Re: [PATCH bpf v2 4/7] bpf: Free dynamically allocated bits in bpf_iter_bits_destroy()

[Hou Tao <houtao@xxxxxxxxxxxxxxx>]

Hi,

On 10/21/2024 9:40 AM, Hou Tao wrote:
> From: Hou Tao <houtao1@xxxxxxxxxx>
>
> bpf_iter_bits_destroy() uses "kit->nr_bits <= 64" to check whether the
> bits are dynamically allocated. However, the check is incorrect and may
> cause a kmemleak as shown below:
>
> unreferenced object 0xffff88812628c8c0 (size 32):
>   comm "swapper/0", pid 1, jiffies 4294727320
>   hex dump (first 32 bytes):
> 	b0 c1 55 f5 81 88 ff ff f0 f0 f0 f0 f0 f0 f0 f0  ..U.............
> 	f0 f0 f0 f0 f0 f0 f0 f0 00 00 00 00 00 00 00 00  ................
>   backtrace (crc 781e32cc):
> 	[<00000000c452b4ab>] kmemleak_alloc+0x4b/0x80
> 	[<0000000004e09f80>] __kmalloc_node_noprof+0x480/0x5c0
> 	[<00000000597124d6>] __alloc.isra.0+0x89/0xb0
> 	[<000000004ebfffcd>] alloc_bulk+0x2af/0x720
> 	[<00000000d9c10145>] prefill_mem_cache+0x7f/0xb0
> 	[<00000000ff9738ff>] bpf_mem_alloc_init+0x3e2/0x610
> 	[<000000008b616eac>] bpf_global_ma_init+0x19/0x30
> 	[<00000000fc473efc>] do_one_initcall+0xd3/0x3c0
> 	[<00000000ec81498c>] kernel_init_freeable+0x66a/0x940
> 	[<00000000b119f72f>] kernel_init+0x20/0x160
> 	[<00000000f11ac9a7>] ret_from_fork+0x3c/0x70
> 	[<0000000004671da4>] ret_from_fork_asm+0x1a/0x30
>
> That is because nr_bits will be set as zero in bpf_iter_bits_next()
> after all bits have been iterated.
>
> Fix the problem by not setting nr_bits to zero in bpf_iter_bits_next().
> Instead, use "bits >= nr_bits" to indicate when iteration is completed
> and still use "nr_bits > 64" to indicate when bits are dynamically
> allocated.
>
> Fixes: 4665415975b0 ("bpf: Add bits iterator")
> Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx>
> ---
>  kernel/bpf/helpers.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 1a43d06eab28..62349e206a29 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2888,7 +2888,7 @@ bpf_iter_bits_new(struct bpf_iter_bits *it, const u64 *unsafe_ptr__ign, u32 nr_w
>  
>  	kit->nr_bits = 0;
>  	kit->bits_copy = 0;
> -	kit->bit = -1;
> +	kit->bit = 0;

Sent the patch out in a hurry and didn't run the related test.

The change above will break "fewer words" test in verifier_bits_iter,
because it will skip bit 0 in the bit. The correct fix should be as below:

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 1a43d06eab28..190b730e0f86 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2934,15 +2934,13 @@ __bpf_kfunc int *bpf_iter_bits_next(struct
bpf_iter_bits *it)
        const unsigned long *bits;
        int bit;

-       if (nr_bits == 0)
+       if (kit->bit >= 0 && kit->bit >= nr_bits)
                return NULL;

        bits = nr_bits == 64 ? &kit->bits_copy : kit->bits;
        bit = find_next_bit(bits, nr_bits, kit->bit + 1);
-       if (bit >= nr_bits) {
-               kit->nr_bits = 0;
+       if (bit >= nr_bits)
                return NULL;
-       }

        kit->bit = bit;
        return &kit->bit;

>  
>  	if (!unsafe_ptr__ign || !nr_words)
>  		return -EINVAL;
> @@ -2934,15 +2934,13 @@ __bpf_kfunc int *bpf_iter_bits_next(struct bpf_iter_bits *it)
>  	const unsigned long *bits;
>  	int bit;
>  
> -	if (nr_bits == 0)
> +	if (kit->bit >= nr_bits)
>  		return NULL;
>  
>  	bits = nr_bits == 64 ? &kit->bits_copy : kit->bits;
>  	bit = find_next_bit(bits, nr_bits, kit->bit + 1);
> -	if (bit >= nr_bits) {
> -		kit->nr_bits = 0;
> +	if (bit >= nr_bits)
>  		return NULL;
> -	}
>  
>  	kit->bit = bit;
>  	return &kit->bit;