Re: [PATCH 1/2] bpf: Fix uninit read for non-fetch atomics on partially spilled slots

[Kumar Kartikeya Dwivedi]

On Fri Sep 25, 2026 at 4:23 AM CEST, Kumar Kartikeya Dwivedi wrote:
> On Thu Sep 24, 2026 at 3:13 PM CEST, Hao Sun wrote:
>> check_stack_read_fixed_off() skips partial spill checks for non-fetch
>> atomics; the following prog can be loaded:
>>
>> 0: (b7) r1 = 1                        ; R1=1
>> 1: (63) *(u32 *)(r10 -8) = r1         ; R1=1 R10=fp0 fp-8=????1
>> 2: (db) lock *(u64 *)(r10 -8) += r1   ; R1=1 R10=fp0 fp-8=mmmmmmmm
>> 3: (79) r0 = *(u64 *)(r10 -8)         ; R0=scalar() R10=fp0 fp-8=mmmmmmmm
>> 4: (77) r0 >>= 32                     ; R0=scalar(smin=0,smax=umax=0xffffffff,var_off=(0x0; 0xffffffff))
>> 5: (95) exit
>>
>> When test run:
>> 	retval=4294967295
>>
>> Note fp-8 is ????1 at #1, yet it becomes fp-8=mmmmmmmm after the
>> non-fetching atomic add at #2; hence the high 32 bits are leaked.
>>
>> Fix by applying the partial load check; after the patch, the
>> prog is rejected:
>>
>> Verification failed: Memory Safety: Uninitialized stack read
>>
>> Reason:
>>   This rejected read uses 8 bytes at stack offset -8, but byte 4 in that range is uninitialized on
>>   this path. Programs loaded with CAP_PERFMON can be allowed to read uninitialized stack bytes, but
>>   this program is being rejected without that allowance.
>>
>> At:
>>   ...
>>       0 | (b7) r1 = 1
>>       1 | (63) *(u32 *)(r10 -8) = r1
>>   >>> 2 | (db) lock *(u64 *)(r10 -8) += r1
>>       3 | (79) r0 = *(u64 *)(r10 -8)
>>       4 | (77) r0 >>= 32
>>
>> This affects CAP_BPF only.
>>
>> Fixes: 354e8f1970f8 ("bpf: Support <8-byte scalar spill and refill")
>> Signed-off-by: Hao Sun <sunhao.th@xxxxxxxxx>
>>
>> ---
>
> The same root cause is also reachable through check_stack_range_initialized().
> It takes the spilled register path for every byte of a slot holding a spilled
> scalar, without looking at the byte's own slot_type, so a helper or kfunc memory
> argument spanning a narrow spill still reads the uninitialized half without
> CAP_PERFMON:
>
>   r1 = 1;
>   *(u32 *)(r10 - 8) = r1;
>   r1 = map_ringbuf ll;
>   r2 = r10;
>   r2 += -8;
>   r3 = 8;
>   r4 = 0;
>   call bpf_ringbuf_output;
>
> loads with CAP_BPF alone and copies four uninitialized bytes of kernel stack to
> user space. The fix is to only take that path when *stype == STACK_SPILL, so the
> rest of the slot goes through the usual MISC/ZERO/INVALID checks like any other
> slot. Could you add that to v2 with a matching test (same Fixes: tag), since
> it's the same bug on the helper path?
>
> Locally I tried something like this and it addressed the problem.
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 200ec0f71617..9166dccef95a 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -7481,7 +7481,12 @@ static int check_stack_range_initialized(
>                         goto mark;
>                 }
>
> -               if (bpf_is_spilled_reg(ss) &&
> +               /*
> +                * Only the bytes marked STACK_SPILL hold the spilled register.
> +                * The rest of a narrowly spilled slot keeps its previous type
> +                * and must be initialized on its own.
> +                */
> +               if (*stype == STACK_SPILL &&
>                     (ss->spilled_ptr.type == SCALAR_VALUE ||
>                      env->allow_ptr_leaks)) {
>                         if (clobber) {
>
> Please also target bpf-next in next revision.
>

Actually, I just applied these two, so you can just follow up with extra fix +
test.

> pw-bot: cr