linux-kernel - Re: [PATCH 1/2] vfs: implement fchmodat2() syscall

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Date:   Tue, 28 Feb 2017 12:23:01 -0600
From:   Eric Blake <eblake@...hat.com>
To:     Greg Kurz <groug@...d.org>, viro@...iv.linux.org.uk
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] vfs: implement fchmodat2() syscall

On 02/28/2017 11:03 AM, Greg Kurz wrote:
> According to the POSIX.1-2008 manual page [1], the fchmodat() function has
> a flag argument which may be passed the following value:
> 
> AT_SYMLINK_NOFOLLOW
>     If path names a symbolic link, then the mode of the symbolic link is
>     changed.
> 
> and the following error may be returned:
> 
> [EOPNOTSUPP]
>     The AT_SYMLINK_NOFOLLOW bit is set in the flag argument, path names a
>     symbolic link, and the system does not support changing the mode of a
>     symbolic link.
> 
> The linux kernel doesn't support changing the mode of a symbolic link, but
> the current implementation doesn't even have a flag argument. It is then
> up to userspace to deal with that. Unfortunately, it is impossible to
> implement the POSIX behavior in a race-free manner.
> 
> This patch introduces a new fchmodat2() syscall with a flag argument to
> address the issue.
> 
> [1] http://pubs.opengroup.org/onlinepubs/9699919799/functions/chmod.html
> 
> Signed-off-by: Greg Kurz <groug@...d.org>
> ---

Might also be worth mentioning that this patch is required in order to
solve CVE-2016-9602, per discussion at
https://lists.gnu.org/archive/html/qemu-devel/2017-02/msg06089.html

> +++ b/include/linux/syscalls.h
> @@ -775,6 +775,8 @@ asmlinkage long sys_futimesat(int dfd, const char __user *filename,
>  asmlinkage long sys_faccessat(int dfd, const char __user *filename, int mode);
>  asmlinkage long sys_fchmodat(int dfd, const char __user * filename,
>  			     umode_t mode);
> +asmlinkage long sys_fchmodat2(int dfd, const char __user *filename,
> +			      umode_t mode, int flag);
>  asmlinkage long sys_fchownat(int dfd, const char __user *filename, uid_t user,
>  			     gid_t group, int flag);

Is the indentation off here?

Reviewed-by: Eric Blake <eblake@...hat.com>


-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org



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