crypto: ccp: Reject SEV commands with mismatching command buffer

WARN on and reject SEV commands that provide a valid data pointer, but do
not have a known, non-zero length.  And conversely, reject commands that
take a command buffer but none is provided (data is null).

Aside from sanity checking input, disallowing a non-null pointer without
a non-zero size will allow a future patch to cleanly handle vmalloc'd
data by copying the data to an internal __pa() friendly buffer.

Note, this also effectively prevents callers from using commands that
have a non-zero length and are not known to the kernel.  This is not an
explicit goal, but arguably the side effect is a good thing from the
kernel's perspective.

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>
Message-Id: <20210406224952.4177376-4-seanjc@google.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>
Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

drivers/crypto/ccp/sev-dev.c

@@ -142,6 +142,7 @@ static int __sev_do_cmd_locked(int cmd, void *data, int *psp_ret)
 	struct sev_device *sev;
 	unsigned int phys_lsb, phys_msb;
 	unsigned int reg, ret = 0;
+	int buf_len;
 
 	if (!psp || !psp->sev_data)
 		return -ENODEV;
@@ -151,6 +152,10 @@ 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))
+		return -EINVAL;
+
 	if (data && WARN_ON_ONCE(!virt_addr_valid(data)))
 		return -EINVAL;
 
@@ -162,7 +167,7 @@ static int __sev_do_cmd_locked(int cmd, void *data, int *psp_ret)
 		cmd, phys_msb, phys_lsb, psp_timeout);
 
 	print_hex_dump_debug("(in):  ", DUMP_PREFIX_OFFSET, 16, 2, data,
-			     sev_cmd_buffer_len(cmd), false);
+			     buf_len, false);
 
 	iowrite32(phys_lsb, sev->io_regs + sev->vdata->cmdbuff_addr_lo_reg);
 	iowrite32(phys_msb, sev->io_regs + sev->vdata->cmdbuff_addr_hi_reg);
@@ -198,7 +203,7 @@ static int __sev_do_cmd_locked(int cmd, void *data, int *psp_ret)
 	}
 
 	print_hex_dump_debug("(out): ", DUMP_PREFIX_OFFSET, 16, 2, data,
-			     sev_cmd_buffer_len(cmd), false);
+			     buf_len, false);
 
 	return ret;
 }