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

[Nicolas Bouchinet]

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@xxxxxxxxxxx wrote:
> > > > From: Nicolas Bouchinet <nicolas.bouchinet@xxxxxxxxxxx>
> > > >
> > > > 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@xxxxxxxxxxx/),
> > 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@xxxxxxxxxxx>
> > > > ---
> > > >    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