[PATCH 04/16] mm/shmem: honour F_SEAL_EXEC when an ACL is set

["David Hildenbrand (Arm)" <david@kernel.org>]

From: Chris Mason <mason@kernel.org>

The owner of a memfd sealed with F_SEAL_EXEC can still change its
execute bits by setting a POSIX access ACL. With CONFIG_TMPFS_POSIX_ACL,
this lets an unprivileged owner make a non-executable memfd executable,
bypassing vm.memfd_noexec=2.

simple_set_acl() updates i_mode directly, bypassing the seal check in
shmem_setattr(). Add shmem_set_acl() to check the resulting mode. Share
the F_SEAL_EXEC check with shmem_setattr() using memfd_check_seals_mode()
in include/linux/memfd.h.

Fix generated by an LLM and sent by Chris Mason in private.

Fixes: 6fd7353829ca ("mm/memfd: add F_SEAL_EXEC")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Mason <mason@kernel.org>
[ david: use common helper; simplify description ]
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 include/linux/memfd.h |  9 +++++++++
 mm/shmem.c            | 35 ++++++++++++++++++++++++++++++-----
 2 files changed, 39 insertions(+), 5 deletions(-)

diff --git a/include/linux/memfd.h b/include/linux/memfd.h
index c159e40e3f34a..272479317cf10 100644
--- a/include/linux/memfd.h
+++ b/include/linux/memfd.h
@@ -2,10 +2,19 @@
 #ifndef __LINUX_MEMFD_H
 #define __LINUX_MEMFD_H
 
+#include <linux/fcntl.h>
 #include <linux/file.h>
 
 #define MEMFD_ANON_NAME "[memfd]"
 
+static inline int memfd_check_seals_mode(unsigned int seals, umode_t old_mode,
+		umode_t new_mode)
+{
+	if ((seals & F_SEAL_EXEC) && ((old_mode ^ new_mode) & 0111))
+		return -EPERM;
+	return 0;
+}
+
 #ifdef CONFIG_MEMFD_CREATE
 extern long memfd_fcntl(struct file *file, unsigned int cmd, unsigned int arg);
 struct folio *memfd_alloc_folio(struct file *memfd, pgoff_t idx);
diff --git a/mm/shmem.c b/mm/shmem.c
index d97fc32e7c942..86b9d19f26c7e 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -77,6 +77,7 @@ static struct vfsmount *shm_mnt __ro_after_init;
 #include <linux/magic.h>
 #include <linux/syscalls.h>
 #include <linux/fcntl.h>
+#include <linux/memfd.h>
 #include <uapi/linux/memfd.h>
 #include <linux/rmap.h>
 #include <linux/uuid.h>
@@ -1549,10 +1550,11 @@ static int shmem_setattr(const struct mnt_idmap *idmap,
 	if (error)
 		return error;
 
-	if ((info->seals & F_SEAL_EXEC) && (attr->ia_valid & ATTR_MODE)) {
-		if ((inode->i_mode ^ attr->ia_mode) & 0111) {
-			return -EPERM;
-		}
+	if (attr->ia_valid & ATTR_MODE) {
+		error = memfd_check_seals_mode(info->seals, inode->i_mode,
+					       attr->ia_mode);
+		if (error)
+			return error;
 	}
 
 	if (S_ISREG(inode->i_mode) && (attr->ia_valid & ATTR_SIZE)) {
@@ -5400,12 +5402,35 @@ static const struct file_operations shmem_file_operations = {
 #endif
 };
 
+#ifdef CONFIG_TMPFS_POSIX_ACL
+static int shmem_set_acl(const struct mnt_idmap *idmap, struct dentry *dentry,
+		struct posix_acl *acl, int type)
+{
+	struct inode *inode = d_inode(dentry);
+	umode_t mode = inode->i_mode;
+	int error;
+
+	if (type == ACL_TYPE_ACCESS) {
+		error = posix_acl_equiv_mode(acl, &mode);
+		if (error < 0)
+			return error;
+		error = memfd_check_seals_mode(SHMEM_I(inode)->seals,
+					       inode->i_mode, mode);
+		if (error)
+			return error;
+	}
+	return simple_set_acl(idmap, dentry, acl, type);
+}
+#else
+#define shmem_set_acl NULL
+#endif
+
 static const struct inode_operations shmem_inode_operations = {
 	.getattr	= shmem_getattr,
 	.setattr	= shmem_setattr,
 #ifdef CONFIG_TMPFS_XATTR
 	.listxattr	= shmem_listxattr,
-	.set_acl	= simple_set_acl,
+	.set_acl	= shmem_set_acl,
 	.fileattr_get	= shmem_fileattr_get,
 	.fileattr_set	= shmem_fileattr_set,
 #endif

-- 
2.43.0