Re: [PATCH v3 1/1] psi: Fix uaf issue when psi trigger is destroyed while being polled

Eric Biggers
2022-01-12T18:45:29Z

On Wed, Jan 12, 2022 at 10:26:08AM -0800, Suren Baghdasaryan wrote:
> On Wed, Jan 12, 2022 at 10:16 AM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Wed, Jan 12, 2022 at 09:49:00AM -0800, Suren Baghdasaryan wrote:
> > > > This happens with the following config:
> > > >
> > > > CONFIG_CGROUPS=n
> > > > CONFIG_PSI=y
> > > >
> > > > With cgroups disabled these functions are defined as non-static but
> > > > are not defined in the header
> > > > (https://elixir.bootlin.com/linux/latest/source/include/linux/psi.h#L28)
> > > > since the only external user cgroup.c is disabled. The cleanest way to
> > > > fix these I think is by doing smth like this in psi.c:
> >
> > A cleaner way to solve these is simply:
> >
> > #ifndef CONFIG_CGROUPS
> > static struct psi_trigger *psi_trigger_create(...);
> > ...
> > #endif
> >
> > I tested this works:
> >
> > $ cat foo5.c
> > static int psi(void *);
> >
> > int psi(void *x)
> > {
> > return (int)(long)x;
> > }
> >
> > int bar(void *x)
> > {
> > return psi(x);
> > }
> > $ gcc -W -Wall -O2 -c -o foo5.o foo5.c
> > $ readelf -s foo5.o
> >
> > Symbol table '.symtab' contains 4 entries:
> > Num: Value Size Type Bind Vis Ndx Name
> > 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
> > 1: 0000000000000000 0 FILE LOCAL DEFAULT ABS foo5.c
> > 2: 0000000000000000 0 SECTION LOCAL DEFAULT 1 .text
> > 3: 0000000000000000 3 FUNC GLOBAL DEFAULT 1 bar
> >
>
> Thanks Matthew!
> That looks much cleaner. I'll post a separate patch to fix these. My
> main concern was whether it's worth adding more code to satisfy this
> warning but with this approach the code changes are minimal, so I'll
> go ahead and post it shortly.

Why not simply move the declarations of psi_trigger_create() and
psi_trigger_destroy() in include/linux/psi.h outside of the
'#ifdef CONFIG_CGROUPS' block, to match the .c file?

They *could* be static when !CONFIG_CGROUPS, but IMO it's not worth bothering.

- Eric