Re: [PATCH 3/5] crypto: ccp: Play nice with vmalloc'd memory for SEV command structs

Christophe Leroy
2021-04-04T06:48:27Z

Le 03/04/2021 à 01:37, Sean Christopherson a écrit :
> Copy vmalloc'd data to an internal buffer instead of rejecting outright
> so that callers can put SEV command buffers on the stack without running
> afoul of CONFIG_VMAP_STACK=y. Currently, the largest supported command
> takes a 68 byte buffer, i.e. pretty much every command can be put on the
> stack. Because sev_cmd_mutex is held for the entirety of a transaction,
> only a single bounce buffer is required.
>
> Use a flexible array for the buffer, sized to hold the largest known
> command. Alternatively, the buffer could be a union of all known
> command structs, but that would incur a higher maintenance cost due to
> the need to update the union for every command in addition to updating
> the existing sev_cmd_buffer_len().
>
> Align the buffer to an 8-byte boundary, mimicking the alignment that
> would be provided by the compiler if any of the structs were embedded
> directly. Note, sizeof() correctly incorporates this alignment.
>
> Cc: Brijesh Singh <brijesh.singh@amd.com>
> Cc: Borislav Petkov <bp@suse.de>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> drivers/crypto/ccp/sev-dev.c | 33 +++++++++++++++++++++++++++------
> drivers/crypto/ccp/sev-dev.h | 7 +++++++
> 2 files changed, 34 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
> index 4c513318f16a..6d5882290cfc 100644
> --- a/drivers/crypto/ccp/sev-dev.c
> +++ b/drivers/crypto/ccp/sev-dev.c
> @@ -135,13 +135,14 @@ static int sev_cmd_buffer_len(int cmd)
> return 0;
> }
>
> -static int __sev_do_cmd_locked(int cmd, void *data, int *psp_ret)
> +static int __sev_do_cmd_locked(int cmd, void *__data, int *psp_ret)
> {
> struct psp_device *psp = psp_master;
> struct sev_device *sev;
> unsigned int phys_lsb, phys_msb;
> unsigned int reg, ret = 0;
> int buf_len;
> + void *data;
>
> if (!psp || !psp->sev_data)
> return -ENODEV;
> @@ -152,11 +153,21 @@ static int __sev_do_cmd_locked(int cmd, void *data, int *psp_ret)
> sev = psp->sev_data;
>
> buf_len = sev_cmd_buffer_len(cmd);
> - if (WARN_ON_ONCE(!!data != !!buf_len))
> + if (WARN_ON_ONCE(!!__data != !!buf_len))
> return -EINVAL;
>
> - if (WARN_ON_ONCE(data && is_vmalloc_addr(data)))
> - return -EINVAL;
> + if (__data && is_vmalloc_addr(__data)) {
> + /*
> + * If the incoming buffer is virtually allocated, copy it to
> + * the driver's scratch buffer as __pa() will not work for such
> + * addresses, vmalloc_to_page() is not guaranteed to succeed,
> + * and vmalloc'd data may not be physically contiguous.
> + */
> + data = sev->cmd_buf;
> + memcpy(data, __data, buf_len);
> + } else {
> + data = __data;
> + }

I don't know how big commands are, but if they are small, it would probably be more efficient to
inconditionnally copy them to the buffer rather then doing the test.

>
> /* Get the physical address of the command buffer */
> phys_lsb = data ? lower_32_bits(__psp_pa(data)) : 0;
> @@ -204,6 +215,13 @@ static int __sev_do_cmd_locked(int cmd, void *data, int *psp_ret)
> print_hex_dump_debug("(out): ", DUMP_PREFIX_OFFSET, 16, 2, data,
> buf_len, false);
>
> + /*
> + * Copy potential output from the PSP back to __data. Do this even on
> + * failure in case the caller wants to glean something from the error.
> + */
> + if (__data && data != __data)
> + memcpy(__data, data, buf_len);
> +
> return ret;
> }
>
> @@ -978,9 +996,12 @@ int sev_dev_init(struct psp_device *psp)
> {
> struct device *dev = psp->dev;
> struct sev_device *sev;
> - int ret = -ENOMEM;
> + int ret = -ENOMEM, cmd_buf_size = 0, i;
>
> - sev = devm_kzalloc(dev, sizeof(*sev), GFP_KERNEL);
> + for (i = 0; i < SEV_CMD_MAX; i++)
> + cmd_buf_size = max(cmd_buf_size, sev_cmd_buffer_len(i));
> +
> + sev = devm_kzalloc(dev, sizeof(*sev) + cmd_buf_size, GFP_KERNEL);
> if (!sev)
> goto e_err;
>
> diff --git a/drivers/crypto/ccp/sev-dev.h b/drivers/crypto/ccp/sev-dev.h
> index dd5c4fe82914..b43283ce2d73 100644
> --- a/drivers/crypto/ccp/sev-dev.h
> +++ b/drivers/crypto/ccp/sev-dev.h
> @@ -52,6 +52,13 @@ struct sev_device {
> u8 api_major;
> u8 api_minor;
> u8 build;
> +
> + /*
> + * Buffer used for incoming commands whose physical address cannot be
> + * resolved via __pa(), e.g. stack pointers when CONFIG_VMAP_STACK=y.
> + * Note, alignment isn't strictly required.
> + */
> + u8 cmd_buf[] __aligned(8);
> };
>
> int sev_dev_init(struct psp_device *psp);
>