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AJvYcCWW1p0oPhBiwMeOLcYe/TkIGT+rtjcm9Az3jE3Zml5noA/tXaKZlikOfKP144fE7ZSj5Q3BWSKz5EE=@lists.xenproject.org X-Gm-Message-State: AOJu0YymuzaP4MuW6vuydEnb06R5fRpam86xHPnfMVqrc6Oe57Om8vla MeA2DaXm8XeOnR9TfMtLUojAUvmsAODUU5sIX0p4DP7dCbbx0Ecag8QgJf4AKjI= X-Google-Smtp-Source: AGHT+IHVRu9ApD05Kx6SMO8rwf4GfcjT5kvw7cewb7eAVmD5EAk5CiWrDwyEU6xnnJx7h1d935JJeA== X-Received: by 2002:a05:6402:274f:b0:5bf:2577:4346 with SMTP id 4fb4d7f45d1cf-5c413e1b2a3mr3480355a12.15.1726166647023; Thu, 12 Sep 2024 11:44:07 -0700 (PDT) Message-ID: <49c43c98-a8b2-4943-af89-dfbf2030e478@citrix.com> Date: Thu, 12 Sep 2024 19:44:05 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] Fix two problems in the microcode parsers To: Demi Marie Obenour , xen-devel@lists.xenproject.org Cc: Jan Beulich , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= , =?UTF-8?Q?Marek_Marczykowski-G=C3=B3recki?= References: <06edbbb7831620bc04c6453b95aad80eaebb20a1.1726162000.git.demi@invisiblethingslab.com> Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 12/09/2024 6:39 pm, Demi Marie Obenour wrote: > The microcode might come from a questionable source, so it is necessary > for the parsers to treat it as untrusted. The CPU will validate the > microcode before applying it, so loading microcode from unofficial > sources is actually a legitimate thing to do in some cases. While true, the better argument is that ucode blobs are mostly encrypted data, so a mis-step tends to fall over a very long way. > > Signed-off-by: Demi Marie Obenour > --- > xen/arch/x86/cpu/microcode/amd.c | 1 + > xen/arch/x86/cpu/microcode/intel.c | 5 +++-- > 2 files changed, 4 insertions(+), 2 deletions(-) > > diff --git a/xen/arch/x86/cpu/microcode/amd.c b/xen/arch/x86/cpu/microcode/amd.c > index d2a26967c6dbc4695602dd46d5836a6d88e15072..31ee5717c5f1c7d0b7e29d990cf4d1024d775900 100644 > --- a/xen/arch/x86/cpu/microcode/amd.c > +++ b/xen/arch/x86/cpu/microcode/amd.c > @@ -338,6 +338,7 @@ static struct microcode_patch *cf_check cpu_request_microcode( > if ( size < sizeof(*et) || > (et = buf)->type != UCODE_EQUIV_CPU_TABLE_TYPE || > size - sizeof(*et) < et->len || > + et->len < sizeof(et->eq[0]) || > et->len % sizeof(et->eq[0]) || > et->eq[(et->len / sizeof(et->eq[0])) - 1].installed_cpu ) > { This is complicated by AMD having no spec in the slightest on their container format, but this change makes Xen reject a case that I reasoned was well formed (if unwise). IMO, the following binary is well formed:     0x00414d44 /* MAGIC */     0x00000000 /* type=equiv */     0x00000000 /* len=0, == no entries */                /* no equiv entries */                /* no ucode entries */     0x00414d44 /* MAGIC */     ... and I believe that Xen would parse it correctly.  Unless you think there's another case I've failed to consider? AFACT the check you put in rejects the len < "1 entry" case. There is no shortage of ambiguity here; the equiv table has both an explicit length, and commonly has a NULL entry at the end.  But, we can parse with a length of 0. > diff --git a/xen/arch/x86/cpu/microcode/intel.c b/xen/arch/x86/cpu/microcode/intel.c > index 6f6957058684d7275d62e525e88ff678db9eb6d2..7a383adbdf1b5cb58f2e4c89e3a1c11ecc053993 100644 > --- a/xen/arch/x86/cpu/microcode/intel.c > +++ b/xen/arch/x86/cpu/microcode/intel.c > @@ -158,8 +158,9 @@ static int microcode_sanity_check(const struct microcode_patch *patch) > * Total size must be a multiple of 1024 bytes. Data size and the header > * must fit within it. > */ > - if ( (total_size & 1023) || > - data_size > (total_size - MC_HEADER_SIZE) ) > + if ( (total_size & 1023) || (total_size < MC_HEADER_SIZE) || > + data_size > (total_size - MC_HEADER_SIZE) || > + (data_size % 4) != 0 ) The caller has already checked some properties. Furthermore, total_size of 0 in the header is a special case for pre-Pentium4 ucode and is substituted with a constant, so can never be 0 in the variable here.  Therefore, I don't think the (total_size < MC_HEADER_SIZE) check can ever fail. data_size being 4-byte aligned probably should be checked, although I think the later cross-check (ext_sigtable fits exactly in the remaining space) should catch this before the function wanders off the buffer. ~Andrew