Re: [PATCH v5 bpf-next 01/10] lib/buildid: harden build ID parsing logic

[Jann Horn <jannh@google.com>]

On Wed, Aug 14, 2024 at 1:21 AM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
> On Tue, Aug 13, 2024 at 1:59 PM Jann Horn <jannh@google.com> wrote:
> >
> > On Tue, Aug 13, 2024 at 2:29 AM Andrii Nakryiko <andrii@kernel.org> wrote:
> > > Harden build ID parsing logic, adding explicit READ_ONCE() where it's
> > > important to have a consistent value read and validated just once.
> > >
> > > Also, as pointed out by Andi Kleen, we need to make sure that entire ELF
> > > note is within a page bounds, so move the overflow check up and add an
> > > extra note_size boundaries validation.
> > >
> > > Fixes tag below points to the code that moved this code into
> > > lib/buildid.c, and then subsequently was used in perf subsystem, making
> > > this code exposed to perf_event_open() users in v5.12+.
> >
> > Sorry, I missed some things in previous review rounds:
> >
> > [...]
> > > @@ -18,31 +18,37 @@ static int parse_build_id_buf(unsigned char *build_id,
> > [...]
> > >                 if (nhdr->n_type == BUILD_ID &&
> > > -                   nhdr->n_namesz == sizeof("GNU") &&
> > > -                   !strcmp((char *)(nhdr + 1), "GNU") &&
> > > -                   nhdr->n_descsz > 0 &&
> > > -                   nhdr->n_descsz <= BUILD_ID_SIZE_MAX) {
> > > -                       memcpy(build_id,
> > > -                              note_start + note_offs +
> > > -                              ALIGN(sizeof("GNU"), 4) + sizeof(Elf32_Nhdr),
> > > -                              nhdr->n_descsz);
> > > -                       memset(build_id + nhdr->n_descsz, 0,
> > > -                              BUILD_ID_SIZE_MAX - nhdr->n_descsz);
> > > +                   name_sz == note_name_sz &&
> > > +                   strcmp((char *)(nhdr + 1), note_name) == 0 &&
> >
> > Please change this to something like "memcmp((char *)(nhdr + 1),
> > note_name, note_name_sz) == 0" to ensure that we can't run off the end
> > of the page if there are no null bytes in the rest of the page.
>
> I did switch this to strncmp() at some earlier point, but then
> realized that there is no point because note_name is controlled by us
> and will ensure there is a zero at byte (note_name_sz - 1). So I don't
> think memcmp() buys us anything.

There are two reasons why using strcmp() here makes me uneasy.


First: We're still operating on shared memory that can concurrently change.

Let's say strcmp is implemented like this, this is the generic C
implementation in the kernel (which I think is the implementation
that's used for x86-64):

int strcmp(const char *cs, const char *ct)
{
        unsigned char c1, c2;

        while (1) {
                c1 = *cs++;
                c2 = *ct++;
                if (c1 != c2)
                        return c1 < c2 ? -1 : 1;
                if (!c1)
                        break;
        }
        return 0;
}

No READ_ONCE() or anything like that - it's not designed for being
used on concurrently changing memory.

And let's say you call it like strcmp(<shared memory>, "GNU"), and
we're now in the fourth iteration. If the compiler decides to re-fetch
the value of "c1" from memory for each of the two conditions, then it
could be that the "if (c1 != c2)" sees c1='\0' and c2='\0', so the
condition evaluates as false; but then at the "if (!c1)", the value in
memory changed, and we see c1='A'. So now in the next round, we'll be
accessing out-of-bounds memory behind the 4-byte string constant
"GNU".

So I don't think strcmp() on memory that can concurrently change is allowed.

(It actually seems like the generic memcmp() is also implemented
without READ_ONCE(), maybe we should change that...)


Second: You are assuming that if one side of the strcmp() is at most
four bytes long (including null terminator), then strcmp() also won't
access more than 4 bytes of the other string, even if that string does
not have a null terminator at index 4. I don't think that's part of
the normal strcmp() API contract.