Messages in this thread | | | Subject | Re: BUG: unable to handle kernel paging request in bpf_dispatcher_xdp | From | Daniel Borkmann <> | Date | Fri, 9 Dec 2022 23:41:11 +0100 |
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On 12/9/22 10:53 PM, Jiri Olsa wrote: > On Fri, Dec 09, 2022 at 12:31:06PM -0800, Yonghong Song wrote: >> >> >> On 12/9/22 7:20 AM, Jiri Olsa wrote: >>> On Fri, Dec 09, 2022 at 02:50:55PM +0100, Jiri Olsa wrote: >>>> On Fri, Dec 09, 2022 at 12:22:37PM +0100, Jiri Olsa wrote: >>>> >>>> SBIP >>>> >>>>>>>>>>> >>>>>>>>>>> I'm trying to understand the severity of the issues and >>>>>>>>>>> whether we need to revert that commit asap since the merge window >>>>>>>>>>> is about to start. >>>>>>>>>> >>>>>>>>>> Jiri, Peter, >>>>>>>>>> >>>>>>>>>> ping. >>>>>>>>>> >>>>>>>>>> cc-ing Thorsten, since he's tracking it now. >>>>>>>>>> >>>>>>>>>> The config has CONFIG_X86_KERNEL_IBT=y. >>>>>>>>>> Is it related? >>>>>>>>> >>>>>>>>> sorry for late reply.. I still did not find the reason, >>>>>>>>> but I did not try with IBT yet, will test now >>>>>>>> >>>>>>>> no difference with IBT enabled, can't reproduce the issue >>>>>>>> >>>>>>> >>>>>>> ok, scratch that.. the reproducer got stuck on wifi init :-\ >>>>>>> >>>>>>> after I fix that I can now reproduce on my local config with >>>>>>> IBT enabled or disabled.. it's something else >>>>>> >>>>>> I'm getting the error also when reverting the static call change, >>>>>> looking for good commit, bisecting >>>>>> >>>>>> I'm getting fail with: >>>>>> f0c4d9fc9cc9 (tag: v6.1-rc4) Linux 6.1-rc4 >>>>>> >>>>>> v6.1-rc1 is ok >>>>> >>>>> so far I narrowed it down between rc1 and rc3.. bisect got me nowhere so far >>>>> >>>>> attaching some more logs >>>> >>>> looking at the code.. how do we ensure that code running through >>>> bpf_prog_run_xdp will not get dispatcher image changed while >>>> it's being exetuted >>>> >>>> we use 'the other half' of the image when we add/remove programs, >>>> but could bpf_dispatcher_update race with bpf_prog_run_xdp like: >>>> >>>> >>>> cpu 0: cpu 1: >>>> >>>> bpf_prog_run_xdp >>>> ... >>>> bpf_dispatcher_xdp_func >>>> start exec image at offset 0x0 >>>> >>>> bpf_dispatcher_update >>>> update image at offset 0x800 >>>> bpf_dispatcher_update >>>> update image at offset 0x0 >>>> >>>> still in image at offset 0x0 >>>> >>>> >>>> that might explain why I wasn't able to trigger that on >>>> bare metal just in qemu >>> >>> I tried patch below and it fixes the issue for me and seems >>> to confirm the race above.. but not sure it's the best fix >>> >>> jirka >>> >>> >>> --- >>> diff --git a/kernel/bpf/dispatcher.c b/kernel/bpf/dispatcher.c >>> index c19719f48ce0..6a2ced102fc7 100644 >>> --- a/kernel/bpf/dispatcher.c >>> +++ b/kernel/bpf/dispatcher.c >>> @@ -124,6 +124,7 @@ static void bpf_dispatcher_update(struct bpf_dispatcher *d, int prev_num_progs) >>> } >>> __BPF_DISPATCHER_UPDATE(d, new ?: (void *)&bpf_dispatcher_nop_func); >>> + synchronize_rcu_tasks(); >>> if (new) >>> d->image_off = noff; >> >> This might work. In arch/x86/kernel/alternative.c, we have following >> code and comments. For text_poke, synchronize_rcu_tasks() might be able >> to avoid concurrent execution and update. > > so my idea was that we need to ensure all the current callers of > bpf_dispatcher_xdp_func (which should have rcu read lock, based > on the comment in bpf_prog_run_xdp) are gone before and new ones > execute the new image, so the next call to the bpf_dispatcher_update > will be safe to overwrite the other half of the image
If v6.1-rc1 was indeed okay, then it looks like this may be related to the trampoline patching for the static_call? Did it repro on v6.1-rc1 just with dbe69b299884 ("bpf: Fix dispatcher patchable function entry to 5 bytes nop") cherry-picked?
>> /** >> * text_poke_copy - Copy instructions into (an unused part of) RX memory >> * @addr: address to modify >> * @opcode: source of the copy >> * @len: length to copy, could be more than 2x PAGE_SIZE >> * >> * Not safe against concurrent execution; useful for JITs to dump >> * new code blocks into unused regions of RX memory. Can be used in >> * conjunction with synchronize_rcu_tasks() to wait for existing >> * execution to quiesce after having made sure no existing functions >> * pointers are live. >> */ >> void *text_poke_copy(void *addr, const void *opcode, size_t len) >> { >> unsigned long start = (unsigned long)addr; >> size_t patched = 0; >> >> if (WARN_ON_ONCE(core_kernel_text(start))) >> return NULL; >> >> mutex_lock(&text_mutex); >> while (patched < len) { >> unsigned long ptr = start + patched; >> size_t s; >> >> s = min_t(size_t, PAGE_SIZE * 2 - offset_in_page(ptr), len - >> patched); >> >> __text_poke(text_poke_memcpy, (void *)ptr, opcode + patched, >> s); >> patched += s; >> } >> mutex_unlock(&text_mutex); >> return addr; >> }
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