On Fri, 31 Aug 2007, Rusty Russell wrote:
> On Thu, 2007-08-30 at 21:44 -0700, Linus Torvalds wrote: > > > > Hmm.. This *really* cannot happen with a normal kernel - it implies that > > the stack has crossed into an invalid page. > > AFAICT, a corrupt stack could lead us to touch a page which isn't > mapped. If we assume the stack isn't corrupt, we don't have to do the > valid_stack_ptr() check at all... Fair enough. That said, you seem to see this even without a corrupt stack. > > Why is that allowed with lguest? What kind of code could validly *ever* > > come in here and cause problems? > > head.S pushes a "$0" on the stack to stop the unwinder, lguest doesn't. The unwinder should stop when it sees an invalid frame pointer, and even without the push 0 I'd have expected it to be invalid. But I suspect lguest triggers another thing: you actually make the stack start at the *very*top* of the stack area. Afaik, normal x86 does not. A normal x86 kernel will start off with a pt_regs[] setup, I think - ie the kernel stack is always set up so that it has the "return to user mode" information. And *that* difference may be what triggers this for lguest, even though it can never trigger for a "real" kernel. But your patch does improve the sanity checking of the frame pointer. That said, I think the following patch improves it more: does this also work for you? (Totally untested, but it looks like the RightThing(tm) to do) Linus --- diff --git a/arch/i386/kernel/traps.c b/arch/i386/kernel/traps.c index cfffe3d..b9998f3 100644 --- a/arch/i386/kernel/traps.c +++ b/arch/i386/kernel/traps.c @@ -100,10 +100,10 @@ asmlinkage void machine_check(void); int kstack_depth_to_print = 24; static unsigned int code_bytes = 64; -static inline int valid_stack_ptr(struct thread_info *tinfo, void *p) +static inline int valid_stack_ptr(struct thread_info *tinfo, void *p, unsigned size) { return p > (void *)tinfo && - p < (void *)tinfo + THREAD_SIZE - 3; + p <= (void *)tinfo + THREAD_SIZE - size; } static inline unsigned long print_context_stack(struct thread_info *tinfo, @@ -113,7 +113,7 @@ static inline unsigned long print_context_stack(struct thread_info *tinfo, unsigned long addr; #ifdef CONFIG_FRAME_POINTER - while (valid_stack_ptr(tinfo, (void *)ebp)) { + while (valid_stack_ptr(tinfo, (void *)ebp, 2*sizeof(unsigned long))) { unsigned long new_ebp; addr = *(unsigned long *)(ebp + 4); ops->address(data, addr); @@ -129,7 +129,7 @@ static inline unsigned long print_context_stack(struct thread_info *tinfo, ebp = new_ebp; } #else - while (valid_stack_ptr(tinfo, stack)) { + while (valid_stack_ptr(tinfo, stack, sizeof(*stack))) { addr = *stack++; if (__kernel_text_address(addr)) ops->address(data, addr); - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/