Hi Mike, Zhi,
On 01/03/2023 23:20, Zhi Wang wrote:
> On Mon, 20 Feb 2023 12:38:43 -0600
> Michael Roth <michael.roth@xxxxxxx> wrote:
>
>> From: Brijesh Singh <brijesh.singh@xxxxxxx>
>>
>> Add support to decrypt guest encrypted memory. These API interfaces can
>> be used for example to dump VMCBs on SNP guest exit.
>>
>
> What kinds of check will be applied from firmware when VMM decrypts this
> page? I suppose there has to be kinda mechanism to prevent VMM to decrypt
> any page in the guest. It would be nice to have some introduction about
> it in the comments.
>
The SNP ABI spec says (section 8.27.2 SNP_DBG_DECRYPT):
The firmware checks that the guest's policy allows debugging. If not,
the firmware returns POLICY_FAILURE.
and in the Guest Policy (section 4.3):
Bit 19 - DEBUG
0: Debugging is disallowed.
1: Debugging is allowed.
In the kernel, that firmware error code is defined as
SEV_RET_POLICY_FAILURE.
>> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
>> Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
>> [mdr: minor commit fixups]
>> Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
>> ---
>> drivers/crypto/ccp/sev-dev.c | 32 ++++++++++++++++++++++++++++++++
>> include/linux/psp-sev.h | 22 ++++++++++++++++++++--
>> 2 files changed, 52 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
>> index e65563bc8298..bf5167b2acfc 100644
>> --- a/drivers/crypto/ccp/sev-dev.c
>> +++ b/drivers/crypto/ccp/sev-dev.c
>> @@ -2017,6 +2017,38 @@ int sev_guest_df_flush(int *error)
>> }
>> EXPORT_SYMBOL_GPL(sev_guest_df_flush);
>>
>> +int snp_guest_dbg_decrypt_page(u64 gctx_pfn, u64 src_pfn, u64 dst_pfn, int *error)
>> +{
>> + struct sev_data_snp_dbg data = {0};
>> + struct sev_device *sev;
>> + int ret;
>> +
>> + if (!psp_master || !psp_master->sev_data)
>> + return -ENODEV;
>> +
>> + sev = psp_master->sev_data;
>> +
>> + if (!sev->snp_initialized)
>> + return -EINVAL;
>> +
>> + data.gctx_paddr = sme_me_mask | (gctx_pfn << PAGE_SHIFT);
>> + data.src_addr = sme_me_mask | (src_pfn << PAGE_SHIFT);
>> + data.dst_addr = sme_me_mask | (dst_pfn << PAGE_SHIFT);
I guess this works, but I wonder why we need to turn on sme_me_mask on
teh dst_addr. I thought that the firmware decrypts the guest page
(src_addr) to a plaintext page. Couldn't find this requirement in the
SNP spec.
>> +
>> + /* The destination page must be in the firmware state. */
>> + if (rmp_mark_pages_firmware(data.dst_addr, 1, false))
>> + return -EIO;
>> +
>> + ret = sev_do_cmd(SEV_CMD_SNP_DBG_DECRYPT, &data, error);
>> +
>> + /* Restore the page state */
>> + if (snp_reclaim_pages(data.dst_addr, 1, false))
>> + ret = -EIO;
>> +
>> + return ret;
>> +}
>> +EXPORT_SYMBOL_GPL(snp_guest_dbg_decrypt_page);
>> +
>> int snp_guest_ext_guest_request(struct sev_data_snp_guest_request *data,
>> unsigned long vaddr, unsigned long *npages, unsigned long *fw_err)
>> {
>> diff --git a/include/linux/psp-sev.h b/include/linux/psp-sev.h
>> index 81bafc049eca..92116e2b74fd 100644
>> --- a/include/linux/psp-sev.h
>> +++ b/include/linux/psp-sev.h
>> @@ -710,7 +710,6 @@ struct sev_data_snp_dbg {
>> u64 gctx_paddr; /* In */
>> u64 src_addr; /* In */
>> u64 dst_addr; /* In */
>> - u32 len; /* In */
>> } __packed;
The comment above this ^^^ struct still lists the 'len' field, and also
calls the first field 'handle' instead of 'gctx_paddr'.
Also - why is this change happening in this patch? Why was the incorrect
'len' field added in the first place in "[PATCH RFC v8 20/56]
crypto:ccp: Define the SEV-SNP commands" ? (the comment fixes should
probably go there too).
>>
>> /**
>> @@ -913,13 +912,27 @@ int sev_guest_decommission(struct sev_data_decommission *data, int *error);
>> * @error: SEV command return code
>> *
>> * Returns:
>> + * 0 if the sev successfully processed the command
>> + * -%ENODEV if the sev device is not available
>> + * -%ENOTSUPP if the sev does not support SEV
>> + * -%ETIMEDOUT if the sev command timed out
>> + * -%EIO if the sev returned a non-zero return code
>> + */
I think that if the word 'sev' would be 'SEV' in this comment, the diff
will be a bit less misleading (basically this patch should not introduce
changes to sev_do_cmd).
-Dov
>> +int sev_do_cmd(int cmd, void *data, int *psp_ret);
>> +
>> +/**
>> + * snp_guest_dbg_decrypt_page - perform SEV SNP_DBG_DECRYPT command
>> + *
>> + * @sev_ret: sev command return code
>> + *
>> + * Returns:
>> * 0 if the SEV successfully processed the command
>> * -%ENODEV if the SEV device is not available
>> * -%ENOTSUPP if the SEV does not support SEV
>> * -%ETIMEDOUT if the SEV command timed out
>> * -%EIO if the SEV returned a non-zero return code
>> */
>> -int sev_do_cmd(int cmd, void *data, int *psp_ret);
>> +int snp_guest_dbg_decrypt_page(u64 gctx_pfn, u64 src_pfn, u64 dst_pfn, int *error);
>>
>> void *psp_copy_user_blob(u64 uaddr, u32 len);
>> void *snp_alloc_firmware_page(gfp_t mask);
>> @@ -987,6 +1000,11 @@ static inline void *psp_copy_user_blob(u64 __user uaddr, u32 len) { return ERR_P
>>
>> void snp_mark_pages_offline(unsigned long pfn, unsigned int npages) {}
>>
>> +static inline int snp_guest_dbg_decrypt_page(u64 gctx_pfn, u64 src_pfn, u64 dst_pfn, int *error)
>> +{
>> + return -ENODEV;
>> +}
>> +
>> static inline void *snp_alloc_firmware_page(gfp_t mask)
>> {
>> return NULL;
>
