linux-kernel - Re: [PATCH v2 2/3] sysctl: Fix underflow value setting risk in vm

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Message-ID: <2d5447b7-c185-4ce9-852e-b56a28b0306a@clip-os.org>
Date: Tue, 10 Dec 2024 15:58:41 +0100
From: Nicolas Bouchinet <nicolas.bouchinet@...p-os.org>
To: Joel Granados <joel.granados@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
 linux-fsdevel@...r.kernel.org,
 Nicolas Bouchinet <nicolas.bouchinet@....gouv.fr>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Jiri Slaby <jirislaby@...nel.org>, Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 Luis Chamberlain <mcgrof@...nel.org>, Kees Cook <kees@...nel.org>,
 Joel Granados <j.granados@...sung.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Neil Horman <nhorman@...driver.com>, Lin Feng <linf@...gsu.com>,
 Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH v2 2/3] sysctl: Fix underflow value setting risk in
 vm_table

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@...p-os.org wrote:
>> From: Nicolas Bouchinet <nicolas.bouchinet@....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.

>
> 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@....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").

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 ?

Thanks,

Nicolas

>
> Best
>