linux-kernel - Re: [PATCH] sysfs: tightened sysfs permission checks

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Date:	Sat, 02 May 2015 06:45:52 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Gobinda Charan Maji <gobinda.cemk07@...il.com>
Cc:	Linux Next <linux-next@...r.kernel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sysfs: tightened sysfs permission checks

Gobinda Charan Maji <gobinda.cemk07@...il.com> writes:
> There were some inconsistency in restriction to VERIFY_OCTAL_PERMISSIONS().
> Previously the test was "User perms >= group perms >= other perms". The
> permission field of User, Group or Other consists of three bits. LSB is
> EXECUTE permission, MSB is READ permission and the middle bit is WRITE
> permission. But logically WRITE is "more privileged" than READ.
>
> Say for example, permission value is "0430". Here User has only READ
> permission whereas Group has both WRITE and EXECUTE permission.
>
> So, the checks could be tightened and the tests are separated to
> USER_READABLE >= GROUP_READABLE >= OTHER_READABLE,
> USER_WRITABLE >= GROUP_WRITABLE and OTHER_WRITABLE is not permitted.
>
> Signed-off-by: Gobinda Charan Maji <gobinda.cemk07@...il.com>

Thanks, applied!

Cheers,
Rusty.

> ---
>  include/linux/kernel.h | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 3a5b48e..cd54b35 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -818,13 +818,15 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { }
>  #endif
>  
>  /* Permissions on a sysfs file: you didn't miss the 0 prefix did you? */
> -#define VERIFY_OCTAL_PERMISSIONS(perms)					\
> -	(BUILD_BUG_ON_ZERO((perms) < 0) +				\
> -	 BUILD_BUG_ON_ZERO((perms) > 0777) +				\
> -	 /* User perms >= group perms >= other perms */			\
> -	 BUILD_BUG_ON_ZERO(((perms) >> 6) < (((perms) >> 3) & 7)) +	\
> -	 BUILD_BUG_ON_ZERO((((perms) >> 3) & 7) < ((perms) & 7)) +	\
> -	 /* Other writable?  Generally considered a bad idea. */	\
> -	 BUILD_BUG_ON_ZERO((perms) & 2) +				\
> +#define VERIFY_OCTAL_PERMISSIONS(perms)						\
> +	(BUILD_BUG_ON_ZERO((perms) < 0) +					\
> +	 BUILD_BUG_ON_ZERO((perms) > 0777) +					\
> +	 /* USER_READABLE >= GROUP_READABLE >= OTHER_READABLE */		\
> +	 BUILD_BUG_ON_ZERO((((perms) >> 6) & 4) < (((perms) >> 3) & 4)) +	\
> +	 BUILD_BUG_ON_ZERO((((perms) >> 3) & 4) < ((perms) & 4)) +		\
> +	 /* USER_WRITABLE >= GROUP_WRITABLE */					\
> +	 BUILD_BUG_ON_ZERO((((perms) >> 6) & 2) < (((perms) >> 3) & 2)) +	\
> +	 /* OTHER_WRITABLE?  Generally considered a bad idea. */		\
> +	 BUILD_BUG_ON_ZERO((perms) & 2) +					\
>  	 (perms))
>  #endif
> -- 
> 1.8.1.4
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