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docs: add post-mortem (ethereum#23518)
* docs: add post-mortem * Update docs/postmortems/2021-08-22-split-postmortem.md Co-authored-by: Sina Mahmoodi <1591639+s1na@users.noreply.github.com> * Update docs/postmortems/2021-08-22-split-postmortem.md Co-authored-by: Sina Mahmoodi <1591639+s1na@users.noreply.github.com> * Update docs/postmortems/2021-08-22-split-postmortem.md Co-authored-by: Sina Mahmoodi <1591639+s1na@users.noreply.github.com> * Update docs/postmortems/2021-08-22-split-postmortem.md * Update 2021-08-22-split-postmortem.md * Update docs/postmortems/2021-08-22-split-postmortem.md Co-authored-by: Sina Mahmoodi <1591639+s1na@users.noreply.github.com> Co-authored-by: Sina Mahmoodi <1591639+s1na@users.noreply.github.com>
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| # Minority split 2021-08-27 post mortem | ||
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| This is a post-mortem concerning the minority split that occurred on Ethereum mainnet on block [13107518](https://etherscan.io/block/13107518), at which a minority chain split occurred. | ||
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| ## Timeline | ||
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| - 2021-08-17: Guido Vranken submitted bounty report. Investigation started, root cause identified, patch variations discussed. | ||
| - 2021-08-18: Made public announcement over twitter about upcoming security release upcoming Tuesday. Downstream projects were also notified about the upcoming patch-release. | ||
| - 2021-08-24: Released [v1.10.8](https://github.com/ethereum/go-ethereum/releases/tag/v1.10.8) containing the fix on Tuesday morning (CET). Erigon released [v2021.08.04](https://github.com/ledgerwatch/erigon/releases/tag/v2021.08.04). | ||
| - 2021-08-27: At 12:50:07 UTC, issue exploited. Analysis started roughly 30m later, | ||
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| ## Bounty report | ||
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| ### 2021-08-17 RETURNDATA corruption via datacopy | ||
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| On 2021-08-17, Guido Vranken submitted a report to bounty@ethereum.org. This co-incided with a geth-meetup in Berlin, so the geth team could fairly quickly analyse the issue. | ||
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| He submitted a proof of concept which called the `dataCopy` precompile, where the input slice and output slice were overlapping but shifted. Doing a `copy` where the `src` and `dest` overlaps is not a problem in itself, however, the `returnData`slice was _also_ using the same memory as a backing-array. | ||
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| #### Technical details | ||
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| During CALL-variants, `geth` does not copy the input. This was changed at one point, to avoid a DoS attack reported by Hubert Ritzdorf, to avoid copying data a lot on repeated `CALL`s -- essentially combating a DoS via `malloc`. Further, the datacopy precompile also does not copy the data, but just returns the same slice. This is fine so far. | ||
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| After the execution of `dataCopy`, we copy the `ret` into the designated memory area, and this is what causes a problem. Because we're copying a slice of memory over a slice of memory, and this operation modifies (shifts) the data in the source -- the `ret`. So this means we wind up with corrupted returndata. | ||
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| ``` | ||
| 1. Calling datacopy | ||
| memory: [0, 1, 2, 3, 4] | ||
| in (mem[0:4]) : [0,1,2,3] | ||
| out (mem[1:5]): [1,2,3,4] | ||
| 2. dataCopy returns | ||
| returndata (==in, mem[0:4]): [0,1,2,3] | ||
| 3. Copy in -> out | ||
| => memory: [0,0,1,2,3] | ||
| => returndata: [0,0,1,2] | ||
| ``` | ||
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| #### Summary | ||
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| A memory-corruption bug within the EVM can cause a consensus error, where vulnerable nodes obtain a different `stateRoot` when processing a maliciously crafted transaction. This, in turn, would lead to the chain being split: mainnet splitting in two forks. | ||
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| #### Handling | ||
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| On the evening of 17th, we discussed options how to handle it. We made a state test to reproduce the issue, and verified that neither `openethereum`, `nethermind` nor `besu` were affected by the same vulnerability, and started a full-sync with a patched version of `geth`. | ||
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| It was decided that in this specific instance, it would be possible to make a public announcement and a patch release: | ||
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| - The fix can be made pretty 'generically', e.g. always copying data on input to precompiles. | ||
| - The flaw is pretty difficult to find, given a generic fix in the call. The attacker needs to figure out that it concerns the precompiles, specifically the datcopy, and that it concerns the `RETURNDATA` buffer rather than the regular memory, and lastly the special circumstances to trigger it (overlapping but shifted input/output). | ||
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| Since we had merged the removal of `ETH65`, if the entire network were to upgrade, then nodes which have not yet implemented `ETH66` would be cut off from the network. After further discussions, we decided to: | ||
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| - Announce an upcoming security release on Tuesday (August 24th), via Twitter and official channels, plus reach out to downstream projects. | ||
| - Temporarily revert the `ETH65`-removal. | ||
| - Place the fix into the PR optimizing the jumpdest analysis [233381](https://github.com/ethereum/go-ethereum/pull/23381). | ||
| - After 4-8 weeks, release details about the vulnerability. | ||
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| ## Exploit | ||
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| At block [13107518](https://etherscan.io/block/13107518), mined at (Aug-27-2021 12:50:07 PM +UTC), a minority chain split occurred. The discord user @AlexSSD7 notified the allcoredevs-channel on the Eth R&D discord, on Aug 27 13:09 UTC. | ||
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| At 14:09 UTC, it was confirmed that the transaction `0x1cb6fb36633d270edefc04d048145b4298e67b8aa82a9e5ec4aa1435dd770ce4` had triggered the bug, leading to a minority-split of the chain. The term 'minority split' means that the majority of miners continued to mine on the correct chain. | ||
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| At 14:17 UTC, @mhswende tweeted out about the issue [2]. | ||
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| The attack was sent from an account funded from Tornado cash. | ||
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| It was also found that the same attack had been carried out on the BSC chain at roughly the same time -- at a block mined [12 minutes earlier](https://bscscan.com/tx/0xf667f820631f6adbd04a4c92274374034a3e41fa9057dc42cb4e787535136dce), at Aug-27-2021 12:38:30 PM +UTC. | ||
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| The blocks on the 'bad' chain were investigated, and Tim Beiko reached out to those mining operators on the minority chain who could be identified via block extradata. | ||
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| ## Lessons learned | ||
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| ### Disclosure decision | ||
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| The geth-team have an official policy regarding [vulnerability disclosure](https://geth.ethereum.org/docs/vulnerabilities/vulnerabilities). | ||
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| > The primary goal for the Geth team is the health of the Ethereum network as a whole, and the decision whether or not to publish details about a serious vulnerability boils down to minimizing the risk and/or impact of discovery and exploitation. | ||
| In this case, it was decided that public pre-announce + patch would likely lead to sufficient update-window for a critical mass of nodes/miners to upgrade in time before it could be exploited. In hindsight, this was a dangerous decision, and it's unlikely that the same decision would be reached were a similar incident to happen again. | ||
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| ### Disclosure path | ||
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| Several subprojects were informed about the upcoming security patch: | ||
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| - Polygon/Matic | ||
| - MEV | ||
| - Avalanche | ||
| - Erigon | ||
| - BSC | ||
| - EWF | ||
| - Quorum | ||
| - ETC | ||
| - xDAI | ||
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| However, some were 'lost', and only notified later | ||
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| - Optimism | ||
| - Summa | ||
| - Harmony | ||
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| Action point: create a low-volume geth-announce@ethereum.org email list where dependent projects/operators can receive public announcements. | ||
| - This has been done. If you wish to receive release- and security announcements, sign up [here](https://groups.google.com/a/ethereum.org/g/geth-announce/about) | ||
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| ### Fork monitoring | ||
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| The fork monitor behaved 'ok' during the incident, but had to be restarted during the evening. | ||
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| Action point: improve the resiliency of the forkmon, which is currently not performing great when many nodes are connected. | ||
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| Action point: enable push-based alerts to be sent from the forkmon, to speed up the fork detection. | ||
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| ## Links | ||
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| - [1] https://twitter.com/go_ethereum/status/1428051458763763721 | ||
| - [2] https://twitter.com/mhswende/status/1431259601530458112 | ||
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| ## Appendix | ||
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| ### Subprojects | ||
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| The projects were sent variations of the following text: | ||
| ``` | ||
| We have identified a security issue with go-ethereum, and will issue a | ||
| new release (v1.10.8) on Tuesday next week. | ||
| At this point, we will not disclose details about the issue, but | ||
| recommend downstream/dependent projects to be ready to take actions to | ||
| upgrade to the latest go-ethereum codebase. More information about the | ||
| issue will be disclosed at a later date. | ||
| https://twitter.com/go_ethereum/status/1428051458763763721 | ||
| ``` | ||
| ### Patch | ||
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| ```diff | ||
| diff --git a/core/vm/instructions.go b/core/vm/instructions.go | ||
| index f7ef2f900e..6c8c6e6e6f 100644 | ||
| --- a/core/vm/instructions.go | ||
| +++ b/core/vm/instructions.go | ||
| @@ -669,6 +669,7 @@ func opCall(pc *uint64, interpreter *EVMInterpreter, scope *ScopeContext) ([]byt | ||
| } | ||
| stack.push(&temp) | ||
| if err == nil || err == ErrExecutionReverted { | ||
| + ret = common.CopyBytes(ret) | ||
| scope.Memory.Set(retOffset.Uint64(), retSize.Uint64(), ret) | ||
| } | ||
| scope.Contract.Gas += returnGas | ||
| @@ -703,6 +704,7 @@ func opCallCode(pc *uint64, interpreter *EVMInterpreter, scope *ScopeContext) ([ | ||
| } | ||
| stack.push(&temp) | ||
| if err == nil || err == ErrExecutionReverted { | ||
| + ret = common.CopyBytes(ret) | ||
| scope.Memory.Set(retOffset.Uint64(), retSize.Uint64(), ret) | ||
| } | ||
| scope.Contract.Gas += returnGas | ||
| @@ -730,6 +732,7 @@ func opDelegateCall(pc *uint64, interpreter *EVMInterpreter, scope *ScopeContext | ||
| } | ||
| stack.push(&temp) | ||
| if err == nil || err == ErrExecutionReverted { | ||
| + ret = common.CopyBytes(ret) | ||
| scope.Memory.Set(retOffset.Uint64(), retSize.Uint64(), ret) | ||
| } | ||
| scope.Contract.Gas += returnGas | ||
| @@ -757,6 +760,7 @@ func opStaticCall(pc *uint64, interpreter *EVMInterpreter, scope *ScopeContext) | ||
| } | ||
| stack.push(&temp) | ||
| if err == nil || err == ErrExecutionReverted { | ||
| + ret = common.CopyBytes(ret) | ||
| scope.Memory.Set(retOffset.Uint64(), retSize.Uint64(), ret) | ||
| } | ||
| scope.Contract.Gas += returnGas | ||
| diff --git a/core/vm/interpreter.go b/core/vm/interpreter.go | ||
| index 9cf0c4e2c1..9fb83799c9 100644 | ||
| --- a/core/vm/interpreter.go | ||
| +++ b/core/vm/interpreter.go | ||
| @@ -262,7 +262,7 @@ func (in *EVMInterpreter) Run(contract *Contract, input []byte, readOnly bool) ( | ||
| // if the operation clears the return data (e.g. it has returning data) | ||
| // set the last return to the result of the operation. | ||
| if operation.returns { | ||
| - in.returnData = common.CopyBytes(res) | ||
| + in.returnData = res | ||
| } | ||
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| switch { | ||
| ``` | ||
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| ### Statetest to test for the issue | ||
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| ```json | ||
| { | ||
| "trigger-issue": { | ||
| "env": { | ||
| "currentCoinbase": "b94f5374fce5edbc8e2a8697c15331677e6ebf0b", | ||
| "currentDifficulty": "0x20000", | ||
| "currentGasLimit": "0x26e1f476fe1e22", | ||
| "currentNumber": "0x1", | ||
| "currentTimestamp": "0x3e8", | ||
| "previousHash": "0x0000000000000000000000000000000000000000000000000000000000000000" | ||
| }, | ||
| "pre": { | ||
| "0x00000000000000000000000000000000000000bb": { | ||
| "code": "0x6001600053600260015360036002536004600353600560045360066005536006600260066000600060047f7ef0367e633852132a0ebbf70eb714015dd44bc82e1e55a96ef1389c999c1bcaf13d600060003e596000208055", | ||
| "storage": {}, | ||
| "balance": "0x5", | ||
| "nonce": "0x0" | ||
| }, | ||
| "0xa94f5374fce5edbc8e2a8697c15331677e6ebf0b": { | ||
| "code": "0x", | ||
| "storage": {}, | ||
| "balance": "0xffffffff", | ||
| "nonce": "0x0" | ||
| } | ||
| }, | ||
| "transaction": { | ||
| "gasPrice": "0x1", | ||
| "nonce": "0x0", | ||
| "to": "0x00000000000000000000000000000000000000bb", | ||
| "data": [ | ||
| "0x" | ||
| ], | ||
| "gasLimit": [ | ||
| "0x7a1200" | ||
| ], | ||
| "value": [ | ||
| "0x01" | ||
| ], | ||
| "secretKey": "0x45a915e4d060149eb4365960e6a7a45f334393093061116b197e3240065ff2d8" | ||
| }, | ||
| "out": "0x", | ||
| "post": { | ||
| "Berlin": [ | ||
| { | ||
| "hash": "2a38a040bab1e1fa499253d98b2fd363e5756ecc52db47dd59af7116c068368c", | ||
| "logs": "1dcc4de8dec75d7aab85b567b6ccd41ad312451b948a7413f0a142fd40d49347", | ||
| "indexes": { | ||
| "data": 0, | ||
| "gas": 0, | ||
| "value": 0 | ||
| } | ||
| } | ||
| ] | ||
| } | ||
| } | ||
| } | ||
| ``` | ||
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