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Add isolated PCRE match limits as a layer of ReDoS defense #2736

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merged 7 commits into from
May 9, 2023
Merged

Add isolated PCRE match limits as a layer of ReDoS defense #2736

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8c269d3
Update Regex util to support match limits
brandonpayton Mar 22, 2022
8c4b7c1
Fix some style issues in regex util header
brandonpayton Aug 15, 2022
0c42ee2
Switch to simpler PCRE error flags
brandonpayton Dec 2, 2022
f3d8198
Respond to code review feedback
brandonpayton Jan 19, 2023
23a0e26
Give PCRE error vars initial value
brandonpayton Apr 8, 2023
6f1bd27
Move var into conditional block where used
brandonpayton Apr 8, 2023
d875738
Add PCRE error tests for rx operator
brandonpayton Apr 11, 2023
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2 changes: 2 additions & 0 deletions headers/modsecurity/rules_set_properties.h
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Original file line number Diff line number Diff line change
Expand Up @@ -379,6 +379,7 @@ class RulesSetProperties {
from->m_responseBodyLimitAction,
PropertyNotSetBodyLimitAction);

to->m_pcreMatchLimit.merge(&from->m_pcreMatchLimit);
to->m_uploadFileLimit.merge(&from->m_uploadFileLimit);
to->m_uploadFileMode.merge(&from->m_uploadFileMode);
to->m_uploadDirectory.merge(&from->m_uploadDirectory);
Expand Down Expand Up @@ -470,6 +471,7 @@ class RulesSetProperties {
ConfigDouble m_requestBodyLimit;
ConfigDouble m_requestBodyNoFilesLimit;
ConfigDouble m_responseBodyLimit;
ConfigInt m_pcreMatchLimit;
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I'm ok with this size. 'int' should be big enough on any architecture that matters.

Note, however, the disparity between pcre1 and pcre2 (long vs. uint32_t) for this. This may another reason to avoid doing the get_default

ConfigInt m_uploadFileLimit;
ConfigInt m_uploadFileMode;
DebugLog *m_debugLog;
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4 changes: 4 additions & 0 deletions headers/modsecurity/transaction.h
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Expand Up @@ -134,6 +134,8 @@ class TransactionAnchoredVariables {
m_variableInboundDataError(t, "INBOUND_DATA_ERROR"),
m_variableMatchedVar(t, "MATCHED_VAR"),
m_variableMatchedVarName(t, "MATCHED_VAR_NAME"),
m_variableMscPcreError(t, "MSC_PCRE_ERROR"),
m_variableMscPcreLimitsExceeded(t, "MSC_PCRE_LIMITS_EXCEEDED"),
m_variableMultipartBoundaryQuoted(t, "MULTIPART_BOUNDARY_QUOTED"),
m_variableMultipartBoundaryWhiteSpace(t,
"MULTIPART_BOUNDARY_WHITESPACE"),
Expand Down Expand Up @@ -219,6 +221,8 @@ class TransactionAnchoredVariables {
AnchoredVariable m_variableInboundDataError;
AnchoredVariable m_variableMatchedVar;
AnchoredVariable m_variableMatchedVarName;
AnchoredVariable m_variableMscPcreError;
AnchoredVariable m_variableMscPcreLimitsExceeded;
AnchoredVariable m_variableMultipartBoundaryQuoted;
AnchoredVariable m_variableMultipartBoundaryWhiteSpace;
AnchoredVariable m_variableMultipartCrlfLFLines;
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28 changes: 26 additions & 2 deletions src/operators/rx.cc
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Expand Up @@ -51,12 +51,36 @@ bool Rx::evaluate(Transaction *transaction, RuleWithActions *rule,
re = m_re;
}

std::vector<Utils::SMatchCapture> captures;
if (re->hasError()) {
ms_dbg_a(transaction, 3, "Error with regular expression: \"" + re->pattern + "\"");
return false;
}
re->searchOneMatch(input, captures);

Utils::RegexResult regex_result;
std::vector<Utils::SMatchCapture> captures;

if (transaction && transaction->m_rules->m_pcreMatchLimit.m_set) {
unsigned long match_limit = transaction->m_rules->m_pcreMatchLimit.m_value;
regex_result = re->searchOneMatch(input, captures, match_limit);
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If we abandon the get_default functionality, I suspect there wouldn't be a compelling reason to overload this function (or the 'Global' equivalent). A single function could either have the second argument explicitly passed something like 0 or -1 (or the second arg could be declared with a default of one of those).

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I went ahead and added a default value for the match_limit param in the declarations in src/utils/regex.h and left this logic the same. My thinking is that, this way, only regex.cc needs to worry about the default value.

} else {
regex_result = re->searchOneMatch(input, captures);
}

// FIXME: DRY regex error reporting. This logic is currently duplicated in other operators.
if (regex_result != Utils::RegexResult::Ok) {
transaction->m_variableMscPcreError.set("1", transaction->m_variableOffset);
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The error variable gets set to '1' when an error is encountered, but there is no setting to '0' otherwise -- it's just not set to anything.

ModSecurity error variables should have testable value for 'no error' and this is conventionally '0'. Often this the variable is set to either 0 or 1 upon completion of the single task per transaction (such as REQBODY_ERROR). You won't be able to do that here. The best analog is URL_ENCODED_ERROR, which is initialized to 0 in the Transaction constructors.

Same thing with m_variableMscPcreLimitsExceeded

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Thanks for pointing this out. It's definitely good to explicitly set it to a sensible initial value. Done.


std::string regex_error_str = "OTHER";
if (regex_result == Utils::RegexResult::ErrorMatchLimit) {
regex_error_str = "MATCH_LIMIT";
transaction->m_variableMscPcreLimitsExceeded.set("1", transaction->m_variableOffset);
}

ms_dbg_a(transaction, 1, "rx: regex error '" + regex_error_str + "' for pattern '" + re->pattern + "'");


return false;
}

if (rule && rule->hasCaptureAction() && transaction) {
for (const Utils::SMatchCapture& capture : captures) {
Expand Down
24 changes: 23 additions & 1 deletion src/operators/rx_global.cc
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Expand Up @@ -51,8 +51,30 @@ bool RxGlobal::evaluate(Transaction *transaction, RuleWithActions *rule,
re = m_re;
}

Utils::RegexResult regex_result;
std::vector<Utils::SMatchCapture> captures;
re->searchGlobal(input, captures);
if (transaction && transaction->m_rules->m_pcreMatchLimit.m_set) {
unsigned long match_limit = transaction->m_rules->m_pcreMatchLimit.m_value;
regex_result = re->searchGlobal(input, captures, match_limit);
} else {
regex_result = re->searchGlobal(input, captures);
}

// FIXME: DRY regex error reporting. This logic is currently duplicated in other operators.
if (regex_result != Utils::RegexResult::Ok) {
transaction->m_variableMscPcreError.set("1", transaction->m_variableOffset);

std::string regex_error_str = "OTHER";
if (regex_result == Utils::RegexResult::ErrorMatchLimit) {
regex_error_str = "MATCH_LIMIT";
transaction->m_variableMscPcreLimitsExceeded.set("1", transaction->m_variableOffset);
}

ms_dbg_a(transaction, 1, "rxGlobal: regex error '" + regex_error_str + "' for pattern '" + re->pattern + "'");

return false;
}


if (rule && rule->hasCaptureAction() && transaction) {
for (const Utils::SMatchCapture& capture : captures) {
Expand Down
2 changes: 1 addition & 1 deletion src/parser/location.hh
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@@ -1,4 +1,4 @@
// A Bison parser, made by GNU Bison 3.7.6.
// A Bison parser, made by GNU Bison 3.8.2.

// Locations for Bison parsers in C++

Expand Down
2 changes: 1 addition & 1 deletion src/parser/position.hh
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@@ -1,4 +1,4 @@
// A Bison parser, made by GNU Bison 3.7.6.
// A Bison parser, made by GNU Bison 3.8.2.

// Starting with Bison 3.2, this file is useless: the structure it
// used to define is now defined in "location.hh".
Expand Down