Clear sensitive memory without getting optimized out.

isle2983 commented at 0:53 am on March 15, 2017: none

(Previous discussion of this topic in #438 )

explicit_bzero() is used when available on the platform (glibc 2.25+), otherwise it falls back to an inline implementation. There are many possible versions of how the inline could be implemented but I have no reason to prefer one over another.

The commit stack does some refactoring in order to preserve the intended non-‘cleanse’ functionality done by several of the _clear() functions. I constructed the stack as unsquashed, incremental refactoring changes. Please let me know if a squash is preferred.

The measured delta in performance appears negligible. Trials were done inside a minimal headless VMs with Debian 8 in one for glibc 2.19 and the current base of Arch Linux in the other for glibc 2.25. The full session output of the trials was logged for reference and is available as a gist.

For reference to bare metal performance with the same CPU, timing on the underlying native Debian 8 install (with glibc 2.19 and gcc 4.9.2 running with master:

0(wo/ endomorphism)
1$ ./bench_sign
2ecdsa_sign: min 55.3us / avg 55.4us / max 55.5us
3$ ./bench_verify
4ecdsa_verify: min 85.6us / avg 85.9us / max 86.3us
5ecdsa_verify_openssl: min 476us / avg 484us / max 490us

Trials:

master w/ glibc 2.19, gcc 4.9.2 w/ and wo/ endomorphisim

 0(wo/ endomorphism)
 1$ ./bench_sign
 2ecdsa_sign: min 55.9us / avg 56.4us / max 56.7us
 3$ ./bench_verify
 4ecdsa_verify: min 86.2us / avg 86.8us / max 87.7us
 5ecdsa_verify_openssl: min 487us / avg 488us / max 491us
 6
 7(w/ endomorphism)
 8$ ./bench_sign
 9ecdsa_sign: min 56.1us / avg 56.6us / max 57.2us
10$ ./bench_verify
11ecdsa_verify: min 66.5us / avg 67.4us / max 68.8us
12ecdsa_verify_openssl: min 487us / avg 493us / max 504us

a16034039f07572a64bc9704c23abb1fff9d70ad w/ glibc 2.19 gcc 4.9.2 w/ and wo/ endomorphism

 0(wo/ endomorphism)
 1./bench_sign
 2ecdsa_sign: min 55.8us / avg 56.5us / max 57.0us
 3$ ./bench_verify
 4ecdsa_verify: min 86.3us / avg 86.6us / max 86.9us
 5ecdsa_verify_openssl: min 482us / avg 483us / max 484us
 6
 7(w/ endomorphism)
 8$ ./bench_sign
 9ecdsa_sign: min 56.0us / avg 56.5us / max 56.7us
10$ ./bench_verify
11ecdsa_verify: min 65.7us / avg 66.0us / max 66.4us
12ecdsa_verify_openssl: min 482us / avg 484us / max 489us

master w/ glibc 2.25, gcc 6.3.1 w/ and wo/ endomorphisim

 0(wo/ endomorphism)
 1./bench_sign
 2ecdsa_sign: min 57.2us / avg 57.8us / max 58.1us
 3$ ./bench_verify
 4ecdsa_verify: min 76.1us / avg 76.7us / max 77.7us
 5ecdsa_verify_openssl: min 461us / avg 464us / max 482us
 6
 7(w/ endomorphism)
 8$ ./bench_sign
 9ecdsa_sign: min 57.0us / avg 57.2us / max 57.5us
10$ ./bench_verify
11ecdsa_verify: min 54.8us / avg 55.1us / max 55.5us
12ecdsa_verify_openssl: min 461us / avg 462us / max 467us

a16034039f07572a64bc9704c23abb1fff9d70ad w/ glibc 2.25 gcc 6.3.1 w/ and wo/ endomorphism

 0(wo/ endomorphism)
 1./bench_sign
 2ecdsa_sign: min 56.7us / avg 57.0us / max 57.5us
 3$ ./bench_verify
 4ecdsa_verify: min 75.6us / avg 76.1us / max 76.6us
 5ecdsa_verify_openssl: min 460us / avg 460us / max 461us
 6
 7(w/ endomorphism)
 8$ ./bench_sign
 9ecdsa_sign: min 56.5us / avg 56.8us / max 57.1us
10$ ./bench_verify
11ecdsa_verify: min 54.7us / avg 54.8us / max 55.0us
12ecdsa_verify_openssl: min 462us / avg 463us / max 464us

The desired behavior of not getting optimized out was also verified by looking at the resulting assembly. With the inline implementation on glibc 2.19, the end section of secp256k1_ecmult_gen() from gcc 4.9.2 looks like:

 0	movl	%r15d, 120(%rbx)	# infinity, r_7(D)->infinity
 1	movq	128(%rsp), %rax	# %sfp, ivtmp.249
 2	cmpq	$64, %rax	#, ivtmp.249
 3	je	.L83	#,
 4	salq	$34, %rax	#, D.9134
 5	shrq	$37, %rax	#, D.9134
 6	movl	304(%rsp,%rax,4), %r9d	# gnb.d, D.9141
 7	jmp	.L84	#
 8.L83:
 9	movq	$memset, 768(%rsp)	#, volatile_memset
10	leaq	300(%rsp), %rdi	#, tmp5061
11	movq	768(%rsp), %rax	# volatile_memset, D.9135
12	movl	$4, %edx	#,
13	xorl	%esi, %esi	#
14	call	*%rax	# D.9135
15	movq	$memset, 816(%rsp)	#, volatile_memset
16	leaq	976(%rsp), %rdi	#, tmp5062
17	movq	816(%rsp), %rax	# volatile_memset, D.9135
18	movl	$84, %edx	#,
19	xorl	%esi, %esi	#
20	call	*%rax	# D.9135
21	movq	$memset, 864(%rsp)	#, volatile_memset
22	leaq	304(%rsp), %rdi	#, tmp5063
23	movq	864(%rsp), %rax	# volatile_memset, D.9135
24	movl	$32, %edx	#,
25	xorl	%esi, %esi	#
26	call	*%rax	# D.9135
27	addq	$1080, %rsp	#,

With the explicit_bzero() from glibc 2.25 linked and with gcc 6.3.1, the same end section of secp256k1_ecmult_gen looks like:

 0	movl	$0, 240(%rsp)	#, add.infinity
 1	call	secp256k1_gej_add_ge	#
 2	addq	$1, (%rsp)	#, %sfp
 3	movq	8(%rsp), %r10	# %sfp, tmp242
 4	movq	(%rsp), %rax	# %sfp, ivtmp.629
 5	movq	16(%rsp), %r9	# %sfp, _90
 6	movq	24(%rsp), %r8	# %sfp, _85
 7	cmpq	$64, %rax	#, ivtmp.629
 8	jne	.L360	#,
 9	leaq	60(%rsp), %rdi	#, tmp380
10	movl	$4, %esi	#,
11	call	explicit_bzero	#
12	leaq	160(%rsp), %rdi	#, tmp381
13	movl	$88, %esi	#,
14	call	explicit_bzero	#
15	leaq	64(%rsp), %rdi	#, tmp382
16	movl	$32, %esi	#,
17	call	explicit_bzero	#
18	addq	$264, %rsp	#,

isle2983 cross-referenced this on Mar 15, 2017 from issue [trivial] clear stack variable by function by isle2983

in src/util.h:82 in ec7f08d375 outdated

76@@ -76,6 +77,17 @@ static SECP256K1_INLINE void *checked_malloc(const secp256k1_callback* cb, size_
77     return ret;
78 }
79 
80+/* Cleanses memory to prevent leaking sensitive info. Won't be optimized out. */
81+#ifdef HAVE_EXPLICIT_BZERO
82+#define SECP256K1_CLEANSE(v) explicit_bzero(&v, sizeof(v))

sipa commented at 0:15 am on March 22, 2017:

Perhaps use &(v) here so that compound expressions in the macro call work?

isle2983 commented at 8:22 pm on March 27, 2017:

Parens added to the macro as suggested

in src/field_10x26_impl.h:296 in c62bf18cc4 outdated

299@@ -300,7 +300,8 @@ SECP256K1_INLINE static int secp256k1_fe_is_odd(const secp256k1_fe *a) {
300 SECP256K1_INLINE static void secp256k1_fe_clear(secp256k1_fe *a) {
301     int i;
302 #ifdef VERIFY
303-    secp256k1_fe_verify_init(a);
304+    a->magnitude = 0;

sipa commented at 0:17 am on March 22, 2017:

Why inline the call again here (compared to the previous commit)?

in src/field_5x52_impl.h:259 in c62bf18cc4 outdated

262@@ -263,7 +263,8 @@ SECP256K1_INLINE static int secp256k1_fe_is_odd(const secp256k1_fe *a) {
263 SECP256K1_INLINE static void secp256k1_fe_clear(secp256k1_fe *a) {
264     int i;
265 #ifdef VERIFY
266-    secp256k1_fe_verify_init(a);
267+    a->magnitude = 0;

sipa commented at 0:18 am on March 22, 2017:

And here as well?

isle2983 commented at 8:22 pm on March 27, 2017:

It isn’t actually a straight inline. Both values are set to zero as part of the cleanse. The thought was to be a bit pedantic about showing the progression in the commits, but it might have been be overboard.

In the new commit progression with secp256_fe_set_zero(), seeing it side-by-side with secp256k1_fe_clear(), with a->normalized = 1; and a->normalized = 0; respectively is clearer with the intent of the two separated functions.

in src/tests.c:1893 in 3fdd634fd7 outdated

1888@@ -1889,7 +1889,11 @@ void test_ge(void) {
1889     secp256k1_fe zfi2, zfi3;
1890 
1891     secp256k1_gej_set_infinity(&gej[0]);
1892-    secp256k1_ge_clear(&ge[0]);
1893+    memset(&ge[0], 0, sizeof(secp256k1_ge));
1894+#ifdef VERIFY

sipa commented at 0:21 am on March 22, 2017:

And alternative maybe is using secp256k1_ge_set_gej(&ge[0], &gej[0]) ?

isle2983 commented at 8:22 pm on March 27, 2017:

Changed as suggested in d056957095d8153d7b29bb2e30125e7341b763f1

sipa commented at 0:27 am on March 22, 2017: contributor

Overall concept ACK.

I’m not sure I like the separation of secp256k1_fe_verify_init, as it places a responsibility on the caller that they shouldn’t have (as in: the group code ideally shouldn’t need to know there is such a thing as debug variables in secp256k1_fe that need initialization). Given that secp256k1_fe_clear remains, can’t the secp256k1_fe_verify_init logic remain in there? Then you can still use a strict separation between SECP256K1_CLEANSE(fe) (meaning fe won’t be used anymore) and secp256k1_fe_clear(&fe) (meaning fe must get the value 0). You could even rename the latter to secp256k1_fe_set_zero perhaps.

Use AC_USE_SYSTEM_EXTENSIONS to define _GNU_SOURCE in libsecp256k1-config.h

In glibc 2.25, the new explicit_bzero() call is guarded by #ifdef __USE_MISC in
string.h. Via glibc 2.25's features.h, the selectors for __USE_MISC are set
under #ifdef _GNU_SOURCE.

Without this set, the linker still figures out how to link explicit_bzero(),
but it throws a bunch of warnings about a missing prototype for
explicit_bzero().

By setting AC_USE_SYSTEM_EXTENSIONS, the system is detected and
libsecp256k1-config.h is generated with the correct setting for _GNU_SOURCE

It was found that even after this addition, the compile for the bench_internal
executable was still throwing the warning. This was because the <stdio.h> was
included before libsec256k1-config.h (via including src/util.h) and confusing
the preprocessing. It turns out that this <stdio.h> include is redundant since
it is included later from src/util.h in the correct order.

3b32684708

Introduce SECP256K1_CLEANSE()

explicit_bzero() is newly available in glibc 2.25 (released 2017-02-05; current
stable).

The fall-back implementation uses the 'volatile' keyword to signal the compiler
to not optimize away the zeroing in the same way as explicit_bzero().

The wrapper macro SECP256K1_CLEANSE() borrows its name from OPENSSL_cleanse()
in the openssl codebase which serves the same purpose and looks reasonably
clear in code context.

fa474312e8

Replace secp256k1_scalar_clear() calls with SECP256K1_CLEANSE() 407cd76dcd

Separate secp256k1_fe_set_zero() from secp256k1_fe_clear()

There are two uses of the secp256k1_fe_clear() function that are now separated
into these two functions in order to reflect the intent:

1) initializing the memory prior to being used -> converted to fe_set)zero()
2) zeroing the memory after being used such that no sensitive data remains. ->
    remains as fe_clear()

In the latter case, 'magnitude' and 'normalized' need to be overwritten when
VERIFY is enabled.

d743f2fa97

Replace secp256k1_fe_clear() with SECP256K1_CLEANSE() e270ef017b

Setup test ge from the infinity gej

...rather than relying on zero assignment to set the initial state.

Suggested by Pieter Wuille to avoid using secp256k1_ge_clear() for this
initialization need.

d056957095

Replace secp256k1_ge_clear with SECP256K1_CLEANSE() 6e8cb68a3a

Replace secp256k1_gej_clear() with SECP256K1_CLEANSE() ad225e8487

Split secp256k1_ecmult_context_teardown() out of secp256k1_ecmult_context_clear

The 'teardown' naming is to complement the 'build' function name.  To 'build'
allocates and sets up the memory for pre_g and pre_g_128. To 'teardown' frees
the memory.

059012ff03

Replace secp256k1_ecmult_context_clear() with SECP256K1_CLEANSE() 2f05e69300

Split secp256k1_ecmult_gen_context_teardown() out of secp256k1_ecmult_gen_context_clear

The 'teardown' naming is to complement the 'build' function name.  To 'build'
allocates and sets up the memory for ctx->prec. To 'teardown' frees the memory.

0edecfd377

Replace secp256k1_ecmult_gen_context_clear() with SECP256K1_CLEANSE() 8feb9fb352

Cleanse 'bits' variable such that it doesn't get optimized away.

Fixes clang static analysis issue:

--------------------------------------------------------------------------------
An issue has been found in ./src/ecmult_gen_impl.h:150:5
Type:         Dead assignment
Description:  Value stored to 'bits' is never read

0: ./src/ecmult_gen_impl.h:153:5 - Value stored to 'bits' is never read
--------------------------------------------------------------------------------

95824cd2e6

Use SECP256K1_CLEANSE() to zero stack memory instead of memset()

All of these conversions:

1) operate on stack memory.
2) happen after the function is done with the variable
3) had an existing memset() action to be replaced

These were found by visual inspection and may not be the total set of
places where SECP256K1_CLEANSE should ideally be applied.

9acb45db56

isle2983 force-pushed on Mar 27, 2017

isle2983 commented at 8:21 pm on March 27, 2017: none

Great suggestions. I introduced secp256k1_fe_set_zero() and re-worked the commit history to avoid the wrangling of the #define VERIFY stuff by converting the remaining clients of secp256k1_fe_set_zero() over first. secp256k1_fe_clear() is then replaced with SECP256K1_CLEANSE() along with the others. secp256k1_fe_set_zero() remains and encapsulates the (former) verify_init() initialization logic.

gmaxwell commented at 6:35 pm on April 29, 2017: contributor

Concept ACK. I worry about the CLEANSE macro using sizeof, this is sort of begging to do the wrong thing with arrays, no?

real-or-random commented at 9:05 am on April 10, 2019: contributor

@isle2983 I considered to work on a similar PR and then found this one, which seems to be pretty mature already. :) Are you still willing to work on this PR?

gmaxwell commented at 4:50 am on May 22, 2019: contributor

@real-or-random you could also just pick up this PR, rebase, make your fixups, and just note it in the commit messages.

real-or-random commented at 8:28 am on May 23, 2019: contributor

Yes, that’s my plan; consider this PR adopted by me. I just couldn’t find time to work on this so far.

real-or-random cross-referenced this on May 30, 2019 from issue Memory zeroization improvements by gmaxwell

real-or-random cross-referenced this on Jun 6, 2019 from issue Clear sensitive memory without getting optimized out by real-or-random

sipa commented at 7:16 pm on July 23, 2019: contributor

Closing as superseded by #636.

sipa closed this on Jul 23, 2019

Clear sensitive memory without getting optimized out. #448