The "Hg" GDB packet is used to select the current thread, and can fail. GDB doesn't not check for failure and emits further packets that can access and dereference s->c_cpu or s->g_cpu.
Add a check that returns "E22" (EINVAL) when those pointers are not set. Peter Maydell reported: GDB doesn't check the "Hg" packet failures and emit a "qXfer:features:read" packet which accesses th Looking at the protocol trace from the gdb end: (gdb) set debug remote 1 (gdb) target remote :1234 Remote debugging using :1234 Sending packet: $qSupported:multiprocess+;swbreak+;hwbreak+;qRelocInsn+;fork-events+;vfork-events+;exec-events+;vContSupported+;QThreadEvents+;no-resumed+;xmlRegisters=i386#6a...Ack Packet received: PacketSize=1000;qXfer:features:read+;multiprocess+ Packet qSupported (supported-packets) is supported Sending packet: $vMustReplyEmpty#3a...Ack Packet received: Sending packet: $Hgp0.0#ad...Ack Packet received: E22 Sending packet: $qXfer:features:read:target.xml:0,ffb#79...Ack [here is where QEMU crashes] What seems to happen is that GDB's attempt to set the current thread with the "Hg" command fails because gdb_get_cpu() fails, and so we return an E22 error status. GDB (incorrectly) ignores this and issues a general command anyway, and then QEMU crashes because we don't handle s->g_cpu being NULL in this command's handling code. Fixes: c145eeae1cc Reported-by: Peter Maydell <[email protected]> Signed-off-by: Philippe Mathieu-Daudé <[email protected]> --- RFC because these are many if..break but I can't think of a cleaner way today. The protocol isn't specific about the error, can it be "E03" for ESRCH (No such process)? --- gdbstub.c | 40 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 40 insertions(+) diff --git a/gdbstub.c b/gdbstub.c index bfc7afb509..480005b094 100644 --- a/gdbstub.c +++ b/gdbstub.c @@ -1306,6 +1306,10 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf) put_packet(s, "OK"); break; case '?': + if (!s->c_cpu) { + put_packet(s, "E22"); + break; + } /* TODO: Make this return the correct value for user-mode. */ snprintf(buf, sizeof(buf), "T%02xthread:%s;", GDB_SIGNAL_TRAP, gdb_fmt_thread_id(s, s->c_cpu, thread_id, sizeof(thread_id))); @@ -1394,6 +1398,10 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf) /* Detach packet */ pid = 1; + if (!s->c_cpu || !s->g_cpu) { + put_packet(s, "E22"); + break; + } if (s->multiprocess) { unsigned long lpid; if (*p != ';') { @@ -1429,6 +1437,10 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf) put_packet(s, "OK"); break; case 's': + if (!s->s_cpu) { + put_packet(s, "E22"); + break; + } if (*p != '\0') { addr = strtoull(p, (char **)&p, 16); gdb_set_cpu_pc(s, addr); @@ -1441,6 +1453,10 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf) target_ulong ret; target_ulong err; + if (!s->s_cpu) { + put_packet(s, "E22"); + break; + } ret = strtoull(p, (char **)&p, 16); if (*p == ',') { p++; @@ -1463,6 +1479,10 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf) } break; case 'g': + if (!s->g_cpu) { + put_packet(s, "E22"); + break; + } cpu_synchronize_state(s->g_cpu); len = 0; for (addr = 0; addr < s->g_cpu->gdb_num_g_regs; addr++) { @@ -1473,6 +1493,10 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf) put_packet(s, buf); break; case 'G': + if (!s->g_cpu) { + put_packet(s, "E22"); + break; + } cpu_synchronize_state(s->g_cpu); registers = mem_buf; len = strlen(p) / 2; @@ -1485,6 +1509,10 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf) put_packet(s, "OK"); break; case 'm': + if (!s->g_cpu) { + put_packet(s, "E22"); + break; + } addr = strtoull(p, (char **)&p, 16); if (*p == ',') p++; @@ -1504,6 +1532,10 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf) } break; case 'M': + if (!s->g_cpu) { + put_packet(s, "E22"); + break; + } addr = strtoull(p, (char **)&p, 16); if (*p == ',') p++; @@ -1642,6 +1674,10 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf) put_packet(s, "OK"); break; } else if (strcmp(p,"C") == 0) { + if (!s->g_cpu) { + put_packet(s, "E22"); + break; + } /* * "Current thread" remains vague in the spec, so always return * the first thread of the current process (gdb returns the @@ -1745,6 +1781,10 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf) const char *xml; target_ulong total_len; + if (!s->g_cpu) { + put_packet(s, "E22"); + break; + } process = gdb_get_cpu_process(s, s->g_cpu); cc = CPU_GET_CLASS(s->g_cpu); if (cc->gdb_core_xml_file == NULL) { -- 2.17.2
