On Sat, Jul 19, 2025 at 4:13 AM Abhinav Saxena xandfury@gmail.com wrote:
Add is_memfd_file() function to reliably detect memfd files by checking for "memfd:" prefix in dentry names on shmem-backed files. This distinguishes true memfd files from regular shmem files.
Move domain_is_scoped() to domain.c for reuse across subsystems. Add comprehensive kunit tests for memfd detection edge cases.
Signed-off-by: Abhinav Saxena xandfury@gmail.com
security/landlock/domain.c | 67 +++++++++++++++ security/landlock/domain.h | 4 + security/landlock/fs.c | 210 +++++++++++++++++++++++++++++++++++++++++++++ security/landlock/task.c | 67 --------------- 4 files changed, 281 insertions(+), 67 deletions(-)
...
+/**
- is_memfd_file - Check if file was created via memfd_create()
- @file: File to check
- Returns true if @file was created via memfd_create(), false otherwise.
- memfd files are shmem-backed files with "memfd:" prefix in their dentry name.
- This is the definitive way to distinguish memfd files from regular shmem
- files.
- */
+static bool is_memfd_file(struct file *file) +{
const struct dentry *dentry;const unsigned char *name;size_t name_len;/* Fast path: basic validation */if (unlikely(!file))return false;/* Must be shmem-backed first - this is the cheapest definitive check */if (!shmem_file(file))return false;+#ifdef CONFIG_MEMFD_CREATE
/* Validate dentry and get name info */dentry = file->f_path.dentry;if (unlikely(!dentry))return false;name_len = dentry->d_name.len;name = dentry->d_name.name;/* memfd files always have "memfd:" prefix (6 characters) */if (name_len < 6 || unlikely(!name))return false;/* Check for exact "memfd:" prefix */return memcmp(name, "memfd:", 6) == 0;+#else
return false;+#endif
I was trying to do something similar early this year but didn't hear feedback from the linux-mm folks. https://lore.kernel.org/linux-security-module/20250129203932.22165-1-wufan@k...
I have considered this approach but didn't use it. My concern is, potentially a malicious user can create a file in a shmem fs, e.g. tmpfs , with the "memfd:" prefix, which can be used to bypass security policy. (Resending this message due to a misconfiguration with my email client. Apologies for any inconvenience.)
-Fan