Re: + utsname-completely-overwrite-prior-information.patch added to -mm tree

["Serge E. Hallyn" <serue@us.ibm.com>]

Quoting akpm@linux-foundation.org (akpm@linux-foundation.org):
> 
> The patch titled
>      utsname: completely overwrite prior information
> has been added to the -mm tree.  Its filename is
>      utsname-completely-overwrite-prior-information.patch
> 
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> 
> ------------------------------------------------------
> Subject: utsname: completely overwrite prior information
> From: Vegard Nossum <vegard.nossum@gmail.com>
> 
> On sethostname() and setdomainname(), previous information may be retained
> if it was longer than than the new hostname/domainname.
> 
> This can be demonstrated trivially by calling sethostname() first with a
> long name, then with a short name, and then calling uname() to retrieve
> the full buffer that contains the hostname (and possibly parts of the old
> hostname), one just has to look past the terminating zero.
> 
> I don't know if we should really care that much (hence the RFC); the only
> scenarios I can possibly think of is administrator putting something
> sensitive in the hostname (or domain name) by accident, and changing it
> back will not undo the mistake entirely, though it's not like we can
> recover gracefully from "rm -rf /" either...  The other scenario is
> namespaces (CLONE_NEWUTS) where some information may be unintentionally
> "inherited" from the previous namespace (a program wants to hide the
> original name and does clone + sethostname, but some information is still
> left).
> 
> I think the patch may be defended on grounds of the principle of least
> surprise.  But I am not adamant :-)
> 
> (I guess the question now is whether userspace should be able to
> write embedded NULs into the buffer or not...)
> 
> At least the observation has been made and the patch has been presented.
> 
> Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>
> Cc: "Eric W. Biederman" <ebiederm@xmission.com>
> Cc: "Serge E. Hallyn" <serue@us.ibm.com>

Agreed it seems an implausible attack vector :)  But especially for
something as rare as setting a hostname, seems worthwhile.

Acked-by: Serge Hallyn <serue@us.ibm.com>

Thanks, Vegard.

-serge

> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>  kernel/sys.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff -puN kernel/sys.c~utsname-completely-overwrite-prior-information kernel/sys.c
> --- a/kernel/sys.c~utsname-completely-overwrite-prior-information
> +++ a/kernel/sys.c
> @@ -1416,7 +1416,8 @@ asmlinkage long sys_sethostname(char __u
>  	errno = -EFAULT;
>  	if (!copy_from_user(tmp, name, len)) {
>  		memcpy(utsname()->nodename, tmp, len);
> -		utsname()->nodename[len] = 0;
> +		memset(utsname()->nodename + len, 0,
> +			sizeof(utsname()->nodename) - len);
>  		errno = 0;
>  	}
>  	up_write(&uts_sem);
> @@ -1462,7 +1463,8 @@ asmlinkage long sys_setdomainname(char _
>  	errno = -EFAULT;
>  	if (!copy_from_user(tmp, name, len)) {
>  		memcpy(utsname()->domainname, tmp, len);
> -		utsname()->domainname[len] = 0;
> +		memset(utsname()->domainname + len, 0,
> +			sizeof(utsname()->domainname) - len);
>  		errno = 0;
>  	}
>  	up_write(&uts_sem);
> _
> 
> Patches currently in -mm which might be from vegard.nossum@gmail.com are
> 
> origin.patch
> linux-next.patch
> ia64-avoid-invoking-irq-handlers-on-offline-cpus.patch
> utsname-completely-overwrite-prior-information.patch
> kernel-sysc-improve-code-generation.patch
> 
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