secure_clear (update to N2599)C Document number: N2631
C++ Document number: P1315R6 [latest]
Date: 2020-12-18
Author: Miguel Ojeda <ojeda@ojeda.dev>
Project: ISO JTC1/SC22/WG14: Programming Language C
Project: ISO JTC1/SC22/WG21: Programming Language C++
Sensitive data, like passwords or keying data, should be cleared from memory as soon as they are not needed. This requires ensuring the compiler will not optimize the memory overwrite away. This proposal adds a memset_explicit function (C) and a memset_explicit function template (C++) that guarantee users that a memory area is cleared.
N2631 & P1315R6 – Update to N2599:
When manipulating sensitive data, like passwords in memory or keying data, there is a need for library and application developers to clear the memory after the data is not needed anymore [1][2][3][4], in order to minimize the time window where it is possible to capture it (e.g., ending in a core dump or probed by a malicious actor). Recent vulnerabilities (e.g., Meltdown, Spectre-class, Rowhammer-class…) have made this requirement even more prominent.
To ensure that the memory is cleared, the developer needs to inform the compiler that it must not optimize away the memory write, even if it can prove it has no observable behavior. For C++, extra care is needed to consider all exceptional return paths.
For instance, the following C function may be vulnerable, since the compiler may optimize the memset call away because the password buffer is not read from before it goes out of scope:
void f(void)
{
char password[SIZE];
// Acquire some sensitive data
get_password_from_user(password, SIZE);
// Use it for some operations
use_password(password, SIZE);
// Attempt to clear the sensitive data
memset(password, 0, SIZE);
}
On top of that, in C++, use_password could throw an exception so memset is never called (i.e., assume the stack is not overwritten and/or that the memory is held in the free store).
There are many ways that developers may use to try to ensure the memory is cleared as expected (i.e., avoiding the optimizer):
memset_s from Annex K, if available.volatile pointer (e.g., decaf_bzero [5] from OpenSSL).memset through a volatile function pointer (e.g., OPENSSL_cleanse C implementation [6]).extern variable into it (e.g., CRYPTO_malloc implementation [7] from OpenSSL).OPENSSL_cleanse SPARC implementation [8]).memzero_explicit implementation [9] from the Linux Kernel).-fno-builtin-memset [10] in GCC).Or they may use a pre-existing solution whenever available:
explicit_bzero [11] from OpenBSD & FreeBSD, SecureZeroMemory [12] from Windows).memzero_explicit [13][9] from the Linux Kernel, OPENSSL_cleanse [14][6]).Regardless of how it is done, none of these ways is — at the same time — portable, easy to recognize the intent (and/or grep for it), readily available and avoiding compiler implementation details. The topic may generate discussions in major projects on which is the best way to proceed and whether an specific approach ensures that the memory is actually cleansed (e.g., [15][16][17][18][19]). Sometimes, such a way is not effective for a particular compiler (e.g., [20]). In the worst case, bugs happen (e.g., [21][22]).
C11 (and C++17 as it was based on it) added memset_s (K.3.7.4.1) to give a standard solution to this problem [4][23]. However, it is an optional extension (Annex K) and, at the time of writing, several major compiler vendors do not define __STDC_LIB_EXT1__ (GCC [24], Clang [25], MSVC [26], icc [27]). Therefore, in practical terms, there is no widely-available, portable solution yet for C nor C++. A 2015 paper on this topic, WG14’s N1967 “Field Experience With Annex K — Bounds Checking Interfaces” [28], concludes that “Annex K should be removed from the C standard”. On the other hand, a 2019 paper, WG14’s N2336 “Bounds-checking Interfaces: Field Experience and Future Directions” [29], examines the arguments both for and against the bounds-checked interfaces, evaluates possible solutions for actual problems with Annex K, and propounds the repair and retention of Annex K for C2x.
Other languages offer similar facilities in their standard libraries or as external projects:
secrecy, secstr and zeroize libraries in Rust [32][33][34].SecureString class in .NET Core, .NET Framework and Xamarin [35].securemem package in Haskell [36].We can standardize current practise by providing a memset_explicit function that takes a memory range (a pointer and a size) to be erased, plus a value, guaranteeing that it won’t be optimized away:
memset_explicit(password, 0, size);
As well as a memset_explicit function template for C++ that takes a reference to a non-pointer trivially_copyable object to scrub it entirely:
std::memset_explicit(password);
There are several reasonable design alternatives for such a function with respect to its interface, semantics, wording, naming, etc. Both WG14 and WG21 discussed them in several instances on previous revisions of this proposal. In the end, the C committee decided to go for a function with the same interface as memset for simplicity and familiarity.
While it is a good practise to ensure that the memory is cleared as soon as possible, there are other potential improvements when handling sensitive data in memory:
Most of these extra improvements require either non-portable code, operating-system-specific calls, or compiler support as well, which also makes them a good target for standardization. A subset (e.g., encryption-at-rest and locking/pinning) may be relatively easy to tackle in the near future since libraries/projects and other languages handle them already. Other improvements, however, are well in the research stage on compilers, e.g.:
Furthermore, discussing the addition of any security-related features to the C and C++ languages is a complex task. Therefore, this paper attempts to only spearhead the work on this area, providing access to users to the well-known “guaranteed memory clearing” function already used in the industry as explained in the previous sections. For context, it is important to mention that, recently, there seems to be a resurgence of interest in these topics: for C, N2485 proposed to add similar functionality [42]; while within C++ there are ongoing discussions about creating a safety and security SG or SIG.
Some other related improvements based on the basic building blocks can be also thought of, too:
memset_explicit is called on destruction/move, plus some other features (explicit read/modify/write access, locking/pinning, encryption-at-rest, etc.). This wrapper is a good approach to ensure memory is cleared even when dealing with exceptions. During the WG21 LEWGI review, it was acknowledged that similar types are in use by third-parties. Some libraries and languages feature similar types, as mentioned in the previous section.secure_string) may be considered useful (e.g., for passwords).There are, as well, other related library features that some languages provide for handling sensitive data:
getpass module in Python [44]).This proposal adds the memset_explicit function which follows the memset interface, semantics and naming; proposing three main wording alternatives.
The proposed wordings are with respect to N2596 C2x Working Draft.
After “7.24.6.1 The memset function”, add:
7.24.6.2 The
memset_explicitfunctionSynopsis
#include <string.h> void *memset_explicit(void *s, int c, size_t n);Description
The
memset_explicitfunction copies the value ofc(converted to anunsigned char) into each of the firstncharacters of the object pointed to bys. The purpose of this function is to make sensitive information stored in the object inaccessible. (Footnote 1)(Footnote 1) The intention is that the memory store is always performed (i.e., never elided), regardless of optimizations. This is in contrast to calls to the
memsetfunction (7.24.6.1).Recommended Practice
A call to the
memset_explicitfunction should have similar performance to a call to amemsetfunction call.Returns
The
memset_explicitfunction returns the value ofs.
In “B.23 String handling <string.h>”, after the memset line, add:
void *memset_explicit(void *s, int c, size_t n);
In “J.6.1 Rule based identifiers”, after the memset line, add:
memset_explicit
After “7.24.6.1 The memset function”, add:
7.24.6.2 The
memset_explicitfunctionSynopsis
#include <string.h> void *memset_explicit(void *s, int c, size_t n);Description
The
memset_explicitfunction copies the value ofc(converted to anunsigned char) into each of the firstncharacters of the object pointed to bysas a needed side effect (5.1.2.3) (Footnote 1). The purpose of this function is to make sensitive information stored in the object inaccessible.(Footnote 1) The intention is that the memory store is always performed (i.e., never elided), regardless of optimizations. This is in contrast to calls to the
memsetfunction (7.24.6.1). The implementation might not clear other copies of the data (e.g., intermediate values, cache lines, spilled registers, etc.).Recommended Practice
A call to the
memset_explicitfunction should have similar performance to a call to amemsetfunction call.Returns
The
memset_explicitfunction returns the value ofs.
In “B.23 String handling <string.h>”, after the memset line, add:
void *memset_explicit(void *s, int c, size_t n);
In “J.6.1 Rule based identifiers”, after the memset line, add:
memset_explicit
After “7.24.6.1 The memset function”, add:
7.24.6.2 The
memset_explicitfunctionSynopsis
#include <string.h> void *memset_explicit(void *s, int c, size_t n);Description
The
memset_explicitfunction copies the value ofc(converted to anunsigned char) into each of the firstncharacters of the object pointed to bysas a needed side effect (5.1.2.3) (Footnote 1). The purpose of this function is to make sensitive information stored in the object inaccessible.(Footnote 1) The intention is that the memory store is always performed (i.e., never elided), regardless of optimizations. This is in contrast to calls to the
memsetfunction (7.24.6.1). The implementation might not clear other copies of the data (e.g., intermediate values, cache lines, spilled registers, etc.).It is implementation-defined what actions in the actual semantics of the execution environment (e.g., system calls) are taken to realize the side effect. The evaluation of an invocation of the function causes these implementation-defined actions to occur. In the abstract machine, the evaluation of the function is considered to not necessarily return to the caller [NOTE 1]. On completing the actions in the actual semantics, the abstract machine continues with the function returning control to its caller.
NOTE 1 This means that the actual semantics associated with the function call cannot be affected by undefined behavior from an evaluation sequenced after the call, even if said evaluation would manifestly occur should the function return. The actual semantics may nevertheless be affected by asynchronous actions.
Recommended Practice
A call to the
memset_explicitfunction should have similar performance to a call to amemsetfunction call.Returns
The
memset_explicitfunction returns the value ofs.
In “B.23 String handling <string.h>”, after the memset line, add:
void *memset_explicit(void *s, int c, size_t n);
In “J.6.1 Rule based identifiers”, after the memset line, add:
memset_explicit
This proposal adds the memset_explicit function template which would rely on the C2x function:
namespace std {
template <class T>
requires is_trivially_copyable_v<T>
and (not is_pointer_v<T>)
void memset_explicit(T & object) noexcept;
}
The memset_explicit function template is equivalent to a call to memset_explicit(addressof(object), sizeof(object)).
The not is_pointer_v<T> constraint is intended to prevent unintended calls that would have cleared a pointer rather than the object it points to:
char buf[size];
char * ptr = buf;
std::memset_explicit(buf); // OK, clears the array
std::memset_explicit(ptr); // Error
std::memset_explicit(ptr, sizeof(ptr)); // OK, explicit
No wording is provided yet, since so far the C function has not been added to the C2x Working Draft nor the C++2y standard has updated its normative references section to import the C2x standard library.
A trivial example implementation (i.e., relying on OS-specific functions) can be found at [45].
Thanks to Aaron Ballman, JF Bastien, David Keaton and Billy O’Neal for providing guidance about the WG14 and WG21 standardization processes. Thanks to Aaron Ballman, Peter Sewell, David Svoboda, Hubert Tong, Martin Uecker and others for their input on wording and the abstract machine. Thanks to Robert C. Seacord for his review and proofreading of a previous revision. Thanks to Ryan McDougall for presenting an early revision at WG21 Kona 2019. Thanks to Graham Markall for his input regarding the SECURE project and the current state of compiler support for related features. Thanks to Martin Sebor for pointing out the SECURE project. Thanks to BSI for suggesting constraining the template to non-pointers. Thanks to Philipp Klaus Krause for raising the discussion in the OpenBSD list. Thanks to everyone else that joined all the different discussions.
memset function call (…)” — https://www.viva64.com/en/w/v597/memset_s() to clear memory, without fear of removal” — http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1381.pdfopenssl/crypto/ec/curve448/utils.c (old code)” — https://github.com/openssl/openssl/blob/f8385b0fc0215b378b61891582b0579659d0b9f4/crypto/ec/curve448/utils.cOPENSSL_cleanse (implementation)” — https://github.com/openssl/openssl/blob/master/crypto/mem_clr.copenssl/crypto/mem.c (old code)” — https://github.com/openssl/openssl/blob/104ce8a9f02d250dd43c255eb7b8747e81b29422/crypto/mem.c#L143openssl/crypto/sparccpuid.S (example of assembly implementation)” — https://github.com/openssl/openssl/blob/master/crypto/sparccpuid.S#L363memzero_explicit (implementation)” — https://elixir.bootlin.com/linux/v4.18.5/source/lib/string.c#L706bzero, explicit_bzero - zero a byte string” — http://man7.org/linux/man-pages/man3/bzero.3.htmlSecureZeroMemory function” — https://msdn.microsoft.com/en-us/library/windows/desktop/aa366877(v=vs.85).aspxmemzero_explicit” — https://www.kernel.org/doc/htmldocs/kernel-api/API-memzero-explicit.htmlOPENSSL_cleanse” — https://www.openssl.org/docs/man1.1.1/man3/OPENSSL_cleanse.htmlOPENSSL_cleanse() #455” — https://github.com/openssl/openssl/pull/455SecureZeroMemory / RtlSecureZeroMemory?” — https://stackoverflow.com/questions/13299420/memset() calls?” — https://gcc.gnu.org/ml/gcc-help/2014-10/msg00047.htmlmemset optimized out in random.c” — https://bugzilla.kernel.org/show_bug.cgi?id=82041memset_s in gcc 8.2 at Godbolt” — https://godbolt.org/g/M7MyRgmemset_s in clang 6.0.0 at Godbolt” — https://godbolt.org/g/ZwbkgYmemset_s in MSVC 19 2017 at Godbolt” — https://godbolt.org/g/FtrVJ8memset_s in icc 18.0.0 at Godbolt” — https://godbolt.org/g/vHZNrWsecrecy Rust library” — https://crates.io/crates/secrecysecstr Rust library” — https://crates.io/crates/secstrzeroize Rust library” — https://crates.io/crates/zeroizeSecureString Class” — https://docs.microsoft.com/en-us/dotnet/api/system.security.securestringsecuremem: abstraction to an auto scrubbing and const time eq, memory chunk.” — https://hackage.haskell.org/package/securememsecure_val: a secure-clear-on-move type” — http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1315r1.htmlsecure_clear Example implementation” — https://github.com/ojeda/secure_clear/tree/master/example-implementation