Re: [pbs-devel] [PATCH proxmox-backup v3 2/4] pbs-config: cache verified API token secrets

[Samuel Rufinatscha <s.rufinatscha@proxmox.com>]

On 1/14/26 11:44 AM, Fabian Grünbichler wrote:
> On January 2, 2026 5:07 pm, Samuel Rufinatscha wrote:
>> Currently, every token-based API request reads the token.shadow file and
>> runs the expensive password hash verification for the given token
>> secret. This shows up as a hotspot in /status profiling (see
>> bug #7017 [1]).
>>
>> This patch introduces an in-memory cache of successfully verified token
>> secrets. Subsequent requests for the same token+secret combination only
>> perform a comparison using openssl::memcmp::eq and avoid re-running the
>> password hash. The cache is updated when a token secret is set and
>> cleared when a token is deleted. Note, this does NOT include manual
>> config changes, which will be covered in a subsequent patch.
>>
>> This patch is part of the series which fixes bug #7017 [1].
>>
>> [1] https://bugzilla.proxmox.com/show_bug.cgi?id=7017
>>
>> Signed-off-by: Samuel Rufinatscha <s.rufinatscha@proxmox.com>
>> ---
>> Changes from v1 to v2:
>>
>> * Replace OnceCell with LazyLock, and std::sync::RwLock with
>> parking_lot::RwLock.
>> * Add API_MUTATION_GENERATION and guard cache inserts
>> to prevent “zombie inserts” across concurrent set/delete.
>> * Refactor cache operations into cache_try_secret_matches,
>> cache_try_insert_secret, and centralize write-side behavior in
>> apply_api_mutation.
>> * Switch fast-path cache access to try_read/try_write (best-effort).
>>
>> Changes from v2 to v3:
>>
>> * Replaced process-local cache invalidation (AtomicU64
>> API_MUTATION_GENERATION) with a cross-process shared generation via
>> ConfigVersionCache.
>> * Validate shared generation before/after the constant-time secret
>> compare; only insert into cache if the generation is unchanged.
>> * invalidate_cache_state() on insert if shared generation changed.
>>
>>   Cargo.toml                     |   1 +
>>   pbs-config/Cargo.toml          |   1 +
>>   pbs-config/src/token_shadow.rs | 157 ++++++++++++++++++++++++++++++++-
>>   3 files changed, 158 insertions(+), 1 deletion(-)
>>
>> diff --git a/Cargo.toml b/Cargo.toml
>> index 1aa57ae5..821b63b7 100644
>> --- a/Cargo.toml
>> +++ b/Cargo.toml
>> @@ -143,6 +143,7 @@ nom = "7"
>>   num-traits = "0.2"
>>   once_cell = "1.3.1"
>>   openssl = "0.10.40"
>> +parking_lot = "0.12"
>>   percent-encoding = "2.1"
>>   pin-project-lite = "0.2"
>>   regex = "1.5.5"
>> diff --git a/pbs-config/Cargo.toml b/pbs-config/Cargo.toml
>> index 74afb3c6..eb81ce00 100644
>> --- a/pbs-config/Cargo.toml
>> +++ b/pbs-config/Cargo.toml
>> @@ -13,6 +13,7 @@ libc.workspace = true
>>   nix.workspace = true
>>   once_cell.workspace = true
>>   openssl.workspace = true
>> +parking_lot.workspace = true
>>   regex.workspace = true
>>   serde.workspace = true
>>   serde_json.workspace = true
>> diff --git a/pbs-config/src/token_shadow.rs b/pbs-config/src/token_shadow.rs
>> index 640fabbf..fa84aee5 100644
>> --- a/pbs-config/src/token_shadow.rs
>> +++ b/pbs-config/src/token_shadow.rs
>> @@ -1,6 +1,8 @@
>>   use std::collections::HashMap;
>> +use std::sync::LazyLock;
>>   
>>   use anyhow::{bail, format_err, Error};
>> +use parking_lot::RwLock;
>>   use serde::{Deserialize, Serialize};
>>   use serde_json::{from_value, Value};
>>   
>> @@ -13,6 +15,18 @@ use crate::{open_backup_lockfile, BackupLockGuard};
>>   const LOCK_FILE: &str = pbs_buildcfg::configdir!("/token.shadow.lock");
>>   const CONF_FILE: &str = pbs_buildcfg::configdir!("/token.shadow");
>>   
>> +/// Global in-memory cache for successfully verified API token secrets.
>> +/// The cache stores plain text secrets for token Authids that have already been
>> +/// verified against the hashed values in `token.shadow`. This allows for cheap
>> +/// subsequent authentications for the same token+secret combination, avoiding
>> +/// recomputing the password hash on every request.
>> +static TOKEN_SECRET_CACHE: LazyLock<RwLock<ApiTokenSecretCache>> = LazyLock::new(|| {
>> +    RwLock::new(ApiTokenSecretCache {
>> +        secrets: HashMap::new(),
>> +        shared_gen: 0,
>> +    })
>> +});
>> +
>>   #[derive(Serialize, Deserialize)]
>>   #[serde(rename_all = "kebab-case")]
>>   /// ApiToken id / secret pair
>> @@ -54,9 +68,27 @@ pub fn verify_secret(tokenid: &Authid, secret: &str) -> Result<(), Error> {
>>           bail!("not an API token ID");
>>       }
>>   
>> +    // Fast path
>> +    if cache_try_secret_matches(tokenid, secret) {
>> +        return Ok(());
>> +    }
>> +
>> +    // Slow path
>> +    // First, capture the shared generation before doing the hash verification.
>> +    let gen_before = token_shadow_shared_gen();
>> +
>>       let data = read_file()?;
>>       match data.get(tokenid) {
>> -        Some(hashed_secret) => proxmox_sys::crypt::verify_crypt_pw(secret, hashed_secret),
>> +        Some(hashed_secret) => {
>> +            proxmox_sys::crypt::verify_crypt_pw(secret, hashed_secret)?;
>> +
>> +            // Try to cache only if nothing changed while verifying the secret.
>> +            if let Some(gen) = gen_before {
>> +                cache_try_insert_secret(tokenid.clone(), secret.to_owned(), gen);
>> +            }
>> +
>> +            Ok(())
>> +        }
>>           None => bail!("invalid API token"),
>>       }
>>   }
>> @@ -82,6 +114,8 @@ fn set_secret(tokenid: &Authid, secret: &str) -> Result<(), Error> {
>>       data.insert(tokenid.clone(), hashed_secret);
>>       write_file(data)?;
>>   
>> +    apply_api_mutation(tokenid, Some(secret));
>> +
>>       Ok(())
>>   }
>>   
>> @@ -97,5 +131,126 @@ pub fn delete_secret(tokenid: &Authid) -> Result<(), Error> {
>>       data.remove(tokenid);
>>       write_file(data)?;
>>   
>> +    apply_api_mutation(tokenid, None);
>> +
>>       Ok(())
>>   }
>> +
>> +struct ApiTokenSecretCache {
>> +    /// Keys are token Authids, values are the corresponding plain text secrets.
>> +    /// Entries are added after a successful on-disk verification in
>> +    /// `verify_secret` or when a new token secret is generated by
>> +    /// `generate_and_set_secret`. Used to avoid repeated
>> +    /// password-hash computation on subsequent authentications.
>> +    secrets: HashMap<Authid, CachedSecret>,
>> +    /// Shared generation to detect mutations of the underlying token.shadow file.
>> +    shared_gen: usize,
>> +}
>> +
>> +/// Cached secret.
>> +struct CachedSecret {
>> +    secret: String,
>> +}
>> +
>> +fn cache_try_insert_secret(tokenid: Authid, secret: String, shared_gen_before: usize) {
>> +    let Some(mut cache) = TOKEN_SECRET_CACHE.try_write() else {
>> +        return;
>> +    };
>> +
>> +    let Some(shared_gen_now) = token_shadow_shared_gen() else {
>> +        return;
>> +    };
>> +
>> +    // If this process missed a generation bump, its cache is stale.
>> +    if cache.shared_gen != shared_gen_now {
>> +        invalidate_cache_state(&mut cache);
>> +        cache.shared_gen = shared_gen_now;
>> +    }
>> +
>> +    // If a mutation happened while we were verifying the secret, do not insert.
>> +    if shared_gen_now == shared_gen_before {
>> +        cache.secrets.insert(tokenid, CachedSecret { secret });
>> +    }
>> +}
>> +
>> +// Tries to match the given token secret against the cached secret.
>> +// Checks the generation before and after the constant-time compare to avoid a
>> +// TOCTOU window. If another process rotates/deletes a token while we're validating
>> +// the cached secret, the generation will change, and we
>> +// must not trust the cache for this request.
>> +fn cache_try_secret_matches(tokenid: &Authid, secret: &str) -> bool {
>> +    let Some(cache) = TOKEN_SECRET_CACHE.try_read() else {
>> +        return false;
>> +    };
>> +    let Some(entry) = cache.secrets.get(tokenid) else {
>> +        return false;
>> +    };
>> +
>> +    let cache_gen = cache.shared_gen;
>> +
>> +    let Some(gen1) = token_shadow_shared_gen() else {
>> +        return false;
>> +    };
>> +    if gen1 != cache_gen {
>> +        return false;
>> +    }
>> +
>> +    let eq = openssl::memcmp::eq(entry.secret.as_bytes(), secret.as_bytes());
> 
> should we invalidate the cache here for this particular authid in case
> of a mismatch, to avoid making brute forcing too easy/cheap?
>

We are not doing a cheap reject, in mismatch we do still fall through to
verify_crypt_pw(). Evicting on mismatch could however enable cache
thrashing where wrong secrets for a known tokenid would evict cached
entries. So I think we should not invalidate here on mismatch.

>> +    let Some(gen2) = token_shadow_shared_gen() else {
>> +        return false;
>> +    };
>> +
>> +    eq && gen2 == cache_gen
>> +}
>> +
>> +fn apply_api_mutation(tokenid: &Authid, new_secret: Option<&str>) {
>> +    // Signal cache invalidation to other processes (best-effort).
>> +    let new_shared_gen = bump_token_shadow_shared_gen();
>> +
>> +    let mut cache = TOKEN_SECRET_CACHE.write();
>> +
>> +    // If we cannot read/bump the shared generation, we cannot safely trust the cache.
>> +    let Some(gen) = new_shared_gen else {
>> +        invalidate_cache_state(&mut cache);
>> +        cache.shared_gen = 0;
>> +        return;
>> +    };
>> +
>> +    // Update to the post-mutation generation.
>> +    cache.shared_gen = gen;
>> +
>> +    // Apply the new mutation.
>> +    match new_secret {
>> +        Some(secret) => {
>> +            cache.secrets.insert(
>> +                tokenid.clone(),
>> +                CachedSecret {
>> +                    secret: secret.to_owned(),
>> +                },
>> +            );
>> +        }
>> +        None => {
>> +            cache.secrets.remove(tokenid);
>> +        }
>> +    }
>> +}
>> +
>> +/// Get the current shared generation.
>> +fn token_shadow_shared_gen() -> Option<usize> {
>> +    crate::ConfigVersionCache::new()
>> +        .ok()
>> +        .map(|cvc| cvc.token_shadow_generation())
>> +}
>> +
>> +/// Bump and return the new shared generation.
>> +fn bump_token_shadow_shared_gen() -> Option<usize> {
>> +    crate::ConfigVersionCache::new()
>> +        .ok()
>> +        .map(|cvc| cvc.increase_token_shadow_generation() + 1)
>> +}
>> +
>> +/// Invalidates the cache state and only keeps the shared generation.
> 
> both calls to this actually set the cached generation to some value
> right after, so maybe this should take a generation directly and set it?
>

patch 3/4 doesn’t always update the gen on cache invalidation
(shadow_mtime_len() error branch in apply_api_mutation) but most other
call sites do. Agreed this can be refactored, maybe:

fn invalidate_cache_state(cache: &mut ApiTokenSecretCache) {
     cache.secrets.clear();
     // clear other cache fields (mtime/len/last_checked) as needed
}

fn invalidate_cache_state_and_set_gen(cache: &mut ApiTokenSecretCache, 
gen: usize) {
     invalidate_cache_state(cache);
     cache.shared_gen = gen;
}

We could also do a single helper with Option<usize> but two helpers make 
the call sites more explicit.

>> +fn invalidate_cache_state(cache: &mut ApiTokenSecretCache) {
>> +    cache.secrets.clear();
>> +}
>> -- 
>> 2.47.3
>>
>>
>>
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> 
> 
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