[PATCH 2/2] tpm_emulator: Read control channel response in 2 passes

[Stefan Berger <stefanb@linux.ibm.com>]

Error responses from swtpm are always only 4 bytes long. Therefore, read
the entire response in 2 passes and stop if the first 4 bytes indicate an
error response with no subsequent bytes readable. Read the rest in a 2nd
pass, if needed. This avoids getting stuck while waiting for too many bytes
if only 4 bytes were sent due to an error message. The 'getting stuck'
condition has not been observed in practice so far, though.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2615
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
---
 backends/tpm/tpm_emulator.c | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/backends/tpm/tpm_emulator.c b/backends/tpm/tpm_emulator.c
index b0e2fb3fc7..1b2e890668 100644
--- a/backends/tpm/tpm_emulator.c
+++ b/backends/tpm/tpm_emulator.c
@@ -129,6 +129,9 @@ static int tpm_emulator_ctrlcmd(TPMEmulator *tpm, unsigned long cmd, void *msg,
     uint32_t cmd_no = cpu_to_be32(cmd);
     ssize_t n = sizeof(uint32_t) + msg_len_in;
     uint8_t *buf = NULL;
+    ptm_res res;
+    off_t o = 0;
+    int to_read;
 
     WITH_QEMU_LOCK_GUARD(&tpm->mutex) {
         buf = g_alloca(n);
@@ -140,11 +143,29 @@ static int tpm_emulator_ctrlcmd(TPMEmulator *tpm, unsigned long cmd, void *msg,
             return -1;
         }
 
-        if (msg_len_out != 0) {
-            n = qemu_chr_fe_read_all(dev, msg, msg_len_out);
+        /*
+         * Any response will be at least 4 bytes long. Error responses are only
+         * 4 bytes (=sizeof(ptm_res)), though. Therefore, read response in 2
+         * passes, returning when an error response is encountered.
+         */
+        to_read = sizeof(res);
+        while (msg_len_out > 0) {
+            n = qemu_chr_fe_read_all(dev, (uint8_t *)msg + o, to_read);
             if (n <= 0) {
                 return -1;
             }
+            msg_len_out -= to_read;
+            if (msg_len_out == 0) {
+                return 0;
+            }
+
+            memcpy(&res, msg, sizeof(res));
+            if (res) {
+                return 0;
+            }
+
+            o = to_read;
+            to_read = msg_len_out;
         }
     }
 
-- 
2.46.2