Re: [PATCH v2 2/3] sysctl: Fix underflow value setting risk in vm_table

[Nicolas Bouchinet <nicolas.bouchinet@clip-os.org>]

On 12/18/24 14:21, Joel Granados wrote:
> On Tue, Dec 10, 2024 at 03:58:41PM +0100, Nicolas Bouchinet wrote:
>> Hi Joel,
>>
>>
>> Thank's for your reply.
>>
>> I apologize for the reply delay, I wasn't available late weeks.
>>
>> On 11/20/24 1:53 PM, Joel Granados wrote:
>>> On Thu, Nov 14, 2024 at 05:25:51PM +0100, nicolas.bouchinet@clip-os.org wrote:
>>>> From: Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>
>>>>
>>>> Commit 3b3376f222e3 ("sysctl.c: fix underflow value setting risk in
>>>> vm_table") fixes underflow value setting risk in vm_table but misses
>>>> vdso_enabled sysctl.
>>>>
>>>> vdso_enabled sysctl is initialized with .extra1 value as SYSCTL_ZERO to
>>>> avoid negative value writes but the proc_handler is proc_dointvec and not
>>>> proc_dointvec_minmax and thus do not uses .extra1 and .extra2.
>>>>
>>>> The following command thus works :
>>>>
>>>> `# echo -1 > /proc/sys/vm/vdso_enabled`
>>> It would be interesting to know what happens when you do a
>>> # echo (INT_MAX + 1) > /proc/sys/vm/vdso_enabled
>> Great question, I'll check that.
>>
>>> This is the reasons why I'm interested in such a test:
>>>
>>> 1. Both proc_dointvec and proc_dointvec_minmax (calls proc_dointvec) have a
>>>      overflow check where they will return -EINVAL if what is given by the user is
>>>      greater than (unsiged long)INT_MAX; this will evaluate can evaluate to true
>>>      or false depending on the architecture where we are running.
>> Indeed, I'll run tests to avouch behaviors of proc handlers bound checks
>> with
>> different architectures.
>>
>>> 2. I noticed that vdso_enabled is an unsigned long. And so the expectation is
>>>      that the range is 0 to ULONG_MAX, which in some cases (depending on the arch)
>>>      would not be the case.
>> Yep, it is. As I've tried to explain in the cover letter
>> (https://lore.kernel.org/all/20241112131357.49582-1-nicolas.bouchinet@clip-os.org/),
>> there are numerous places where sysctl data type differs from the proc
>> handler
>> return type.
>>
>> AFAIK, for proc_dointvec there is more than 10 different sysctl where it
>> happens. The three I've patched represents three common mistakes using
>> proc_handlers.
> It would be useful to analyze the others. Do you have more outstanding
> patches for these?

I've started to analyze them more in depth it this monday, will send a 
patchset when
it seems ok.
I'm focusing on proc_dointvec for now.

>
>>> So my question is: What is the expected range for this value? Because you might
>>> not be getting the whole range in the cases where int is 32 bit and long is 64
>>> bit.
>>>
>>>> This patch properly sets the proc_handler to proc_dointvec_minmax.
>>>>
>>>> Fixes: 3b3376f222e3 ("sysctl.c: fix underflow value setting risk in vm_table")
>>>> Signed-off-by: Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>
>>>> ---
>>>>    kernel/sysctl.c | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
>>>> index 79e6cb1d5c48f..37b1c1a760985 100644
>>>> --- a/kernel/sysctl.c
>>>> +++ b/kernel/sysctl.c
>>>> @@ -2194,7 +2194,7 @@ static struct ctl_table vm_table[] = {
>>>>    		.maxlen		= sizeof(vdso_enabled),
>>>>    #endif
>>>>    		.mode		= 0644,
>>>> -		.proc_handler	= proc_dointvec,
>>>> +		.proc_handler	= proc_dointvec_minmax,
>>>>    		.extra1		= SYSCTL_ZERO,
>>> Any reason why extra2 is not defined. I know that it was not defined before, but
>>> this does not mean that it will not have an upper limit. The way that I read the
>>> situation is that this will be bounded by the overflow check done in
>>> proc_dointvec and will have an upper limit of INT_MAX.
>> Yes, it is bounded by the overflow checks done in proc_dointvec, I've not
>> changed the current sysctl behavior but we should bound it between 0
>> and 1 since it seems vdso compat is not supported anymore since
>> Commit b0b49f2673f011cad ("x86, vdso: Remove compat vdso support").
> I think you have already done this in your V3
>
>> This is the behavior of vdso32_enabled exposed under the abi sysctl
>> node.
>>
>>> Please correct me if I have read the situation incorrectly.
>> You perfectly understood the problematic of it, thanks a lot for your
>> review.
>>
>> I'll reply to above questions after I've run more tests.
>>
>> I saw GKH already merged the third commit of this patchset and
>> backported it to stable branches.
>> Should I evict it from future version of this patchset ?
> Yes. You should remove what has already been merged into main
> line. thx.
>
> Best
>