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

Andrii Nakryiko
2024-08-14T17:06:52Z

On Wed, Aug 14, 2024 at 9:14 AM Jann Horn <jannh@google.com> wrote:
>
> 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.

Ok, I'm convinced, all fair points. I'll switch to memcmp(), there is
no downside to that anyways.