Re: [pbs-devel] [PATCH proxmox-backup v4 2/4] pbs-config: cache verified API token secrets

[Christian Ebner <c.ebner@proxmox.com>]

one suggestion below

On 1/21/26 4:13 PM, Samuel Rufinatscha wrote:
> Adds 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.
> 
> Signed-off-by: Samuel Rufinatscha <s.rufinatscha@proxmox.com>
> ---
> Changes from v3 to v4:
> * Add gen param to invalidate_cache_state()
> * Validates the generation bump after obtaining write lock in
> apply_api_mutation
> * Pass lock to apply_api_mutation
> * Remove unnecessary gen check cache_try_secret_matches
> * Adjusted commit message
> 
> 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.
> 
> 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).
> 
>   Cargo.toml                     |   1 +
>   pbs-config/Cargo.toml          |   1 +
>   pbs-config/src/token_shadow.rs | 160 ++++++++++++++++++++++++++++++++-
>   3 files changed, 159 insertions(+), 3 deletions(-)
> 
> diff --git a/Cargo.toml b/Cargo.toml
> index 0da18383..aed66fe3 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..d5aa5de2 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"),
>       }
>   }
> @@ -75,13 +107,15 @@ fn set_secret(tokenid: &Authid, secret: &str) -> Result<(), Error> {
>           bail!("not an API token ID");
>       }
>   
> -    let _guard = lock_config()?;
> +    let guard = lock_config()?;
>   
>       let mut data = read_file()?;
>       let hashed_secret = proxmox_sys::crypt::encrypt_pw(secret)?;
>       data.insert(tokenid.clone(), hashed_secret);
>       write_file(data)?;
>   
> +    apply_api_mutation(guard, tokenid, Some(secret));
> +
>       Ok(())
>   }
>   
> @@ -91,11 +125,131 @@ pub fn delete_secret(tokenid: &Authid) -> Result<(), Error> {
>           bail!("not an API token ID");
>       }
>   
> -    let _guard = lock_config()?;
> +    let guard = lock_config()?;
>   
>       let mut data = read_file()?;
>       data.remove(tokenid);
>       write_file(data)?;
>   
> +    apply_api_mutation(guard, 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_and_set_gen(&mut cache, 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.
> +///
> +/// Verifies the generation/version before doing the constant-time
> +/// comparison to reduce TOCTOU risk. During token rotation or deletion
> +/// tokens for in-flight requests may still validate against the previous
> +/// generation.
> +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 Some(current_gen) = token_shadow_shared_gen() else {
> +        return false;
> +    };
> +
> +    if current_gen == cache.shared_gen {
> +        return openssl::memcmp::eq(entry.secret.as_bytes(), secret.as_bytes());
> +    }
> +
> +    false
> +}
> +
> +fn apply_api_mutation(_guard: BackupLockGuard, tokenid: &Authid, new_secret: Option<&str>) {
> +    // Signal cache invalidation to other processes (best-effort).
> +    let bumped_gen = bump_token_shadow_shared_gen();
> +
> +    let mut cache = TOKEN_SECRET_CACHE.write();
> +
> +    // If we cannot get the current generation, we cannot trust the cache
> +    let Some(current_gen) = token_shadow_shared_gen() else {
> +        invalidate_cache_state_and_set_gen(&mut cache, 0);
> +        return;
> +    };
> +
> +    // If we cannot bump the shared generation, or if it changed after
> +    // obtaining the cache write lock, we cannot trust the cache
> +    if bumped_gen != Some(current_gen) {
> +        invalidate_cache_state_and_set_gen(&mut cache, current_gen);
> +        return;
> +    }
> +
> +    // Update to the post-mutation generation.
> +    cache.shared_gen = current_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 local cache contents and sets/updates the cached generation.
> +fn invalidate_cache_state_and_set_gen(cache: &mut ApiTokenSecretCache, gen: usize) {
> +    cache.secrets.clear();
> +    cache.shared_gen = gen;
> +}

above function operates on the chache, so why not make it a method 
thereof? And also bundle the generation bumps, so they might not be 
forgotten.

Something along the lines of the following diff on top of this patch:

diff --git a/pbs-config/src/token_shadow.rs b/pbs-config/src/token_shadow.rs
index d5aa5de28..a8104f142 100644
--- a/pbs-config/src/token_shadow.rs
+++ b/pbs-config/src/token_shadow.rs
@@ -136,6 +136,11 @@ pub fn delete_secret(tokenid: &Authid) -> 
Result<(), Error> {
      Ok(())
  }

+/// Cached secret.
+struct CachedSecret {
+    secret: String,
+}
+
  struct ApiTokenSecretCache {
      /// Keys are token Authids, values are the corresponding plain 
text secrets.
      /// Entries are added after a successful on-disk verification in
@@ -147,9 +152,22 @@ struct ApiTokenSecretCache {
      shared_gen: usize,
  }

-/// Cached secret.
-struct CachedSecret {
-    secret: String,
+impl ApiTokenSecretCache {
+    /// Invalidates local cache contents and sets/updates the cached 
generation.
+    fn invalidate_state_and_set_gen(&mut self, gen: usize) {
+        self.secrets.clear();
+        self.shared_gen = gen;
+    }
+
+    fn insert_and_set_gen(&mut self, tokenid: Authid, secret: 
CachedSecret, gen: usize) {
+        self.secrets.insert(tokenid.clone(), secret);
+        self.shared_gen = gen;
+    }
+
+    fn evict_and_set_gen(&mut self, tokenid: &Authid, gen: usize) {
+        self.secrets.remove(tokenid);
+        self.shared_gen = gen;
+    }
  }

  fn cache_try_insert_secret(tokenid: Authid, secret: String, 
shared_gen_before: usize) {
@@ -163,12 +181,12 @@ fn cache_try_insert_secret(tokenid: Authid, 
secret: String, shared_gen_before: u

      // If this process missed a generation bump, its cache is stale.
      if cache.shared_gen != shared_gen_now {
-        invalidate_cache_state_and_set_gen(&mut cache, shared_gen_now);
+        cache.invalidate_state_and_set_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 });
+        cache.insert_and_set_gen(tokenid, CachedSecret { secret }, 
shared_gen_now);
      }
  }

@@ -204,33 +222,24 @@ fn apply_api_mutation(_guard: BackupLockGuard, 
tokenid: &Authid, new_secret: Opt

      // If we cannot get the current generation, we cannot trust the cache
      let Some(current_gen) = token_shadow_shared_gen() else {
-        invalidate_cache_state_and_set_gen(&mut cache, 0);
+        cache.invalidate_state_and_set_gen(0);
          return;
      };

      // If we cannot bump the shared generation, or if it changed after
      // obtaining the cache write lock, we cannot trust the cache
      if bumped_gen != Some(current_gen) {
-        invalidate_cache_state_and_set_gen(&mut cache, current_gen);
+        cache.invalidate_state_and_set_gen(current_gen);
          return;
      }

-    // Update to the post-mutation generation.
-    cache.shared_gen = current_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);
+            let cached_secret = CachedSecret { secret: secret.to_owned() };
+            cache.insert_and_set_gen(tokenid.clone(), cached_secret, 
current_gen);
          }
+        None => cache.evict_and_set_gen(tokenid, current_gen),
      }
  }

@@ -248,8 +257,3 @@ fn bump_token_shadow_shared_gen() -> Option<usize> {
          .map(|cvc| cvc.increase_token_shadow_generation() + 1)
  }

-/// Invalidates local cache contents and sets/updates the cached 
generation.
-fn invalidate_cache_state_and_set_gen(cache: &mut ApiTokenSecretCache, 
gen: usize) {
-    cache.secrets.clear();
-    cache.shared_gen = gen;
-}