parser: fix possible attack vector #4297
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The force_eof rule used in the parser allowed for arbitrary statements to be appended to a DDL after a ';', which gets passed through to mysql without any validation. This basically means that someone could append a statement that bypasses the ACL checks in vttablets. This is not a big deal because people that are allowed to execute DDLs generally have all privileges. But it's better to play this safe. Also, the skip to end was looking for ';' characters instead of an actual ';' token, which could also lead to exploitation. I had to refactor the code a bit because we also ignore all errors on partial DDL parse. We have to make sure there are no new tokens past the ';' before accepting partial ddls. In the process, I've renamed 'force eof' -> 'skip to end'. Signed-off-by: Sugu Sougoumarane <ssougou@gmail.com>
| // we should not accept partially parsed DDLs. They | ||
| // should instead result in parser errors. See the | ||
| // Parse function to see how this is handled. | ||
| tkn.partialDDL = nil |
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What is this particular case trying to protect against? It's not immediately obvious to me why this is necessary since the check in Parse() seems like it would be sufficient.
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Without this reset, a statement like this would get quietly accepted by vitess: incomplete ddl; evil query.
This is the case where a ; tirggers a parser error. That will cause the Parse function to detect that it was a partial DDL. So, it will accept the entire string, including evil query. It's the second test case in TestSkipToEnd.
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I guess I'm confused about the specific sequence we expect to happen in that evil example above.
From my read of the code, it flows like this:
- The incomplete DDL sets
SkipToEnd - The next time through the lexer it should scan ahead to the
;character usingskipStatement, but no further - It then returns to
Parsewith the partialDDL flag set. - Parse then calls
Scanagain expecting only a 0 but finds more tokens fromevil queryand fails then.
Presumably I'm missing something but can you clarify where my logic is wrong?
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After some DM discussion in Slack I now understand.
| if tkn.lastChar != ';' { | ||
| tkn.skipStatement() | ||
| } | ||
| tkn.skipStatement() |
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Why did you remove the if tkn.lastChar != ';' guard here?
Would some input likeselect invalid; select ok from table; end up skipping the 2nd statement?
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This is because skipStatement has been refactored to take care of that same check.
The force_eof rule used in the parser allowed for arbitrary
statements to be appended to a DDL after a ';', which gets
passed through to mysql without any validation.
This basically means that someone could append a statement
that bypasses the ACL checks in vttablets. This is not a big
deal because people that are allowed to execute DDLs generally
have all privileges. But it's better to play this safe.
Also, the skip to end was looking for ';' characters instead
of an actual ';' token, which could also lead to exploitation.
I had to refactor the code a bit because we also ignore all
errors on partial DDL parse. We have to make sure there are
no new tokens past the ';' before accepting partial ddls.
In the process, I've renamed 'force eof' -> 'skip to end'.
Signed-off-by: Sugu Sougoumarane ssougou@gmail.com