netdev - Re: [PATCH v17 08/15] seccomp: add system call filtering using BPF

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Date:	Sat, 31 Mar 2012 13:14:25 -0500
From:	Will Drewry <wad@...omium.org>
To:	Vladimir Murzin <murzin.v@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-doc@...r.kernel.org, kernel-hardening@...ts.openwall.com,
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	keescook@...omium.org, jmorris@...ei.org
Subject: Re: [PATCH v17 08/15] seccomp: add system call filtering using BPF

2012/3/30 Vladimir Murzin <murzin.v@...il.com>:
> Hi Will,
>
> On Thu, Mar 29, 2012 at 03:01:53PM -0500, Will Drewry wrote:
>
> snipped
>
>> +
>> +/* Limit any path through the tree to 256KB worth of instructions. */
>> +#define MAX_INSNS_PER_PATH ((1 << 18) / sizeof(struct sock_filter))
>> +
>> +static void seccomp_filter_log_failure(int syscall)
>> +{
>> +     int compat = 0;
>> +#ifdef CONFIG_COMPAT
>> +     compat = is_compat_task();
>> +#endif
>> +     pr_info("%s[%d]: %ssystem call %d blocked at 0x%lx\n",
>> +             current->comm, task_pid_nr(current),
>> +             (compat ? "compat " : ""),
>> +             syscall, KSTK_EIP(current));
>> +}
>
> snipped
>
>> +/**
>> + * seccomp_attach_user_filter - attaches a user-supplied sock_fprog
>> + * @user_filter: pointer to the user data containing a sock_fprog.
>> + *
>> + * Returns 0 on success and non-zero otherwise.
>> + */
>> +long seccomp_attach_user_filter(char __user *user_filter)
>> +{
>> +     struct sock_fprog fprog;
>> +     long ret = -EFAULT;
>> +
>> +#ifdef CONFIG_COMPAT
>> +     if (is_compat_task()) {
>> +             struct compat_sock_fprog fprog32;
>> +             if (copy_from_user(&fprog32, user_filter, sizeof(fprog32)))
>> +                     goto out;
>> +             fprog.len = fprog32.len;
>> +             fprog.filter = compat_ptr(fprog32.filter);
>> +     } else /* falls through to the if below. */
>> +#endif
>> +     if (copy_from_user(&fprog, user_filter, sizeof(fprog)))
>> +             goto out;
>> +     ret = seccomp_attach_filter(&fprog);
>> +out:
>> +     return ret;
>> +}
>
> Do we really need to surround is_compat_task() with CNFIG_COMPAT?
> It seems that this case has already handled in compat.h [1]
>
> [1] http://lxr.linux.no/#linux+v3.3/include/linux/compat.h#L566

In log_failure, it's not needed at all, but that function disappears
in a subsequent patch in the series that converts over to using the
auditing framework.  For the next use of CONFIG_COMPAT, it is more
important for guarding the compat_sock_fprog struct (which isn't
doubly defined) rather than the is_compat_task.

Thanks!
will
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