linux-kernel - Re: [PATCH RFC 2/4] landlock: implement memfd detection

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Message-ID: <87v7nj7p1d.fsf@gmail.com>
Date: Tue, 22 Jul 2025 15:56:38 -0600
From: Abhinav Saxena <xandfury@...il.com>
To: Fan Wu <wufan@...nel.org>
Cc: Mickaël Salaün <mic@...ikod.net>, Günther Noack <gnoack@...gle.com>,
 Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>, "Serge
 E. Hallyn" <serge@...lyn.com>, Shuah Khan <shuah@...nel.org>, Nathan
 Chancellor <nathan@...nel.org>, Nick Desaulniers
 <nick.desaulniers+lkml@...il.com>, Bill Wendling <morbo@...gle.com>,
 Justin Stitt <justinstitt@...gle.com>,
 linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-kselftest@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH RFC 2/4] landlock: implement memfd detection

Fan Wu <wufan@...nel.org> writes:

> On Sat, Jul 19, 2025 at 4:13 AM Abhinav Saxena <xandfury@...il.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@...il.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@kernel.org/>
>
> 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

Hi Fan,

Thanks for your comments.

I agree that an LSM hook into memfd_create() would be a much better
solution. In the absence of such a function, do you think adding a
`d_unlinked(dentry)` check could serve as an additional verification?

I say things since I *think* that legitimate memfd files are always
unlinked while spoofed tmpfs files remain linked. I could be wrong
though.

In any case, we can test this approach using kprobes to validate
the behavior.

-Abhinav