Undefined behavior in CK_ATTRIBUTE::try_from or Session::get_attributes

[nwalfield]

While testing sequoia-cryptoki using SoftHSM2, I encountered a panic:

unsafe precondition(s) violated: slice::from_raw_parts requires the pointer to be aligned and non-null, and the total size of the slice not to exceed `isize::MAX`

The error message suggests that there is undefined behavior. Examining the backtrace, CK_ATTRIBUTE::try_from, was executing which is called from Session::get_attributes, which is immediately preceded by an unsafe block:

        // Allocating a chunk of memory where to put the attributes value.
        let attrs_memory: Vec<(AttributeType, Vec<u8>)> = attrs_info
            .iter()
            .zip(attributes.iter())
            .filter_map(|(attr_info, attr_type)| {
                if let AttributeInfo::Available(size) = attr_info {
                    Some((*attr_type, vec![0; *size]))
                } else {
                    None
                }
            })
            .collect();

        let mut template: Vec<CK_ATTRIBUTE> = attrs_memory
            .iter()
            .map(|(attr_type, memory)| {
                Ok(CK_ATTRIBUTE {
                    type_: (*attr_type).into(),
                    pValue: memory.as_ptr() as *mut std::ffi::c_void,
                    ulValueLen: memory.len().try_into()?,
                })
            })
            .collect::<Result<Vec<CK_ATTRIBUTE>>>()?;

        // This should only return OK as all attributes asked should be
        // available. Concurrency problem?
        unsafe {
            Rv::from(get_pkcs11!(self.client(), C_GetAttributeValue)(
                self.handle(),
                object.handle(),
                template.as_mut_ptr(),
                template.len().try_into()?,
            ))
            .into_result(Function::GetAttributeValue)?;
        }

        // Convert from CK_ATTRIBUTE to Attribute
        template.into_iter().map(|attr| attr.try_into()).collect()

In my code, the call to Session::get_attributes is here.

Instrumenting my code, I see the error occurs while fetching the AllowedMechanisms attribute whose attribute info was Available(0).

The following shows how to reproduce the crash. Note that sequoia-cryptoki uses rust-cryptoki's HEAD, which is currently 2702bba.

I'm compiling with rustc 1.85.1 and the issue is only triggered is debug mode; compiling with --release doesn't trigger the issue. I'm on Debian Stable and am using softhsm2 2.6.1-3.

$ git clone https://gitlab.com/sequoia-pgp/sequoia-cryptoki.git
Cloning into 'sequoia-cryptoki'...
remote: Enumerating objects: 1313, done.
remote: Counting objects: 100% (88/88), done.
remote: Compressing objects: 100% (16/16), done.
remote: Total 1313 (delta 84), reused 72 (delta 72), pack-reused 1225 (from 1)
Receiving objects: 100% (1313/1313), 400.48 KiB | 2.00 MiB/s, done.
Resolving deltas: 100% (834/834), done.
us@alice:/tmp/cryptoki (0)$ git checkout 537f0f84
fatal: not a git repository (or any parent up to mount point /)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
$ cd sequoia-cryptoki/
$ git checkout 537f0f84
Note: switching to '537f0f84'.
...
HEAD is now at 537f0f8 tools: Improve PIN long help
$ cargo build --features sequoia-openpgp/crypto-openssl --no-default-features
...
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 9.59s
$ rm -rf /tmp/softhsm/
config.file  tokens/      
$ cat /tmp/softhsm/config.file 
directories.tokendir = /tmp/softhsm/tokens/
objectstore.backend = file

# ERROR, WARNING, INFO, DEBUG
log.level = ERROR

# If CKF_REMOVABLE_DEVICE flag should be set
slots.removable = false
$ rm -rf /tmp/softhsm/tokens/
$ mkdir -p /tmp/softhsm/tokens/
$ SOFTHSM2_CONF=/tmp/softhsm/config.file target/debug/sq-cryptoki --module /usr/lib/softhsm/libsofthsm2.so token init --new-token-label foo
Token not yet initialized.  Performing initial initialization.
Please enter a PIN to protect the SO PIN: 
                   Please repeat the PIN: 
Please enter a PIN to protect the user PIN: 
                     Please repeat the PIN: 
$ SOFTHSM2_CONF=/tmp/softhsm/config.file target/debug/sq-cryptoki --module /usr/lib/softhsm/libsofthsm2.so key generate --token-label foo --random-key-label --can-certify --new-cert
Please enter the PIN to unlock the user PIN: 
Warning: /usr/lib/softhsm/libsofthsm2.so's slot 1 is unusable: Function::OpenSession: PKCS11 error: The Cryptoki library and/or slot does not recognize the token in the slot.
Warning: /usr/lib/softhsm/libsofthsm2.so's slot 1 is unusable: Function::OpenSession: PKCS11 error: The Cryptoki library and/or slot does not recognize the token in the slot.
-----BEGIN PGP PUBLIC KEY BLOCK-----
Cryptoki-ID: pgp:v4:eddsa:20251118T090408Z:scrap-heat-mouse
Cryptoki-Label: scrap-heat-mouse

xjMEaRw2iBYJKwYBBAHaRw8BAQdAtMqd22erF066DFOOdGKtVLdKn3otDCmie07t
1Dbfwo/CwAsEHxYKAH0FgmkcNogDCwkHCRALbiC0oZbPHEcUAAAAAAAeACBzYWx0
QG5vdGF0aW9ucy5zZXF1b2lhLXBncC5vcmfGYoMI2Cjo3UWb8//gUtRp2Kv0mL/5
ZDgn/9wzrOKUYwMVCggCmwECHgkWIQSOCXstP5hJzIyg5UgLbiC0oZbPHAAAozgB
APBx80X6nhUPl4bvNLq0UP9W0iH+GLJXYmnhQVzmvWIwAP0fiwPV1RyxCHi32oF1
w9qGFmuV4y7drOTLIK9Id8w3DA==
=fM3c
-----END PGP PUBLIC KEY BLOCK-----
$ RUST_BACKTRACE=full SOFTHSM2_CONF=/tmp/softhsm/config.file target/debug/sq-cryptoki --module /usr/lib/softhsm/libsofthsm2.so token list --key-label scrap-heat-mouse

thread 'main' panicked at library/core/src/panicking.rs:218:5:
unsafe precondition(s) violated: slice::from_raw_parts requires the pointer to be aligned and non-null, and the total size of the slice not to exceed `isize::MAX`
stack backtrace:
   0:     0x56250730a73a - std::backtrace_rs::backtrace::libunwind::trace::h2740d05102fd9881
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/../../backtrace/src/backtrace/libunwind.rs:116:5
   1:     0x56250730a73a - std::backtrace_rs::backtrace::trace_unsynchronized::h93ae2edf130065c8
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x56250730a73a - std::sys::backtrace::_print_fmt::h7660a544e6110dd9
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/sys/backtrace.rs:66:9
   3:     0x56250730a73a - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::hfc616348d9ad0abc
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/sys/backtrace.rs:39:26
   4:     0x56250732e073 - core::fmt::rt::Argument::fmt::h3a92517f8117fcd5
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/core/src/fmt/rt.rs:177:76
   5:     0x56250732e073 - core::fmt::write::h7ca648217bc79799
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/core/src/fmt/mod.rs:1440:21
   6:     0x562507307623 - std::io::Write::write_fmt::h7960c58bfa5ccbcb
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/io/mod.rs:1887:15
   7:     0x56250730a582 - std::sys::backtrace::BacktraceLock::print::h3fb349e80cbe0423
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/sys/backtrace.rs:42:9
   8:     0x56250730b5f0 - std::panicking::default_hook::{{closure}}::h3366e5842cba645d
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/panicking.rs:295:22
   9:     0x56250730b3d0 - std::panicking::default_hook::hd7573a5d4879884b
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/panicking.rs:322:9
  10:     0x56250730bd52 - std::panicking::rust_panic_with_hook::h66e909d048c263a9
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/panicking.rs:828:13
  11:     0x56250730bac6 - std::panicking::begin_panic_handler::{{closure}}::h8d9aa8be7e8634cf
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/panicking.rs:694:13
  12:     0x56250730ac49 - std::sys::backtrace::__rust_end_short_backtrace::h7d7e47ef99abf6aa
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/sys/backtrace.rs:168:18
  13:     0x56250730b78d - rust_begin_unwind
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/panicking.rs:692:5
  14:     0x562506a44f4d - core::panicking::panic_nounwind_fmt::runtime::h19438eeae7eaf031
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/core/src/panicking.rs:117:22
  15:     0x562506a44f4d - core::panicking::panic_nounwind_fmt::h57347130f21a7343
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/core/src/intrinsics/mod.rs:3869:9
  16:     0x562506a44fe2 - core::panicking::panic_nounwind::h2f7749cb358aa979
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/core/src/panicking.rs:218:5
  17:     0x562506d7f2c3 - core::slice::raw::from_raw_parts::precondition_check::h3c75f0263862b151
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/core/src/ub_checks.rs:68:21
  18:     0x562506d7f08f - core::slice::raw::from_raw_parts::h1fcb92e9a7016678
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/core/src/ub_checks.rs:75:17
  19:     0x562506d5f046 - <cryptoki::object::Attribute as core::convert::TryFrom<cryptoki_sys::CK_ATTRIBUTE>>::try_from::hdc4d38e483f52cfb
                               at /home/us/.cargo/git/checkouts/rust-cryptoki-d5dc7807e7b467b4/e702367/cryptoki/src/object.rs:955:21
  20:     0x562506d6c97e - <T as core::convert::TryInto<U>>::try_into::hecd42c0a58963277
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/core/src/convert/mod.rs:800:9
  21:     0x562506d57aa5 - cryptoki::session::object_management::<impl cryptoki::session::Session>::get_attributes::{{closure}}::hf48fdd4ad86c40d7
                               at /home/us/.cargo/git/checkouts/rust-cryptoki-d5dc7807e7b467b4/e702367/cryptoki/src/session/object_management.rs:547:41
  22:     0x562506d741b3 - core::iter::adapters::map::map_try_fold::{{closure}}::h30e3e631e9395c74
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/core/src/iter/adapters/map.rs:95:28
  23:     0x562506d51660 - <alloc::vec::into_iter::IntoIter<T,A> as core::iter::traits::iterator::Iterator>::try_fold::h88e818c6261d99a6
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/alloc/src/vec/into_iter.rs:346:25
  24:     0x562506d73a90 - <core::iter::adapters::map::Map<I,F> as core::iter::traits::iterator::Iterator>::try_fold::h78826b022f8e313c
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/core/src/iter/adapters/map.rs:121:9
  25:     0x562506d6dc6c - <core::iter::adapters::GenericShunt<I,R> as core::iter::traits::iterator::Iterator>::try_fold::h18f91d3c85a7b99d
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/core/src/iter/adapters/mod.rs:191:9
  26:     0x562506d6dace - core::iter::traits::iterator::Iterator::try_for_each::h780c17ea7f5530e3
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/core/src/iter/traits/iterator.rs:2431:9
  27:     0x562506d6dace - <core::iter::adapters::GenericShunt<I,R> as core::iter::traits::iterator::Iterator>::next::h66764564edac8663
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/core/src/iter/adapters/mod.rs:174:14
  28:     0x562506d48451 - <alloc::vec::Vec<T> as alloc::vec::spec_from_iter_nested::SpecFromIterNested<T,I>>::from_iter::haf3e6fb6f2c0758e
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/alloc/src/vec/spec_from_iter_nested.rs:25:32
  29:     0x562506d78905 - <alloc::vec::Vec<T> as alloc::vec::spec_from_iter_nested::SpecFromIterNested<T,I>>::from_iter{{reify.shim}}::h7a12c5bce59ab962
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/alloc/src/vec/spec_from_iter_nested.rs:19:5
  30:     0x562506d4b2be - alloc::vec::in_place_collect::<impl alloc::vec::spec_from_iter::SpecFromIter<T,I> for alloc::vec::Vec<T>>::from_iter::h193cc45ef1110893
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/alloc/src/vec/in_place_collect.rs:246:9
  31:     0x562506d4b9f3 - <alloc::vec::Vec<T> as core::iter::traits::collect::FromIterator<T>>::from_iter::h125c9b82fab0133d
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/alloc/src/vec/mod.rs:3427:9
  32:     0x562506d417a9 - core::iter::traits::iterator::Iterator::collect::h3bebcb28a8355402
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/core/src/iter/traits/iterator.rs:1971:9
  33:     0x562506d417a9 - <core::result::Result<V,E> as core::iter::traits::collect::FromIterator<core::result::Result<A,E>>>::from_iter::{{closure}}::ha72dfe59c9d09579
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/core/src/result.rs:1985:51
  34:     0x562506d6efba - core::iter::adapters::try_process::h2e61e5506f36e223
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/core/src/iter/adapters/mod.rs:160:17
  35:     0x562506d41649 - <core::result::Result<V,E> as core::iter::traits::collect::FromIterator<core::result::Result<A,E>>>::from_iter::h90fa2de9d45a8d71
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/core/src/result.rs:1985:9
  36:     0x562506d73dee - core::iter::traits::iterator::Iterator::collect::h1fc70cf577cb49ac
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/core/src/iter/traits/iterator.rs:1971:9
  37:     0x562506d4fe18 - cryptoki::session::object_management::<impl cryptoki::session::Session>::get_attributes::ha0e17c654666027c
                               at /home/us/.cargo/git/checkouts/rust-cryptoki-d5dc7807e7b467b4/e702367/cryptoki/src/session/object_management.rs:547:9
  38:     0x562506ae2249 - sq_cryptoki::commands::token::list::list::h35abe4f2c0812a87
                               at /tmp/cryptoki/sequoia-cryptoki/tools/src/commands/token/list.rs:685:45
  39:     0x562506a82273 - sq_cryptoki::commands::token::dispatch::hcce07600e7141a7b
                               at /tmp/cryptoki/sequoia-cryptoki/tools/src/commands/token.rs:16:20
  40:     0x562506a780ef - sq_cryptoki::commands::dispatch::h464f15db3271631c
                               at /tmp/cryptoki/sequoia-cryptoki/tools/src/commands.rs:21:13
  41:     0x562506af2b44 - sq_cryptoki::real_main::h144368cd99acf4cf
                               at /tmp/cryptoki/sequoia-cryptoki/tools/src/main.rs:169:11
  42:     0x562506af1b69 - sq_cryptoki::main::ha4fbf0c7d75042b3
                               at /tmp/cryptoki/sequoia-cryptoki/tools/src/main.rs:25:21
  43:     0x562506a6890b - core::ops::function::FnOnce::call_once::h10abfd2b003a13ff
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/core/src/ops/function.rs:250:5
  44:     0x562506acd61e - std::sys::backtrace::__rust_begin_short_backtrace::h2244fbeab20830e4
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/sys/backtrace.rs:152:18
  45:     0x562506af6191 - std::rt::lang_start::{{closure}}::h09212e6202408097
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/rt.rs:195:18
  46:     0x5625072feda7 - core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &F>::call_once::h6889704b556d4c77
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/core/src/ops/function.rs:284:13
  47:     0x5625072feda7 - std::panicking::try::do_call::hb54df035e1ac8c92
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/panicking.rs:584:40
  48:     0x5625072feda7 - std::panicking::try::h58823af4bcc61a96
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/panicking.rs:547:19
  49:     0x5625072feda7 - std::panic::catch_unwind::hc30ff8fe04ef2371
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/panic.rs:358:14
  50:     0x5625072feda7 - std::rt::lang_start_internal::{{closure}}::h527f54f1892dcb9b
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/rt.rs:174:48
  51:     0x5625072feda7 - std::panicking::try::do_call::h41aaba5fff752501
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/panicking.rs:584:40
  52:     0x5625072feda7 - std::panicking::try::hf0596d206f5fce25
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/panicking.rs:547:19
  53:     0x5625072feda7 - std::panic::catch_unwind::h5a2061dbbb8ffdf4
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/panic.rs:358:14
  54:     0x5625072feda7 - std::rt::lang_start_internal::heee0af441e41a6d2
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/rt.rs:174:20
  55:     0x562506af616a - std::rt::lang_start::he0267f468637608e
                               at /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/rt.rs:194:17
  56:     0x562506af45ee - main
  57:     0x7f6423b43ca8 - __libc_start_call_main
                               at ./csu/../sysdeps/nptl/libc_start_call_main.h:58:16
  58:     0x7f6423b43d65 - __libc_start_main_impl
                               at ./csu/../csu/libc-start.c:360:3
  59:     0x562506a45791 - _start
  60:                0x0 - <unknown>
thread caused non-unwinding panic. aborting.
Aborted (core dumped)

[Jakuje]

Running the reproducer with the PKCS#11 SPY, the crash happens with the following attribute:

76: C_GetAttributeValue
P:2127568; T:0x140248153966528 2025-11-19 17:57:57.573
[in] hSession = 0x1
[in] hObject = 0x2
[in] pTemplate[1]: 
    CKA_ALLOWED_MECHANISMS  0000000000000001 / 0
[out] pTemplate[1]: 
    CKA_ALLOWED_MECHANISMS  0000000000000001 / 0
Returned:  0 CKR_OK

This is controversial behavior of the softhsm, where it returns uninitialized pointer + zero length for the CKA_ALLOWED_MECHANISMS attribute, when there is no mechanisms limitation. We have a workaround for this in pkcs11-provider where we hit this previously:

https://github.com/latchset/pkcs11-provider/blob/e4f723ddb1a287165177f2d4f5c1635e916526b1/src/sig/rsasig.c#L260-L266

We did not bother to report this to the SoftHSM as the project looks completely dead, but I will do that as it becomes annoying: softhsm/SoftHSMv2#825

[Jakuje]

To the question what we can do about this in rust-cryptoki. I think we will just need to workaround this bug by not trying to create zero-length objects we get with invalid pointers. Let me see if I can try to put together a test and fix.

[Jakuje]

I guess we can consider the Available(0) attribute as Unavailable. Given that there is no data associated with that attribute, I do not think there is any attribute that would support empty values and that would make any semantic difference just by presence.

[Jakuje]

Ok, won't be that easy. The CK_DATE type can be is by default empty as well as the CKA_ID says it could be empty by default:

https://docs.oasis-open.org/pkcs11/pkcs11-spec/v3.2/pkcs11-spec-v3.2.html#_Toc195693096