xtensa: reset windowbase/windowstart when cloning the VM

When we copy a user thread with CLONE_VM, we also have to reset
windowbase and windowstart to start a pristine stack frame. Otherwise,
overflows can happen using the address 0 as the stack pointer.
Also add a special case for vfork, which continues on the
parent stack until it calls execve. Because this could be a call8, we
need to spill the stack pointer of the previus frame (if still 'live' in
the register file).

Signed-off-by: Chris Zankel <chris@zankel.net>

1 files changed, 25 insertions, 1 deletions

diff --git a/arch/xtensa/kernel/process.c b/arch/xtensa/kernel/process.c
index 1908f6642d31..341c151af919 100644
--- a/arch/xtensa/kernel/process.c
+++ b/arch/xtensa/kernel/process.c
@@ -220,8 +220,32 @@ int copy_thread(unsigned long clone_flags, unsigned long usp,
         if (user_mode(regs)) {
 
                 childregs->areg[1] = usp;
+
+                /* When sharing memory with the parent thread, the child
+                   usually starts on a pristine stack, so we have to reset
+                   windowbase, windowstart and wmask.
+                   (Note that such a new thread is required to always create
+                   an initial call4 frame)
+                   The exception is vfork, where the new thread continues to
+                   run on the parent's stack until it calls execve. This could
+                   be a call8 or call12, which requires a legal stack frame
+                   of the previous caller for the overflow handlers to work.
+                   (Note that it's always legal to overflow live registers).
+                   In this case, ensure to spill at least the stack pointer
+                   of that frame. */
+
                 if (clone_flags & CLONE_VM) {
-                        childregs->wmask = 1;   /* can't share live windows */
+                        /* check that caller window is live and same stack */
+                        int len = childregs->wmask & ~0xf;
+                        if (regs->areg[1] == usp && len != 0) {
+                                int callinc = (regs->areg[0] >> 30) & 3;
+                                int caller_ars = XCHAL_NUM_AREGS - callinc * 4;
+                                put_user(regs->areg[caller_ars+1],
+                                         (unsigned __user*)(usp - 12));
+                        }
+                        childregs->wmask = 1;
+                        childregs->windowstart = 1;
+                        childregs->windowbase = 0;
                 } else {
                         int len = childregs->wmask & ~0xf;
                         memcpy(&childregs->areg[XCHAL_NUM_AREGS - len/4],