{"id":112431,"date":"2023-12-22T21:10:32","date_gmt":"2023-12-22T19:10:33","guid":{"rendered":"https:\/\/prohoster.info\/blog\/novosti-interneta\/mayhem-ataka-iskazhayushhaya-bity-v-pamyati-dlya-obhoda-autentifikaczii-v-sudo-i-openssh"},"modified":"2023-12-22T21:10:32","modified_gmt":"2023-12-22T19:10:33","slug":"mayhem-ataka-iskazhayushhaya-bity-v-pamyati-dlya-obhoda-autentifikaczii-v-sudo-i-openssh","status":"publish","type":"post","link":"https:\/\/prohoster.info\/cs\/blog\/news\/mayhem-ataka-iskazhayushhaya-bity-v-pamyati-dlya-obhoda-autentifikaczii-v-sudo-i-openssh","title":{"rendered":"Mayhem \u2014 \u00fatok, kter\u00fd naru\u0161uje bity v pam\u011bti, aby obch\u00e1zel autentizaci v sudo a OpenSSH","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"
V\u00fdzkumn\u00edci z Worcester Polytechnic Institute (USA) p\u0159edstavili nov\u00fd typ \u00fatoku Mayhem, kter\u00fd vyu\u017e\u00edv\u00e1 metodu po\u0161kozen\u00ed bit\u016f v dynamick\u00e9 opera\u010dn\u00ed pam\u011bti Rowhammer k zm\u011bn\u011b hodnot prom\u011bnn\u00fdch ve stacku, pou\u017e\u00edvan\u00fdch v programu jako p\u0159\u00edznaky pro rozhodov\u00e1n\u00ed o \u00fasp\u011b\u0161nosti autentizace a p\u0159ed\u00e1v\u00e1n\u00ed bezpe\u010dnostn\u00edch kontrol. Praktick\u00e9 p\u0159\u00edklady aplikace \u00fatoku jsou demonstrov\u00e1ny pro obch\u00e1zen\u00ed autentizace v SUDO, OpenSSH a MySQL, stejn\u011b jako pro zm\u011bnu v\u00fdsledku kontrol spojen\u00fdch se zabezpe\u010den\u00edm v knihovn\u011b OpenSSL. <\/p>\n
\u00datok m\u016f\u017ee b\u00fdt uplatn\u011bn na aplikace, kter\u00e9 p\u0159i kontrol\u00e1ch pou\u017e\u00edvaj\u00ed porovn\u00e1n\u00ed odli\u0161nost\u00ed hodnot od nuly. P\u0159\u00edklad zraniteln\u00e9ho k\u00f3du: int auth = 0; … \/\/ kontroln\u00ed k\u00f3d, kter\u00fd m\u011bn\u00ed hodnotu auth v p\u0159\u00edpad\u011b \u00fasp\u011b\u0161n\u00e9 autentizace if(auth != 0) return AUTH_SUCCESS; else return AUTH_FAILURE; <\/p>\n
V kontextu tohoto p\u0159\u00edkladu je pro \u00fasp\u011b\u0161n\u00fd \u00fatok dostate\u010dn\u00e9 dos\u00e1hnout po\u0161kozen\u00ed libovoln\u00e9ho bite v pam\u011bti, kter\u00fd se t\u00fdk\u00e1 32bitov\u00e9 prom\u011bnn\u00e9 auth ve stacku. P\u0159i po\u0161kozen\u00ed jak\u00e9hokoli bitu v prom\u011bnn\u00e9 ji\u017e nebude hodnota rovna nule a podm\u00ednkov\u00fd p\u0159\u00edkaz ur\u010duje \u00fasp\u011b\u0161n\u00e9 proveden\u00ed autentizace. Takov\u00e9 kontroln\u00ed \u0161ablony jsou dost b\u011b\u017en\u00e9 v aplikac\u00edch a objevuj\u00ed se nap\u0159\u00edklad v SUDO, OpenSSH, MySQL a OpenSSL.
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\u00datok m\u016f\u017ee b\u00fdt proveden i na porovn\u00e1n\u00ed typu «if(auth == 1)», av\u0161ak v tomto p\u0159\u00edpad\u011b je jeho proveden\u00ed slo\u017eit\u011bj\u0161\u00ed, proto\u017ee je nutn\u00e9 zkreslit nikoli jak\u00fdkoli bit ze 32, ale posledn\u00ed bit. Tato metoda m\u016f\u017ee b\u00fdt tak\u00e9 pou\u017eita k ovlivn\u011bn\u00ed hodnot prom\u011bnn\u00fdch v procesorov\u00fdch registrech, proto\u017ee obsah registr\u016f m\u016f\u017ee b\u00fdt na chv\u00edli ulo\u017een na z\u00e1sobn\u00edk p\u0159i p\u0159epnut\u00ed kontextu, vol\u00e1n\u00ed funkc\u00ed nebo aktivaci obslu\u017en\u00e9 rutiny sign\u00e1lu. B\u011bhem doby, kdy se hodnoty registr\u016f nach\u00e1zej\u00ed v pam\u011bti, lze do t\u00e9to pam\u011bti zav\u00e9st zkreslen\u00ed a do registru bude obnovena zm\u011bn\u011bn\u00e1 hodnota.
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K deformaci bit\u016f se pou\u017e\u00edv\u00e1 jedna z modifikac\u00ed \u00fatoku t\u0159\u00eddy RowHammer. Vzhledem k tomu, \u017ee pam\u011b\u0165 DRAM p\u0159edstavuje dvourozm\u011brn\u00e9 pole bun\u011bk, z nich\u017e ka\u017ed\u00e1 se skl\u00e1d\u00e1 z kondenz\u00e1toru a tranzistoru, prov\u00e1d\u011bn\u00ed nep\u0159etr\u017eit\u00e9ho \u010dten\u00ed stejn\u00e9 oblasti pam\u011bti vede k fluktuaci nap\u011bt\u00ed a anom\u00e1li\u00edm, kter\u00e9 zp\u016fsobuj\u00ed malou ztr\u00e1tu n\u00e1boje sousedn\u00edch bun\u011bk. Pokud je intenzita \u010dten\u00ed vysok\u00e1, m\u016f\u017ee sousedn\u00ed bu\u0148ka ztratit dostate\u010dn\u00e9 mno\u017estv\u00ed n\u00e1boje a p\u0159\u00ed\u0161t\u00ed regenerace nebude schopna obnovit jej\u00ed p\u016fvodn\u00ed stav, co\u017e povede ke zm\u011bn\u011b hodnoty ulo\u017een\u00fdch dat v bu\u0148ce. Pro ochranu proti RowHammer v\u00fdrobci \u010dip\u016f p\u0159idali mechanismus TRR (Target Row Refresh), kter\u00fd blokuje deformaci bun\u011bk v n\u011bkter\u00fdch p\u0159\u00edpadech, ale nechr\u00e1n\u00ed p\u0159ed v\u0161emi mo\u017en\u00fdmi varianty \u00fatoku. <\/p>\n
Pro ochranu p\u0159ed \u00fatokem Mayhem se doporu\u010duje v porovn\u00e1n\u00edch nepou\u017e\u00edvat posuzov\u00e1n\u00ed odli\u0161nost\u00ed od nuly nebo shod s jedni\u010dkou, ale prov\u00e1d\u011bt kontrolu shod, p\u0159i\u010dem\u017e se pou\u017e\u00edv\u00e1 n\u00e1hodn\u00e1 hodnota seed s nenulov\u00fdmi oktety. V tomto p\u0159\u00edpad\u011b je nutn\u00e9 p\u0159esn\u011b zkreslit podstatn\u00e9 mno\u017estv\u00ed bit\u016f, co\u017e je nerealistick\u00e9, na rozd\u00edl od zkreslen\u00ed jednoho bitu. P\u0159\u00edklad k\u00f3du, kter\u00fd nen\u00ed citliv\u00fd na \u00fatok: int auth = 0xbe406d1a; … \/\/ kontroln\u00ed k\u00f3d, kter\u00fd nastavuje hodnotu auth na 0x23ab8701 v p\u0159\u00edpad\u011b \u00fasp\u011b\u0161n\u00e9 autentizace if(auth == 0x23ab8701) return AUTH_SUCCESS; else return AUTH_FAILURE; <\/p>\n
![\"Mayhem](\"\/wp-content\/uploads\/2023\/12\/ef5dd9463837514f055f9f2610652fb9.png\")
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