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Merge branch 'bc/constant-memequal'

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Validation of push certificate has been made more robust against
timing attacks.

* bc/constant-memequal:
  receive-pack: compilation fix
  builtin/receive-pack: use constant-time comparison for HMAC value
This commit is contained in:
Junio C Hamano 2020-04-28 15:49:57 -07:00
commit 2abd648b17
1 changed files with 22 additions and 1 deletions
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23
builtin/receive-pack.c
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@ -499,12 +499,27 @@ static char *find_header(const char *msg, size_t len, const char *key,
return NULL; return NULL;
} }
/*
* Return zero if a and b are equal up to n bytes and nonzero if they are not.
* This operation is guaranteed to run in constant time to avoid leaking data.
*/
static int constant_memequal(const char *a, const char *b, size_t n)
{
int res = 0;
size_t i;
for (i = 0; i < n; i++)
res |= a[i] ^ b[i];
return res;
}
static const char *check_nonce(const char *buf, size_t len) static const char *check_nonce(const char *buf, size_t len)
{ {
char *nonce = find_header(buf, len, "nonce", NULL); char *nonce = find_header(buf, len, "nonce", NULL);
timestamp_t stamp, ostamp; timestamp_t stamp, ostamp;
char *bohmac, *expect = NULL; char *bohmac, *expect = NULL;
const char *retval = NONCE_BAD; const char *retval = NONCE_BAD;
size_t noncelen;
if (!nonce) { if (!nonce) {
retval = NONCE_MISSING; retval = NONCE_MISSING;
@ -546,8 +561,14 @@ static const char *check_nonce(const char *buf, size_t len)
goto leave; goto leave;
} }
noncelen = strlen(nonce);
expect = prepare_push_cert_nonce(service_dir, stamp); expect = prepare_push_cert_nonce(service_dir, stamp);
if (strcmp(expect, nonce)) { if (noncelen != strlen(expect)) {
/* This is not even the right size. */
retval = NONCE_BAD;
goto leave;
}
if (constant_memequal(expect, nonce, noncelen)) {
/* Not what we would have signed earlier */ /* Not what we would have signed earlier */
retval = NONCE_BAD; retval = NONCE_BAD;
goto leave; goto leave;