linux-kernel - Re: [PATCH] x86/microcode/AMD: check microcode file sanity before loading it

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Date:   Sun, 11 Mar 2018 14:25:32 +0100
From:   "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/microcode/AMD: check microcode file sanity before
 loading it

On 11.03.2018 10:59, Ingo Molnar wrote:
> 
> * Maciej S. Szmigiero <mail@...iej.szmigiero.name> wrote:
> 
>> Currently, it is very easy to make the AMD microcode update driver crash
>> or spin on a malformed microcode file since it does very little
>> consistency checking on data loaded from such file.
>>
>> This commit introduces various checks, mostly on length-type fields, so
>> all corrupted microcode files are (hopefully) correctly rejected instead.
>>
>> Signed-off-by: Maciej S. Szmigiero <mail@...iej.szmigiero.name>
>> ---
>> Test files are at https://pastebin.com/XkKUSmMp
>> One has to enable KASAN in the kernel config and rename a particular
>> test file to name appropriate to the running CPU family to test its
>> loading.
>>
>>  arch/x86/include/asm/microcode_amd.h |   6 ++
>>  arch/x86/kernel/cpu/microcode/amd.c  | 111 ++++++++++++++++++++++++++---------
>>  2 files changed, 89 insertions(+), 28 deletions(-)
> 
> Treating microcode update files as external input and sanity checking the data 
> makes sense I suppose, but there's various small uglies in the patch:
> 
>> +/* if a patch is larger than this the microcode file is surely corrupted */
>> +#define PATCH_MAX_SIZE_ABSOLUTE (16 * 1024 * 1024)
> 
> Please capitalize comments.

Will do.

> 
>>   * Returns the amount of bytes consumed while scanning. @desc contains all the
>>   * data we're going to use in later stages of the application.
>>   */
>> -static ssize_t parse_container(u8 *ucode, ssize_t size, struct cont_desc *desc)
>> +static size_t parse_container(u8 *ucode, size_t size, struct cont_desc *desc)
>>  {
>>  	struct equiv_cpu_entry *eq;
>> -	ssize_t orig_size = size;
>> +	size_t orig_size = size;
>>  	u32 *hdr = (u32 *)ucode;
>> +	u32 eqsize;
>>  	u16 eq_id;
>>  	u8 *buf;
> 
> So we have 'eq_id', but 'eqsize'? Why not 'eq_size' to have fewer random 
> variations of coding style?

Will change.
>>  
>> +	if (size < CONTAINER_HDR_SZ)
>> +		return 0;
> 
> The comment about CONTAINER_HDR_SZ better belongs here, where we use it.

Will move it.
 
>>  	/* Am I looking at an equivalence table header? */
>>  	if (hdr[0] != UCODE_MAGIC ||
>>  	    hdr[1] != UCODE_EQUIV_CPU_TABLE_TYPE ||
>>  	    hdr[2] == 0)
>>  		return CONTAINER_HDR_SZ;
>>  
>> +	eqsize = hdr[2];
>> +	if (eqsize < sizeof(*eq) ||
>> +	    eqsize > UCODE_EQUIV_CPU_TABLE_MAX_SIZE)
>> +		return 0;
>> +
>> +	if (size < CONTAINER_HDR_SZ + eqsize)
>> +		return 0;
>> +
>>  	buf = ucode;
>>  
>>  	eq = (struct equiv_cpu_entry *)(buf + CONTAINER_HDR_SZ);
>>  
>>  	/* Find the equivalence ID of our CPU in this table: */
>> -	eq_id = find_equiv_id(eq, desc->cpuid_1_eax);
>> +	eq_id = find_equiv_id(eq, eqsize / sizeof(*eq), desc->cpuid_1_eax);
> 
> Does eq_size have to be a multiple of sizeof(*eq)? If yes then we should probably 
> check that too.

In principle yes, but having garbage at the end of the equivalence
table in a microcode container file is a non-fatal problem.
I have mixed feelings whether we should be really strict here, especially
that the existing driver would accept such files without any error.

>> -static int install_equiv_cpu_table(const u8 *buf)
>> +static int install_equiv_cpu_table(const u8 *buf, size_t bufsize)
> 
> s/bufsize/buf_size

Will change.

>>  {
>>  	unsigned int *ibuf = (unsigned int *)buf;
>> -	unsigned int type = ibuf[1];
>> -	unsigned int size = ibuf[2];
>> +	unsigned int type, size;
>>  
>> -	if (type != UCODE_EQUIV_CPU_TABLE_TYPE || !size) {
>> -		pr_err("empty section/"
>> -		       "invalid type field in container file section header\n");
>> +	if (bufsize < CONTAINER_HDR_SZ) {
>> +		pr_err("no container header\n");
>> +		return -EINVAL;
>> +	}
>> +
>> +	type = ibuf[1];
>> +	if (type != UCODE_EQUIV_CPU_TABLE_TYPE) {
>> +		pr_err("invalid type field in container file section header\n");
>> +		return -EINVAL;
>> +	}
>> +
>> +	size = ibuf[2];
>> +	if (size < sizeof(struct equiv_cpu_entry) ||
>> +	    size > UCODE_EQUIV_CPU_TABLE_MAX_SIZE) {
>> +		pr_err("equivalent CPU table size invalid\n");
>> +		return -EINVAL;
>> +	}
>> +
>> +	if (bufsize < CONTAINER_HDR_SZ + size) {
>> +		pr_err("equivalent CPU table truncated\n");
>>  		return -EINVAL;
>>  	}
>>  
>> @@ -559,6 +599,7 @@ static int install_equiv_cpu_table(const u8 *buf)
>>  	}
>>  
>>  	memcpy(equiv_cpu_table, buf + CONTAINER_HDR_SZ, size);
>> +	equiv_cpu_table_size = size / sizeof(struct equiv_cpu_entry);
> 
> Btw., 'equiv_cpu_table_size' is an ambiguous name: often _size variables are in 
> byte units - but this is number of entries. So the name should be 
> 'equiv_cpu_table_entries' or so.

Will rename.

> Thanks,
> 
> 	Ingo
> 

Thanks,
Maciej