x86/a.out: Clear the dump structure initially

BugLink: https://bugs.launchpad.net/bugs/1818813

commit 10970e1b upstream.

dump_thread32() in aout_core_dump() does not clear the user32 structure
allocated on the stack as the first thing on function entry.

As a result, the dump.u_comm, dump.u_ar0 and dump.signal which get
assigned before the clearing, get overwritten.

Rename that function to fill_dump() to make it clear what it does and
call it first thing.

This was caught while staring at a patch by Derek Robson
<robsonde@gmail.com>.
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Derek Robson <robsonde@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael Matz <matz@suse.de>
Cc: x86@kernel.org
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/20190202005512.3144-1-robsonde@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

arch/x86/ia32/ia32_aout.c

@@ -50,7 +50,7 @@ static unsigned long get_dr(int n)
 /*
  * fill in the user structure for a core dump..
  */
-static void dump_thread32(struct pt_regs *regs, struct user32 *dump)
+static void fill_dump(struct pt_regs *regs, struct user32 *dump)
 {
 	u32 fs, gs;
 	memset(dump, 0, sizeof(*dump));
@@ -156,10 +156,12 @@ static int aout_core_dump(struct coredump_params *cprm)
 	fs = get_fs();
 	set_fs(KERNEL_DS);
 	has_dumped = 1;
+
+	fill_dump(cprm->regs, &dump);
+
 	strncpy(dump.u_comm, current->comm, sizeof(current->comm));
 	dump.u_ar0 = offsetof(struct user32, regs);
 	dump.signal = cprm->siginfo->si_signo;
-	dump_thread32(cprm->regs, &dump);
 
 	/*
 	 * If the size of the dump file exceeds the rlimit, then see