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fix race condition in string generator helper #19875

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merged 2 commits into from
Mar 31, 2023
Merged

fix race condition in string generator helper #19875

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fa1ce70
fix race condition in string generator helper
fairclothjm Mar 30, 2023
33b6df3
add changelog
fairclothjm Mar 31, 2023
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11 changes: 9 additions & 2 deletions helper/random/string_generator.go
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Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,7 @@ import (
"io"
"math"
"sort"
"sync"
"time"
"unicode"

Expand Down Expand Up @@ -79,7 +80,8 @@ type StringGenerator struct {
Rules serializableRules `mapstructure:"-" json:"rule"` // This is "rule" in JSON so it matches the HCL property type

// CharsetRule to choose runes from. This is computed from the rules, not directly configurable
charset runes
charset runes
charsetLock sync.RWMutex
}

// Generate a random string from the charset and adhering to the provided rules.
Expand Down Expand Up @@ -119,7 +121,10 @@ func (g *StringGenerator) generate(rng io.Reader) (str string, err error) {
// If performance improvements need to be made, this can be changed to read a batch of
// potential strings at once rather than one at a time. This will significantly
// improve performance, but at the cost of added complexity.
candidate, err := randomRunes(rng, g.charset, g.Length)
g.charsetLock.RLock()
charset := g.charset
g.charsetLock.RUnlock()
Comment on lines +124 to +126
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Can you explain why locking is needed for setting a variable like this?

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The lock isn't necessary for the write to the local charset variable, but the read of the g.charset value.

In validateConfig: 
if len(g.charset) == 0 {
    g.charset = getChars(g.Rules)
}

so if there's parallel access to both validateConfig and generate, we will race reading this variable. Since this assignment isn't an atomic write (its a []rune slice, which is a pointer type), you might read a bad pointer (like, half of the pointer's value could be copied over when you read, so you'd point to a garbage memory location) and thus fail.

In particular, I think when you first initialize the group and set it up to do a password rotation, if you have a lot of in-flight requests, you'll see multiple try and write this charset via validating the config I believe, and some which will be past it trying to do this generate read already.

Or maybe it is parallel modification + use of the policy...

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Thanks @cipherboy! That is a great summary.

@robmonte I am happy to talk more about this if you have more questions. Let me know!

candidate, err := randomRunes(rng, charset, g.Length)
if err != nil {
return "", fmt.Errorf("unable to generate random characters: %w", err)
}
Expand Down Expand Up @@ -232,6 +237,8 @@ func (g *StringGenerator) validateConfig() (err error) {
merr = multierror.Append(merr, fmt.Errorf("specified rules require at least %d characters but %d is specified", minLen, g.Length))
}

g.charsetLock.Lock()
defer g.charsetLock.Unlock()
// Ensure we have a charset & all characters are printable
if len(g.charset) == 0 {
// Yes this is mutating the generator but this is done so we don't have to compute this on every generation
Expand Down