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kex.c: improve bounds checking in kex_agree_methods() (#399)

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file: kex.c

notes:
use _libssh2_get_string instead of kex_string_pair which does additional checks
This commit is contained in:
Will Cosgrove
2019-08-27 10:58:52 -07:00
committed by GitHub
parent 9662c963a2
commit 43f24eb152
1 changed files with 24 additions and 41 deletions
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65
src/kex.c
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@@ -3937,35 +3937,10 @@ static int kex_agree_comp(LIBSSH2_SESSION *session,
} }
/* TODO: When in server mode we need to turn this logic on its head /* TODO: When in server mode we need to turn this logic on its head
* The Client gets to make the final call on "agreed methods" * The Client gets to make the final call on "agreed methods"
*/ */
/*
* kex_string_pair() extracts a string from the packet and makes sure it fits
* within the given packet.
*/
static int kex_string_pair(unsigned char **sp, /* parsing position */
unsigned char *data, /* start pointer to packet */
size_t data_len, /* size of total packet */
size_t *lenp, /* length of the string */
unsigned char **strp) /* pointer to string start */
{
unsigned char *s = *sp;
*lenp = _libssh2_ntohu32(s);
/* the length of the string must fit within the current pointer and the
end of the packet */
if(*lenp > (data_len - (s - data) -4))
return 1;
*strp = s + 4;
s += 4 + *lenp;
*sp = s;
return 0;
}
/* kex_agree_methods /* kex_agree_methods
* Decide which specific method to use of the methods offered by each party * Decide which specific method to use of the methods offered by each party
*/ */
@@ -3976,40 +3951,48 @@ static int kex_agree_methods(LIBSSH2_SESSION * session, unsigned char *data,
*mac_cs, *mac_sc; *mac_cs, *mac_sc;
size_t kex_len, hostkey_len, crypt_cs_len, crypt_sc_len, comp_cs_len; size_t kex_len, hostkey_len, crypt_cs_len, crypt_sc_len, comp_cs_len;
size_t comp_sc_len, mac_cs_len, mac_sc_len; size_t comp_sc_len, mac_cs_len, mac_sc_len;
unsigned char *s = data; struct string_buf buf;
/* Skip packet_type, we know it already */ if(data_len < 17)
s++; return -1;
buf.data = (unsigned char *)data;
buf.len = data_len;
buf.dataptr = buf.data;
buf.dataptr++; /* advance past packet type */
/* Skip cookie, don't worry, it's preserved in the kexinit field */ /* Skip cookie, don't worry, it's preserved in the kexinit field */
s += 16; buf.dataptr += 16;
/* Locate each string */ /* Locate each string */
if(kex_string_pair(&s, data, data_len, &kex_len, &kex)) if(_libssh2_get_string(&buf, &kex, &kex_len))
return -1; return -1;
if(kex_string_pair(&s, data, data_len, &hostkey_len, &hostkey)) if(_libssh2_get_string(&buf, &hostkey, &hostkey_len))
return -1; return -1;
if(kex_string_pair(&s, data, data_len, &crypt_cs_len, &crypt_cs)) if(_libssh2_get_string(&buf, &crypt_cs, &crypt_cs_len))
return -1; return -1;
if(kex_string_pair(&s, data, data_len, &crypt_sc_len, &crypt_sc)) if(_libssh2_get_string(&buf, &crypt_sc, &crypt_sc_len))
return -1; return -1;
if(kex_string_pair(&s, data, data_len, &mac_cs_len, &mac_cs)) if(_libssh2_get_string(&buf, &mac_cs, &mac_cs_len))
return -1; return -1;
if(kex_string_pair(&s, data, data_len, &mac_sc_len, &mac_sc)) if(_libssh2_get_string(&buf, &mac_sc, &mac_sc_len))
return -1; return -1;
if(kex_string_pair(&s, data, data_len, &comp_cs_len, &comp_cs)) if(_libssh2_get_string(&buf, &comp_cs, &comp_cs_len))
return -1; return -1;
if(kex_string_pair(&s, data, data_len, &comp_sc_len, &comp_sc)) if(_libssh2_get_string(&buf, &comp_sc, &comp_sc_len))
return -1; return -1;
/* If the server sent an optimistic packet, assume that it guessed wrong. /* If the server sent an optimistic packet, assume that it guessed wrong.
* If the guess is determined to be right (by kex_agree_kex_hostkey) * If the guess is determined to be right (by kex_agree_kex_hostkey)
* This flag will be reset to zero so that it's not ignored */ * This flag will be reset to zero so that it's not ignored */
session->burn_optimistic_kexinit = *(s++); if(_libssh2_check_length(&buf, 1)) {
/* Next uint32 in packet is all zeros (reserved) */ session->burn_optimistic_kexinit = *(buf.dataptr++);
}
else {
return -1;
}
if(data_len < (unsigned) (s - data)) /* Next uint32 in packet is all zeros (reserved) */
return -1; /* short packet */
if(kex_agree_kex_hostkey(session, kex, kex_len, hostkey, hostkey_len)) { if(kex_agree_kex_hostkey(session, kex, kex_len, hostkey, hostkey_len)) {
return -1; return -1;