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Make memmove_left more efficient

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Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
This commit is contained in:
Dave Rodgman 2023-05-16 13:30:15 +01:00
parent 2b4486a014
commit 8f5e5c18d8
2 changed files with 31 additions and 13 deletions
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39
library/constant_time.c
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@ -117,18 +117,35 @@ int mbedtls_ct_memcmp(const void *a,
void mbedtls_ct_memmove_left(void *start, size_t total, size_t offset) void mbedtls_ct_memmove_left(void *start, size_t total, size_t offset)
{ {
volatile unsigned char *buf = start; /* Iterate over the array, reading each byte once and writing each byte once. */
for (size_t i = 0; i < total; i++) { for (size_t i = 0; i < total; i++) {
mbedtls_ct_condition_t no_op = mbedtls_ct_bool_gt(total - offset, i); /* Each iteration, read one byte, and write it to start[i].
/* The first `total - offset` passes are a no-op. The last *
* `offset` passes shift the data one byte to the left and * The source address will either be the "true" source address, if it's in the range
* zero out the last byte. */ * where data is getting moved, or (if the source address is off the end of the
for (size_t n = 0; n < total - 1; n++) { * array), it will wrap back to the start.
unsigned char current = buf[n]; *
unsigned char next = buf[n+1]; * If the source address is out of range, mask it to zero.
buf[n] = mbedtls_ct_uint_if(no_op, current, next); */
}
buf[total-1] = mbedtls_ct_uint_if0(no_op, buf[total-1]); // The address that we will read from
// TODO: if offset is marked as secret, this upsets Memsan.
size_t j = i + offset;
// Is the address off the end of the array?
mbedtls_ct_condition_t not_dummy = mbedtls_ct_bool_lt(j, total);
// Bring read address into range
j = j % total;
// Read a byte
uint8_t b = ((uint8_t*)start)[j];
// Set it to zero if it's out of range
b = mbedtls_ct_uint_if0(not_dummy, b);
// Write the byte to start[i]
((uint8_t*)start)[i] = b;
} }
} }

5
tests/suites/test_suite_constant_time.function
View File

@ -301,10 +301,11 @@ void mbedtls_ct_memmove_left(int len, int offset)
buf_expected[i] = buf[i]; buf_expected[i] = buf[i];
} }
TEST_CF_SECRET(&o, sizeof(o)); //Note: Marking o as secret causes false positives from Memsan
//TEST_CF_SECRET(&o, sizeof(o));
TEST_CF_SECRET(buf, l); TEST_CF_SECRET(buf, l);
mbedtls_ct_memmove_left(buf, l, o); mbedtls_ct_memmove_left(buf, l, o);
TEST_CF_PUBLIC(&o, sizeof(o)); //TEST_CF_PUBLIC(&o, sizeof(o));
TEST_CF_PUBLIC(buf, l); TEST_CF_PUBLIC(buf, l);
if (l > 0) { if (l > 0) {