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

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Message-ID: <c3e800d2-0aff-478e-906a-18f8fc6d756a@clip-os.org>
Date: Tue, 17 Dec 2024 14:57:51 +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,

I've pushed patchset version 3 :
https://lore.kernel.org/all/20241217132908.38096-1-nicolas.bouchinet@clip-os.org/.

On 11/20/24 13:53, 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
>
> 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.
>
> 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.
 From my observations, vdso_enabled is a unsigned int. If one wants to
convert to an unsigned long, proc_doulongvec_minmax should be used
instead.

IMHO, the main issues are that .data variable type can differ from the 
return
type of .proc_handler function. This can lead to undefined behaviors and
eventually vulnerabilities.

.extra1 and .extra2 can also be used with proc_handlers that do not uses 
them.
I think sysctl_check_table() could be enhanced to control this behavior.

>
> 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.
If proc_dointvec or its derivative is used, as you said, range is bounded
by checks in do_proc_dointvec_conv ((unsigned long) INT_MAX).

INT_MAX being based on the max value of an int (((int)(~0U >> 1))),
do_proc_dointvec_conv behavior is thus architecture dependent.
>
>> 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.

I've added an extra2 parameter to restrict vdso_enabled between 0 and 1 
in patchset v3.
https://lore.kernel.org/all/20241217132908.38096-3-nicolas.bouchinet@clip-os.org/

>
> Please correct me if I have read the situation incorrectly.
>
> Best
>
Thanks again for your review,

Best regards,

Nicolas