linux-kernel - Re: [PATCH] compat: fix use of an uninitialized variable in compat_sys_io

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Date:	Mon, 3 Jan 2011 13:19:11 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Namhyung Kim <namhyung@...il.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jeff Moyer <jmoyer@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] compat: fix use of an uninitialized variable in
 compat_sys_io_setup()

On Fri, 24 Dec 2010 18:16:26 +0900
Namhyung Kim <namhyung@...il.com> wrote:

> The upper bytes of ctx64 might contain garbages because it was
> set by get_user() which copied only lower 4 bytes as its second
> argument points to. Since sys_io_setup() requires its argumet
> is properly initialized to 0 we should set it explicitly.
> 
> On x86, this was not a problem since its implementation of
> get_user() does a C assignment so that it can fill upper bytes
> with 0's. But other archs that use __get_user_asm() or something
> might have a problem.
> 
> Signed-off-by: Namhyung Kim <namhyung@...il.com>
> Cc: Jeff Moyer <jmoyer@...hat.com>
> ---
>  fs/compat.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/compat.c b/fs/compat.c
> index 4376e07febbb..b074e9f79148 100644
> --- a/fs/compat.c
> +++ b/fs/compat.c
> @@ -526,7 +526,7 @@ asmlinkage long
>  compat_sys_io_setup(unsigned nr_reqs, u32 __user *ctx32p)
>  {
>  	long ret;
> -	aio_context_t ctx64;
> +	aio_context_t ctx64 = 0;
>  
>  	mm_segment_t oldfs = get_fs();
>  	if (unlikely(get_user(ctx64, ctx32p)))

Well.  What _should_ a get_user(some_u64, some_u32*) do to `some_u64'?

I don't recall it coming up before but I'd say that the sane, expected
and certainly *safe* behaviour would be for the implementation to zero
out the upper 32 bits of `some_u64'.

If that's the rule then those architectures need fixing.  Did you have
any architectures in mind?

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