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

["Fabian Grünbichler" <f.gruenbichler@proxmox.com>]

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?

> +    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?

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