATE;
@@ -118,6 +129,7 @@ static struct inode *anon_inode_make_secure_inode(
}
return inode;
}
+EXPORT_SYMBOL_GPL(anon_inode_make_secure_inode);
static struct file *__anon_inode_getfile(const char *name,
const struct file_operations *fops,
@@ -132,7 +144,8 @@ static struct file *__anon_inode_getfile(const char *name,
return ERR_PTR(-ENOENT);
if (make_inode) {
- inode = anon_inode_make_secure_inode(name, context_inode);
+ inode = anon_inode_make_secure_inode(anon_inode_mnt->mnt_sb,
+ name, context_inode);
if (IS_ERR(inode)) {
file = ERR_CAST(inode);
goto err;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index b085f161ed22..040c0036320f 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3608,6 +3608,8 @@ extern int simple_write_begin(struct file *file, struct address_space *mapping,
extern const struct address_space_operations ram_aops;
extern int always_delete_dentry(const struct dentry *);
extern struct inode *alloc_anon_inode(struct super_block *);
+struct inode *anon_inode_make_secure_inode(struct super_block *sb, const char *name,
+ const struct inode *context_inode);
extern int simple_nosetlease(struct file *, int, struct file_lease **, void **);
extern const struct dentry_operations simple_dentry_operations;
diff --git a/mm/secretmem.c b/mm/secretmem.c
index 589b26c2d553..9a11a38a6770 100644
--- a/mm/secretmem.c
+++ b/mm/secretmem.c
@@ -195,18 +195,11 @@ static struct file *secretmem_file_create(unsigned long flags)
struct file *file;
struct inode *inode;
const char *anon_name = "[secretmem]";
- int err;
- inode = alloc_anon_inode(secretmem_mnt->mnt_sb);
+ inode = anon_inode_make_secure_inode(secretmem_mnt->mnt_sb, anon_name, NULL);
if (IS_ERR(inode))
return ERR_CAST(inode);
- err = security_inode_init_security_anon(inode, &QSTR(anon_name), NULL);
- if (err) {
- file = ERR_PTR(err);
- goto err_free_inode;
- }
-
file = alloc_file_pseudo(inode, secretmem_mnt, "secretmem",
O_RDWR, &secretmem_fops);
if (IS_ERR(file))
--
2.43.0
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