On Mon, Dec 16, 2024 at 05:16:33PM +0100, Daniel Borkmann wrote:
> On 12/16/24 11:46 AM, Andrea Righi wrote:
> > Calling bpf_get_smp_processor_id() in a kernel with CONFIG_SMP disabled
> > can trigger the following bug, as pcpu_hot is unavailable:
> >
> > [ 8.471774] BUG: unable to handle page fault for address: 00000000936a290c
> > [ 8.471849] #PF: supervisor read access in kernel mode
> > [ 8.471881] #PF: error_code(0x0000) - not-present page
> >
> > Fix by preventing the inlining of bpf_get_smp_processor_id() when
> > CONFIG_SMP disabled.
> >
> > Fixes: 1ae6921009e5 ("bpf: inline bpf_get_smp_processor_id() helper")
> > Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
>
> lgtm, but can't we instead do sth like this :
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index f7f892a52a37..761c70899754 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -21281,11 +21281,15 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
> * changed in some incompatible and hard to support
> * way, it's fine to back out this inlining logic
> */
> +#ifdef CONFIG_SMP
> insn_buf[0] = BPF_MOV32_IMM(BPF_REG_0, (u32)(unsigned long)&pcpu_hot.cpu_number);
> insn_buf[1] = BPF_MOV64_PERCPU_REG(BPF_REG_0, BPF_REG_0);
> insn_buf[2] = BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_0, 0);
> cnt = 3;
> -
> +#else
> + BPF_ALU32_REG(BPF_XOR, BPF_REG_0, BPF_REG_0),
> + cnt = 1;
> +#endif
> new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
> if (!new_prog)
> return -ENOMEM;
That works as well (just tested) and it's probably better since we're
basically inlining the return 0. Do you want me to send a v2 with this?
Thanks,
-Andrea
