On Wed, Aug 7, 2024 at 8:12 AM Jann Horn <jannh@xxxxxxxxxx> wrote:
>
> +Matthew and fsdevel list for pagecache question
>
> On Tue, Jul 30, 2024 at 10:39 PM Andrii Nakryiko <andrii@xxxxxxxxxx> wrote:
> > Harden build ID parsing logic, adding explicit READ_ONCE() where it's
> > important to have a consistent value read and validated just once.
> >
> > 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+.
>
> One thing that still seems dodgy to me with this patch applied is the
> call from build_id_parse() to find_get_page(), followed by reading the
> page contents. My understanding of the page cache (which might be
> incorrect) is that find_get_page() can return a page whose contents
> have not been initialized yet, and you're supposed to check for
> PageUptodate() or something like that before reading from it.
>
> Maybe Matthew can check if I understood that right?
>
>
> Also, it might be a good idea to liberally spray READ_ONCE() around
> all the remaining unannotated shared memory accesses in
> build_id_parse(), get_build_id_32(), get_build_id_64() and
> parse_build_id_buf().
Andi was against that, so I kept READ_ONCE() only where strictly
necessary, AFAICT.
>
> > Cc: stable@xxxxxxxxxxxxxxx
> > Cc: Jann Horn <jannh@xxxxxxxxxx>
> > Suggested-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> > Fixes: bd7525dacd7e ("bpf: Move stack_map_get_build_id into lib")
> > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > ---
> > lib/buildid.c | 51 +++++++++++++++++++++++++++------------------------
> > 1 file changed, 27 insertions(+), 24 deletions(-)
> >
> > diff --git a/lib/buildid.c b/lib/buildid.c
> > index e02b5507418b..d21d86f6c19a 100644
> > --- a/lib/buildid.c
> > +++ b/lib/buildid.c
> > @@ -18,28 +18,29 @@ static int parse_build_id_buf(unsigned char *build_id,
> > const void *note_start,
> > Elf32_Word note_size)
> > {
> > + const char note_name[] = "GNU";
> > + const size_t note_name_sz = sizeof(note_name);
> > Elf32_Word note_offs = 0, new_offs;
> > + u32 name_sz, desc_sz;
> > + const char *data;
> >
> > while (note_offs + sizeof(Elf32_Nhdr) < note_size) {
> > Elf32_Nhdr *nhdr = (Elf32_Nhdr *)(note_start + note_offs);
> >
> > + name_sz = READ_ONCE(nhdr->n_namesz);
> > + desc_sz = READ_ONCE(nhdr->n_descsz);
> > 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 &&
> > + desc_sz > 0 && desc_sz <= BUILD_ID_SIZE_MAX) {
> > + data = note_start + note_offs + ALIGN(note_name_sz, 4);
>
> I don't think we have any guarantee here that this addition won't
> result in an OOB pointer?
>
> > + memcpy(build_id, data, desc_sz);
>
> I think this can access OOB data (because "data" can already be OOB
> and because "desc_sz" hasn't been checked against the amount of
> remaining space in the page).
Andi already pointed this out and I fixed it locally, thanks.
>
> > + memset(build_id + desc_sz, 0, BUILD_ID_SIZE_MAX - desc_sz);
> > if (size)
> > - *size = nhdr->n_descsz;
> > + *size = desc_sz;
> > return 0;
> > }
> > - new_offs = note_offs + sizeof(Elf32_Nhdr) +
> > - ALIGN(nhdr->n_namesz, 4) + ALIGN(nhdr->n_descsz, 4);
> > + new_offs = note_offs + sizeof(Elf32_Nhdr) + ALIGN(name_sz, 4) + ALIGN(desc_sz, 4);
> > if (new_offs <= note_offs) /* overflow */
> > break;
>
> You check whether "new_offs" has wrapped here, but then on the next
> loop iteration, you check for "note_offs + sizeof(Elf32_Nhdr) <
> note_size". So if new_offs is 0xffffffff at this point, then I think
> the overflow check here will be passed, the loop condition will be
> true on 32-bit kernels (on 64-bit kernels it won't be because the
> addition happens on 64-bit numbers thanks to sizeof()), and "nhdr"
> will point in front of the note?
Correct, and so I moved this new_offs calculation and overflow check
to the beginning of the loop, which I think should capture this issue.
For the while() condition itself I have:
if (check_add_overflow(note_offs, note_size, ¬e_end))
return -EINVAL;
while (note_offs < note_end - sizeof(Elf32_Nhdr) - note_name_sz) {
...
}
I'll try to post an updated version soon-ish, have been waiting for mm
folks feedback before posting a new version.
>
> > note_offs = new_offs;
