linux-kernel - Re: user pointers and race conditions

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Message-Id: <20070516.220631.07641650.davem@davemloft.net>
Date:	Wed, 16 May 2007 22:06:31 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	skb300@...mail.com
Cc:	linux-kernel@...r.kernel.org
Subject: Re: user pointers and race conditions

From: sk b <skb300@...mail.com>
Date: Wed, 16 May 2007 22:56:22 -0600

> 3:        if (!access_ok(VERIFY_READ,stp,sizeof(struct st)))
> 4:                return;
> 5:        if (!access_ok(VERIFY_WRITE,stp->u,sizeof(int)))
> 6:                return;

This code would not exist in the kernel, the kernel cannot dereference
stp->u.  The stp->u dereference would silently work on x86 and x86_64
but it would generate an exception on sparc64 and other platforms.

User space accesses must go through the proper copy_from_user(),
copy_to_user, get_user(), and put_user() interfaces.

It must first copy stp into a local kernel space copy, then it may
inspect the value of stp->u.

And yes sparse would catch this problem in your code, because the
"__user" annotations would catch the illegal "stp->u" dereference.
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