[PATCH] late checking of permissions during PTRACE_ATTACH

[Alexey Dobriyan <adobriyan@xxxxx>]

ptrace_attach() does permissions check _after_ actual attaching. Given
that utrace_attach is quite non-trivial operation there is no way such
ordering should be allowed -- the following program should crash the box
in less than second.

Something like: ./a.out $(pidof syslogd)

#include <stdlib.h>
#include <sys/ptrace.h>

int main(int argc, char *argv[])
{
	int pid = atoi(argv[1]);

	while (1)
		ptrace(PTRACE_ATTACH, pid, NULL, NULL);
	return 0;
}

Unable to handle kernel NULL pointer dereference at 0000000000000000 RIP: 
 [<0000000000000000>]
PGD 17f3ed067 PUD 17ec80067 PMD 0 
Oops: 0010 [1] PREEMPT SMP 
CPU 1 
Modules linked in: rtc
Pid: 400, comm: udevd Not tainted 2.6.23-rc1-utrace #1
RIP: 0010:[<0000000000000000>]  [<0000000000000000>]
RSP: 0000:ffff81017ee4dcb0  EFLAGS: 00010202
RAX: ffffffff803cdbe0 RBX: ffff81017dfd6350 RCX: ffff81017f7a3080
RDX: 0000000000000000 RSI: ffff81017f7a3080 RDI: ffff81017dfd6350
RBP: 0000000000000021 R08: 0000000000000038 R09: ffffffff8025145e
R10: 0000000000000007 R11: 0000000000000246 R12: 0000000000000020
R13: ffff81017f7a3080 R14: ffff81017ee4def8 R15: ffff81017ecca6e0
FS:  00002b98567836d0(0000) GS:ffff81017fc017c0(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 0000000000000000 CR3: 000000017f1e9000 CR4: 00000000000006a0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process udevd (pid: 400, threadinfo ffff81017ee4c000, task ffff81017f7a3080)
Stack:  ffffffff8025058d ffff81017ecca6f0 ffff81017f7a3080 ffff81017ecca6e0
 0000000000000007 ffff81017ee4dd58 ffff81017ee4def8 ffff81017ee4de78
 ffffffff802506b5 ffff81017ecca700 ffff81017f7a3080 0000000000000007
Call Trace:
 [<ffffffff8025058d>] report_quiescent+0x36/0x13c
 [<ffffffff802506b5>] utrace_quiescent+0x22/0x215
 [<ffffffff80251882>] utrace_get_signal+0x513/0x576
 [<ffffffff802347b1>] get_signal_to_deliver+0x10c/0x3d3
 [<ffffffff8020abe5>] do_notify_resume+0x9c/0x728
 [<ffffffff803b96e0>] _spin_unlock_irqrestore+0x3d/0x69
 [<ffffffff80242d7f>] trace_hardirqs_on+0x116/0x13a
 [<ffffffff803b96ec>] _spin_unlock_irqrestore+0x49/0x69
 [<ffffffff803b8d35>] trace_hardirqs_on_thunk+0x35/0x37
 [<ffffffff80242d7f>] trace_hardirqs_on+0x116/0x13a
 [<ffffffff8020bca6>] retint_signal+0x46/0x90


Code:  Bad RIP value.
RIP  [<0000000000000000>]
 RSP <ffff81017ee4dcb0>
CR2: 0000000000000000
Kernel panic - not syncing: Fatal exception





NOTE, NOTE, NOTE: this is exactly same backtrace as in
"dead_engine_ops vs engine->flags" race, so it's reproducable :)

If by some miracle RHEL5 will also be patched, closes
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=245735

Signed-off-by: Alexey Dobriyan <adobriyan@xxxxx>
---

 kernel/ptrace.c |   42 +++++++++++++++++++++---------------------
 1 file changed, 21 insertions(+), 21 deletions(-)

--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -327,6 +327,8 @@ static int ptrace_attach(struct task_struct *task)
 		goto bad;
 	if (!task->mm)		/* kernel threads */
 		goto bad;
+	if (!ptrace_may_attach(task))
+		goto bad;
 
 	pr_debug("%d ptrace_attach %d state %lu exit_code %x\n",
 		 current->pid, task->pid, task->state, task->exit_code);
@@ -346,29 +348,27 @@ static int ptrace_attach(struct task_struct *task)
 		 current->pid, task->pid, task->state, task->exit_code);
 
 	NO_LOCKS;
-	if (ptrace_may_attach(task)) {
-		state = ptrace_setup(task, engine, current, 0,
-				     capable(CAP_SYS_PTRACE));
-		if (IS_ERR(state))
-			retval = PTR_ERR(state);
-		else {
-			retval = ptrace_setup_finish(task, state);
+	state = ptrace_setup(task, engine, current, 0,
+			capable(CAP_SYS_PTRACE));
+	if (IS_ERR(state))
+		retval = PTR_ERR(state);
+	else {
+		retval = ptrace_setup_finish(task, state);
 
-			pr_debug("%d ptrace_attach %d after ptrace_update (%d)"
-				 " %lu exit_code %x\n",
-				 current->pid, task->pid, retval,
-				 task->state, task->exit_code);
+		pr_debug("%d ptrace_attach %d after ptrace_update (%d)"
+				" %lu exit_code %x\n",
+				current->pid, task->pid, retval,
+				task->state, task->exit_code);
 
-			if (retval) {
-				/*
-				 * It died before we enabled any callbacks.
-				 */
-				if (retval == -EALREADY)
-					retval = -ESRCH;
-				BUG_ON(retval != -ESRCH);
-				ptrace_state_unlink(state);
-				ptrace_done(state);
-			}
+		if (retval) {
+			/*
+			 * It died before we enabled any callbacks.
+			 */
+			if (retval == -EALREADY)
+				retval = -ESRCH;
+			BUG_ON(retval != -ESRCH);
+			ptrace_state_unlink(state);
+			ptrace_done(state);
 		}
 	}
 	NO_LOCKS;