linux-kernel - Re: [Patch] kexec_load: check CAP_SYS

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Date:	Thu, 6 Jan 2011 14:02:47 -0500
From:	Eric Paris <eparis@...isplace.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Amerigo Wang <amwang@...hat.com>, linux-kernel@...r.kernel.org,
	kexec@...ts.infradead.org, eparis@...hat.com
Subject: Re: [Patch] kexec_load: check CAP_SYS_MODULE

On Thu, Jan 6, 2011 at 3:47 AM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
> Amerigo Wang <amwang@...hat.com> writes:
>
>> Eric pointed out that kexec_load() actually allows you to
>> run any code you want in ring0, this is more like CAP_SYS_MODULE.
>
> Let me get this straight you want to make the permission checks
> less stringent by allowing either CAP_SYS_MODULE or CAP_SYS_BOOT?

Nope, read my patch again.  It actually requires BOTH of them.

> CAP_SYS_BOOT is the correct capability.  Sure you can run any
> code but only after rebooting.  I don't see how this differs
> from any other reboot scenario.

The difference is that after a reboot the bootloader and the system
control what code is run.  kexec_load() immediately runs the new
kernel which is not controlled by the bootloader or by the system.
Imagine a situation where the bootloader and the /boot directory are
RO (enforced by hardware).   kexec_load() would let you run any kernel
code you want on the box whereas reboot would not.

>> Reported-by: Eric Paris <eparis@...hat.com>
>> Signed-off-by: WANG Cong <amwang@...hat.com>
>>
>> ---
>> diff --git a/kernel/kexec.c b/kernel/kexec.c
>> index b55045b..c30d613 100644
>> --- a/kernel/kexec.c
>> +++ b/kernel/kexec.c
>> @@ -945,7 +945,7 @@ SYSCALL_DEFINE4(kexec_load, unsigned long, entry, unsigned long, nr_segments,
>>       int result;
>>
>>       /* We only trust the superuser with rebooting the system. */
>> -     if (!capable(CAP_SYS_BOOT))
>> +     if (!capable(CAP_SYS_BOOT) || !capable(CAP_SYS_MODULE))
>>               return -EPERM;
>>
>>       /*
--
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