Patch "ARC: signal handling robustify" has been added to the 3.10-stable tree

This is a note to let you know that I've just added the patch titled

ARC: signal handling robustify

to the 3.10-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
arc-signal-handling-robustify.patch
and it can be found in the queue-3.10 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Vineet Gupta <[email protected]>
Date: Thu, 26 Mar 2015 11:14:41 +0530
Subject: ARC: signal handling robustify

From: Vineet Gupta <[email protected]>

commit e4140819dadc3624accac8294881bca8a3cba4ed upstream.

A malicious signal handler / restorer can DOS the system by fudging the user regs saved on stack, causing weird things such as sigreturn returning to user mode PC but cpu state still being kernel mode....

Ensure that in sigreturn path status32 always has U bit; any other bogosity (gargbage PC etc) will be taken care of by normal user mode exceptions mechanisms.

Reproducer signal handler:

    void handle_sig(int signo, siginfo_t *info, void *context)
    {
	ucontext_t *uc = context;
	struct user_regs_struct *regs = &(uc->uc_mcontext.regs);

	regs->scratch.status32 = 0;
    }

Before the fix, kernel would go off to weeds like below:

    --------->8-----------
    [ARCLinux]$ ./signal-test
    Path: /signal-test
    CPU: 0 PID: 61 Comm: signal-test Not tainted 4.0.0-rc5+ #65
    task: 8f177880 ti: 5ffe6000 task.ti: 8f15c000

    [ECR   ]: 0x00220200 => Invalid Write @ 0x00000010 by insn @ 0x00010698
    [EFA   ]: 0x00000010
    [BLINK ]: 0x2007c1ee
    [ERET  ]: 0x10698
    [STAT32]: 0x00000000 :                                   <--------
    BTA: 0x00010680	 SP: 0x5ffe7e48	 FP: 0x00000000
    LPS: 0x20003c6c	LPE: 0x20003c70	LPC: 0x00000000
    ...
    --------->8-----------

---
 arch/arc/kernel/signal.c |   20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

@@ -131,6 +131,15 @@ SYSCALL_DEFINE0(rt_sigreturn)
 	/* Don't restart from sigreturn */
 	syscall_wont_restart(regs);
 
+	/*
+	 * Ensure that sigreturn always returns to user mode (in case the
+	 * regs saved on user stack got fudged between save and sigreturn)
+	 * Otherwise it is easy to panic the kernel with a custom
+	 * signal handler and/or restorer which clobberes the status32/ret
+	 * to return to a bogus location in kernel mode.
+	 */
+	regs->status32 |= STATUS_U_MASK;
+
 	return regs->r0;
 
 badframe:

@@ -234,8 +243,11 @@ setup_rt_frame(int signo, struct k_sigac
 
 	/*
 	 * handler returns using sigreturn stub provided already by userpsace
+	 * If not, nuke the process right away
 	 */
-	BUG_ON(!(ka->sa.sa_flags & SA_RESTORER));
+	if(!(ka->sa.sa_flags & SA_RESTORER))
+		return 1;
+
 	regs->blink = (unsigned long)ka->sa.sa_restorer;
 
 	/* User Stack for signal handler will be above the frame just carved */

@@ -302,12 +314,12 @@ handle_signal(unsigned long sig, struct
 	      struct pt_regs *regs)
 {
 	sigset_t *oldset = sigmask_to_save();
-	int ret;
+	int failed;
 
 	/* Set up the stack frame */
-	ret = setup_rt_frame(sig, ka, info, oldset, regs);
+	failed = setup_rt_frame(sig, ka, info, oldset, regs);
 
-	if (ret)
+	if (failed)
 		force_sigsegv(sig, current);
 	else
 		signal_delivered(sig, info, ka, regs, 0);

Patches currently in stable-queue which might be from [email protected] are

queue-3.10/arc-signal-handling-robustify.patch