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

[Suren Baghdasaryan]

On Wed, Jan 12, 2022 at 10:44 AM Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxx> 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?

IIRC this was done to avoid another warning that these functions are
not used outside of psi.c when CONFIG_CGROUPS=n

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