Re: [PATCH v2] [gdb] Fix heap-use-after-free in select_event_lwp

[Pedro Alves <pedro@palves.net>]

On 2024-02-19 15:04, Tom de Vries wrote:
> On 2/9/24 16:46, Pedro Alves wrote:
>> On 2024-01-23 11:48, Tom de Vries wrote:
>> It sounds like we were about to report a stop for a thread that isn't marked as stopped?
>> Now it looks to me that _that_ would be the bug to fix.
> 
> This patch adds an assert to catch the bug you mention, and a fix in wait_lwp:
> ...
> diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
> index e91c57ba239..5022da9abd2 100644
> --- a/gdb/linux-nat.c
> +++ b/gdb/linux-nat.c
> @@ -2210,6 +2210,7 @@ wait_lwp (struct lwp_info *lp)
>           core.  Store it in lp->waitstatus, because lp->status
>           would be ambiguous (W_EXITCODE(0,0) == 0).  */
>            lp->waitstatus = host_status_to_waitstatus (status);
> +          lp->stopped = 1;
>            return 0;
>          }
> 
> @@ -3368,6 +3369,7 @@ linux_nat_wait_1 (ptid_t ptid, struct target_waitstatus *ourstatus,
>      }
> 
>    gdb_assert (lp);
> +  gdb_assert (lp->stopped);
> 
>    status = lp->status;
>    lp->status = 0;
> ...
> 
> This fixes the problem observed in the PR, and passes testing on x86_64-linux and aarch64-linux.
> 
> WDYT?

Ah, thanks, so it was an exiting lwp that we're going to report the exit for.

Yes, this is the right thing to do.  We do the same in linux_nat_filter_event:

  /* This LWP is stopped now.  (And if dead, this prevents it from
     ever being continued.)  */
  lp->stopped = 1;


gdbserver has a mark_lwp_dead function to centralize setting up various lwp flags such that the
rest of the code doesn't mishandle them, and it seems like a good idea to do a similar thing in
gdb as well.  Like below.  I ran the testsuite and saw no regressions.

I tried to reproduce the gdb.base/vfork-follow-parent.exp issue described in the bug, but when I
run that with a gdb built with -O0 -fsanitize=thread, the testcase just seemingly hangs starting
up until I ctrl-c:

 Running /home/pedro/rocm/gdb/build-master-sanitize-thread/gdb/testsuite/../../../src/gdb/testsuite/gdb.base/vfork-follow-parent.exp ...
 WARNING: Couldn't set the height to 0
 WARNING: Couldn't set the width to 0.
 ^Cgot a INT signal, interrupted by user

and when I look at gdb.log, I see it is full of the same report over and over:

...
WARNING: ThreadSanitizer: signal-unsafe call inside of a signal (pid=3086741)
    #0 free ../../../../src/libsanitizer/tsan/tsan_interceptors_posix.cpp:711 (libtsan.so.0+0x37ab8)
    #1 __gconv_close_transform iconv/gconv_db.c:800 (libc.so.6+0x2be3a)
    #2 ext_lang_initialization() ../../src/gdb/extension.c:339 (gdb+0x5c6a3f)
    #3 captured_main_1 ../../src/gdb/main.c:1058 (gdb+0x7c94da)
    #4 captured_main ../../src/gdb/main.c:1329 (gdb+0x7ca7be)
    #5 gdb_main(captured_main_args*) ../../src/gdb/main.c:1358 (gdb+0x7ca886)
    #6 main ../../src/gdb/gdb.c:39 (gdb+0xf98b3)

SUMMARY: ThreadSanitizer: signal-unsafe call inside of a signal iconv/gconv_db.c:800 in __gconv_close_transform
...


Does this patch work for you?

From 8cbc62be0dab8bb4b09fe05a52a348ddbfb2e33c Mon Sep 17 00:00:00 2001
From: Pedro Alves <pedro@palves.net>
Date: Wed, 21 Feb 2024 16:23:55 +0000
Subject: [PATCH] mark_lwp_dead

Change-Id: I4a6169976f89bf714c478cbb2b7d4c32365e62a9
---
 gdb/linux-nat.c | 34 +++++++++++++++++++++++++---------
 1 file changed, 25 insertions(+), 9 deletions(-)

diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
index e91c57ba239..b3d319e0a82 100644
--- a/gdb/linux-nat.c
+++ b/gdb/linux-nat.c
@@ -2123,6 +2123,27 @@ wait_for_signal ()
     }
 }
 
+/* Mark LWP dead, with STATUS as exit status pending to report
+   later.  */
+
+static void
+mark_lwp_dead (lwp_info *lp, int status)
+{
+  /* Store the exit status lp->waitstatus, because lp->status would be
+     ambiguous (W_EXITCODE(0,0) == 0).  */
+  lp->waitstatus = host_status_to_waitstatus (status);
+
+  /* If we're processing LP's status, there should be no other event
+     already recorded as pending.  */
+  gdb_assert (lp->status == 0);
+
+  /* Dead LWP's aren't expected to reported a pending sigstop.  */
+  lp->signalled = 0;
+
+  /* Prevent trying to stop it.  */
+  lp->stopped = 1;
+}
+
 /* Wait for LP to stop.  Returns the wait status, or 0 if the LWP has
    exited.  */
 
@@ -2207,9 +2228,8 @@ wait_lwp (struct lwp_info *lp)
 
 	      /* If this is the leader exiting, it means the whole
 		 process is gone.  Store the status to report to the
-		 core.  Store it in lp->waitstatus, because lp->status
-		 would be ambiguous (W_EXITCODE(0,0) == 0).  */
-	      lp->waitstatus = host_status_to_waitstatus (status);
+		 core.  */
+	      mark_lwp_dead (lp, status);
 	      return 0;
 	    }
 
@@ -3013,12 +3033,7 @@ linux_nat_filter_event (int lwpid, int status)
       linux_nat_debug_printf ("LWP %ld exited (resumed=%d)",
 			      lp->ptid.lwp (), lp->resumed);
 
-      /* Dead LWP's aren't expected to reported a pending sigstop.  */
-      lp->signalled = 0;
-
-      /* Store the pending event in the waitstatus, because
-	 W_EXITCODE(0,0) == 0.  */
-      lp->waitstatus = host_status_to_waitstatus (status);
+      mark_lwp_dead (lp, status);
       return;
     }
 
@@ -3368,6 +3383,7 @@ linux_nat_wait_1 (ptid_t ptid, struct target_waitstatus *ourstatus,
     }
 
   gdb_assert (lp);
+  gdb_assert (lp->stopped);
 
   status = lp->status;
   lp->status = 0;

base-commit: 99eeecc8d276e5af745e48825d66efff693a7678
-- 
2.43.2