On Wed, Oct 23, 2024 at 4:29 PM Hou Tao <houtao@xxxxxxxxxxxxxxx> wrote:
>
>
>
> On 10/23/2024 11:17 AM, Yafang Shao wrote:
> > On Mon, Oct 21, 2024 at 9:28 AM Hou Tao <houtao@xxxxxxxxxxxxxxx> wrote:
> >> From: Hou Tao <houtao1@xxxxxxxxxx>
> >>
> >> Check the validity of nr_words in bpf_iter_bits_new(). Without this
> >> check, when multiplication overflow occurs for nr_bits (e.g., when
> >> nr_words = 0x0400-0001, nr_bits becomes 64), stack corruption may occur
> >> due to bpf_probe_read_kernel_common(..., nr_bytes = 0x2000-0008).
> >>
> >> Fix it by limiting the max value of nr_words to 512.
> >>
> >> Fixes: 4665415975b0 ("bpf: Add bits iterator")
> >> Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx>
> >> ---
> >> kernel/bpf/helpers.c | 4 ++++
> >> 1 file changed, 4 insertions(+)
> >>
> >> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> >> index 62349e206a29..c147f75e1b48 100644
> >> --- a/kernel/bpf/helpers.c
> >> +++ b/kernel/bpf/helpers.c
> >> @@ -2851,6 +2851,8 @@ struct bpf_iter_bits {
> >> __u64 __opaque[2];
> >> } __aligned(8);
> >>
> >> +#define BITS_ITER_NR_WORDS_MAX 512
> >> +
> >> struct bpf_iter_bits_kern {
> >> union {
> >> unsigned long *bits;
> >> @@ -2892,6 +2894,8 @@ bpf_iter_bits_new(struct bpf_iter_bits *it, const u64 *unsafe_ptr__ign, u32 nr_w
> >>
> >> if (!unsafe_ptr__ign || !nr_words)
> >> return -EINVAL;
> >> + if (nr_words > BITS_ITER_NR_WORDS_MAX)
> >> + return -E2BIG;
> > It is documented that nr_words cannot exceed 512, not due to overflow
> > concerns, but because of memory allocation limits. It might be better
> > to use 512 directly instead of BITS_ITER_NR_WORDS_MAX. Alternatively,
> > if we decide to keep using the macro, the documentation should be
> > updated accordingly.
>
> Thanks for the explanation. Actually according to the limitation of bpf
> memory allocator, the limitation should be (4096 - 8) / 8 = 511 due to
> the overhead of llist_head in the returned pointer.
If that's the case, we should make the following code change, right ?
diff --git a/kernel/bpf/memalloc.c b/kernel/bpf/memalloc.c
index 1a1b4458114c..64e73579c7d6 100644
--- a/kernel/bpf/memalloc.c
+++ b/kernel/bpf/memalloc.c
@@ -65,7 +65,7 @@ static u8 size_index[24] __ro_after_init = {
static int bpf_mem_cache_idx(size_t size)
{
- if (!size || size > 4096)
+ if (!size || size > (4096 - 8))
return -1;
if (size <= 192)
--
Regards
Yafang
