[PATCH 3.2 90/94] ptrace: change __ptrace_unlink() to clear ->ptrace under ->siglock

From: Ben Hutchings
Date: Thu Dec 28 2017 - 13:26:20 EST


3.2.97-rc1 review patch. If anyone has any objections, please let me know.

------------------

From: Oleg Nesterov <oleg@xxxxxxxxxx>

commit 1333ab03150478df8d6f5673a91df1e50dc6ab97 upstream.

This test-case (simplified version of generated by syzkaller)

#include <unistd.h>
#include <sys/ptrace.h>
#include <sys/wait.h>

void test(void)
{
for (;;) {
if (fork()) {
wait(NULL);
continue;
}

ptrace(PTRACE_SEIZE, getppid(), 0, 0);
ptrace(PTRACE_INTERRUPT, getppid(), 0, 0);
_exit(0);
}
}

int main(void)
{
int np;

for (np = 0; np < 8; ++np)
if (!fork())
test();

while (wait(NULL) > 0)
;
return 0;
}

triggers the 2nd WARN_ON_ONCE(!signr) warning in do_jobctl_trap(). The
problem is that __ptrace_unlink() clears task->jobctl under siglock but
task->ptrace is cleared without this lock held; this fools the "else"
branch which assumes that !PT_SEIZED means PT_PTRACED.

Note also that most of other PTRACE_SEIZE checks can race with detach
from the exiting tracer too. Say, the callers of ptrace_trap_notify()
assume that SEIZED can't go away after it was checked.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: syzkaller <syzkaller@xxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
kernel/ptrace.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -77,12 +77,11 @@ void __ptrace_unlink(struct task_struct
{
BUG_ON(!child->ptrace);

- child->ptrace = 0;
child->parent = child->real_parent;
list_del_init(&child->ptrace_entry);

spin_lock(&child->sighand->siglock);
-
+ child->ptrace = 0;
/*
* Clear all pending traps and TRAPPING. TRAPPING should be
* cleared regardless of JOBCTL_STOP_PENDING. Do it explicitly.