linux-kernel - Re: [PATCH] Capabilities: BUG when an invalid capability is requested

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Date:	Wed, 29 Oct 2008 18:20:00 -0700
From:	"Andrew G. Morgan" <morgan@...nel.org>
To:	"Serge E. Hallyn" <serue@...ibm.com>,
	Eric Paris <eparis@...hat.com>
CC:	linux-kernel@...r.kernel.org, arjan@...radead.org
Subject: Re: [PATCH] Capabilities: BUG when an invalid capability is	requested

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Acked-by: Andrew G. Morgan <morgan@...nel.org>

Cheers

Andrew

Serge E. Hallyn wrote:
> Quoting Eric Paris (eparis@...hat.com):
>> If an invalid (large) capability is requested the capabilities system
>> may panic as it is dereferencing an array of fixed (short) length.  Its
>> possible (and actually often happens) that the capability system
>> accidentally stumbled into a valid memory region but it also regularly
>> happens that it hits invalid memory and BUGs.  If such an operation does
>> get past cap_capable then the selinux system is sure to have problems as
>> it already does a (simple) validity check and BUG.  This is known to
>> happen by the broken and buggy firegl driver.
>>
>> This patch cleanly checks all capable calls and BUG if a call is for an
>> invalid capability.  This will likely break the firegl driver for some
>> situations, but it is the right thing to do.  Garbage into a security
>> system gets you killed/bugged
>>
>> Signed-off-by: Eric Paris <eparis@...hat.com>
> 
> I really don't like this, but I'm not sure we really have a choice.
> 
> Acked-by: Serge Hallyn <serue@...ibm.com>
> 
> I suppose we can think later about whether it's worthwhile (a) having a
> separate capable() function exported, keeping one without the check for
> compiled-in use only, and/or (b) changing the cap_valid() definition to
> be "(((unsigned int)cap) <= CAP_LAST_CAP)" which seems to work and shave
> one whopping instruction.  I suspect the answer will be no to both.
> 
> Thanks, Eric.
> 
> -serge
> 
>> ---
>>
>>  kernel/capability.c |    5 +++++
>>  1 files changed, 5 insertions(+), 0 deletions(-)
>>
>> diff --git a/kernel/capability.c b/kernel/capability.c
>> index 33e51e7..50d9d99 100644
>> --- a/kernel/capability.c
>> +++ b/kernel/capability.c
>> @@ -498,6 +498,11 @@ asmlinkage long sys_capset(cap_user_header_t header, const cap_user_data_t data)
>>   */
>>  int capable(int cap)
>>  {
>> +	if (unlikely(!cap_valid(cap))) {
>> +		printk(KERN_CRIT "capable() called with invalid cap=%u\n", cap);
>> +		BUG();
>> +	}
>> +
>>  	if (has_capability(current, cap)) {
>>  		current->flags |= PF_SUPERPRIV;
>>  		return 1;
>>
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