From f48d9663f19afb88ac0e45b825da523180f1f9c0 Mon Sep 17 00:00:00 2001 From: Alexander Nyberg Date: Thu, 5 May 2005 16:15:03 -0700 Subject: [PATCH] [PATCH] x86 stack initialisation fix The recent change fix-crash-in-entrys-restore_all.patch childregs->esp = esp; p->thread.esp = (unsigned long) childregs; - p->thread.esp0 = (unsigned long) (childregs+1); + p->thread.esp0 = (unsigned long) (childregs+1) - 8; p->thread.eip = (unsigned long) ret_from_fork; introduces an inconsistency between esp and esp0 before the task is run the first time. esp0 is no longer the actual start of the stack, but 8 bytes off. This shows itself clearly in a scenario when a ptracer that is set to also ptrace eventual children traces program1 which then clones thread1. Now the ptracer wants to modify the registers of thread1. The x86 ptrace implementation bases it's knowledge about saved user-space registers upon p->thread.esp0. But this will be a few bytes off causing certain writes to the kernel stack to overwrite a saved kernel function address making the kernel when actually running thread1 jump out into user-space. Very spectacular. The testcase I've used is: /* start with strace -f ./a.out */ #include #include void *do_thread(void *p) { for (;;); } int main() { pthread_t one; pthread_create(&one, NULL, &do_thread, NULL); for (;;); return 0; } So, my solution is to instead of just adjusting esp0 that creates an inconsitent state I adjust where the user-space registers are saved with -8 bytes. This gives us the wanted extra bytes on the start of the stack and esp0 is now correct. This solves the issues I saw from the original testcase from Mateusz Berezecki and has survived testing here. I think this should go into -mm a round or two first however as there might be some cruft around depending on pt_regs lying on the start of the stack. That however would have broken with the first change too! It's actually a 2-line diff but I had to move the comment of why the -8 bytes are there a few lines up. Thanks to Zwane for helping me with this. Signed-off-by: Alexander Nyberg Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- arch/i386/kernel/process.c | 13 +++++++------ 1 file changed, 7 insertions(+), 6 deletions(-) diff --git a/arch/i386/kernel/process.c b/arch/i386/kernel/process.c index 85bd56d44314..96e3ea6b17c7 100644 --- a/arch/i386/kernel/process.c +++ b/arch/i386/kernel/process.c @@ -400,11 +400,6 @@ int copy_thread(int nr, unsigned long clone_flags, unsigned long esp, int err; childregs = ((struct pt_regs *) (THREAD_SIZE + (unsigned long) p->thread_info)) - 1; - *childregs = *regs; - childregs->eax = 0; - childregs->esp = esp; - - p->thread.esp = (unsigned long) childregs; /* * The below -8 is to reserve 8 bytes on top of the ring0 stack. * This is necessary to guarantee that the entire "struct pt_regs" @@ -415,7 +410,13 @@ int copy_thread(int nr, unsigned long clone_flags, unsigned long esp, * "struct pt_regs" is possible, but they may contain the * completely wrong values. */ - p->thread.esp0 = (unsigned long) (childregs+1) - 8; + childregs = (struct pt_regs *) ((unsigned long) childregs - 8); + *childregs = *regs; + childregs->eax = 0; + childregs->esp = esp; + + p->thread.esp = (unsigned long) childregs; + p->thread.esp0 = (unsigned long) (childregs+1); p->thread.eip = (unsigned long) ret_from_fork; -- GitLab