kernel-hardening - Re: [PATCH v16 1/3] fs: Add trusted_for(2) syscall implementation and related sysctl

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Date: Fri, 12 Nov 2021 20:16:01 +0100
From: "Alejandro Colomar (man-pages)" <alx.manpages@...il.com>
To: Mickaël Salaün <mic@...ikod.net>,
 Al Viro <viro@...iv.linux.org.uk>, Andrew Morton <akpm@...ux-foundation.org>
Cc: Aleksa Sarai <cyphar@...har.com>, Andy Lutomirski <luto@...nel.org>,
 Arnd Bergmann <arnd@...db.de>, Casey Schaufler <casey@...aufler-ca.com>,
 Christian Brauner <christian.brauner@...ntu.com>,
 Christian Heimes <christian@...hon.org>,
 Deven Bowers <deven.desai@...ux.microsoft.com>,
 Dmitry Vyukov <dvyukov@...gle.com>, Eric Biggers <ebiggers@...nel.org>,
 Eric Chiang <ericchiang@...gle.com>, Florian Weimer <fweimer@...hat.com>,
 Geert Uytterhoeven <geert@...ux-m68k.org>, James Morris <jmorris@...ei.org>,
 Jan Kara <jack@...e.cz>, Jann Horn <jannh@...gle.com>,
 Jonathan Corbet <corbet@....net>, Kees Cook <keescook@...omium.org>,
 Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
 "Madhavan T . Venkataraman" <madvenka@...ux.microsoft.com>,
 Matthew Garrett <mjg59@...gle.com>, Matthew Wilcox <willy@...radead.org>,
 Miklos Szeredi <mszeredi@...hat.com>, Mimi Zohar <zohar@...ux.ibm.com>,
 Paul Moore <paul@...l-moore.com>,
 Philippe Trébuchet <philippe.trebuchet@....gouv.fr>,
 Scott Shell <scottsh@...rosoft.com>, Shuah Khan <shuah@...nel.org>,
 Steve Dower <steve.dower@...hon.org>, Steve Grubb <sgrubb@...hat.com>,
 Thibaut Sautereau <thibaut.sautereau@....gouv.fr>,
 Vincent Strubel <vincent.strubel@....gouv.fr>,
 Yin Fengwei <fengwei.yin@...el.com>, kernel-hardening@...ts.openwall.com,
 linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-security-module@...r.kernel.org,
 Mickaël Salaün <mic@...ux.microsoft.com>
Subject: Re: [PATCH v16 1/3] fs: Add trusted_for(2) syscall implementation and
 related sysctl

Hi Mickaël,

On 11/10/21 20:06, Mickaël Salaün wrote:
> diff --git a/fs/open.c b/fs/open.c
> index f732fb94600c..96a80abec41b 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -480,6 +482,114 @@ SYSCALL_DEFINE2(access, const char __user *, filename, int, mode)
>   	return do_faccessat(AT_FDCWD, filename, mode, 0);
>   }
>   
> +#define TRUST_POLICY_EXEC_MOUNT			BIT(0)
> +#define TRUST_POLICY_EXEC_FILE			BIT(1)
> +
> +int sysctl_trusted_for_policy __read_mostly;
> +
> +/**
...
> + */
> +SYSCALL_DEFINE3(trusted_for, const int, fd, const enum trusted_for_usage, usage,

Please, don't use enums for interfaces.  They are implementation defined 
types, and vary between compilers and within the same compiler also 
depending on optimization flags.

C17::6.7.2.2.4:
[
Each enumerated type shall be compatible with char,
a signed integer type, or an unsigned integer type.
The choice of type is implementation-defined,130)
but shall be capable of representing the values of
all the members of the enumeration.
]

See also:
<https://stackoverflow.com/questions/366017/what-is-the-size-of-an-enum-in-c>

So, please use only standard integer types for interfaces.

And in the case of enums, since the language specifies that enumeration 
constants (the macro-like identifiers) are of type int, it makes sense 
for functions to use int.

C17::6.7.2.2.3:
[
The identifiers in an enumerator list are declared as constants
that have type int and may appear wherever such are permitted.
]

I'd use an int for the API/ABI, even if it's expected to be assigned 
values of 'enum trusted_for_usage' (that should be specified in the 
manual page in DESCRIPTION, but not in SYNOPSIS, which should specify int).



TL;DR:

ISO C specifies that for the following code:

	enum foo {BAR};

	enum foo foobar;

typeof(foo)    shall be int
typeof(foobar) is implementation-defined

Since foobar = BAR; assigns an int, the best thing to do to avoid 
implementation-defined behavior, is to declare foobar as int too.


> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 528a478dbda8..c535e0e43cc8 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -462,6 +463,7 @@ asmlinkage long sys_fallocate(int fd, int mode, loff_t offset, loff_t len);
>   asmlinkage long sys_faccessat(int dfd, const char __user *filename, int mode);
>   asmlinkage long sys_faccessat2(int dfd, const char __user *filename, int mode,
>   			       int flags);
> +asmlinkage long sys_trusted_for(int fd, enum trusted_for_usage usage, u32 flags);

Same here.

>   asmlinkage long sys_chdir(const char __user *filename);
>   asmlinkage long sys_fchdir(unsigned int fd);
>   asmlinkage long sys_chroot(const char __user *filename);

Thanks,
Alex


-- 
Alejandro Colomar
Linux man-pages comaintainer; http://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/

-- 
Alejandro Colomar
Linux man-pages comaintainer; http://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/
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