Re: Segfault on the i386 enter instruction

From: Chuck Ebbert
Date: Wed May 17 2006 - 04:24:40 EST

In-Reply-To: <200605161132.18610.ak@xxxxxxx>

On Tue, 16 May 2006 11:32:18 +0200, Andi Kleen wrote:

> On Tuesday 16 May 2006 04:29, Chuck Ebbert wrote:
> > In-Reply-To: <44676F42.7080907@xxxxxxxx>
> >
> > On Sun, 14 May 2006 21:56:18 +0400, Stas Sergeev wrote:
> >
> > > Andi Kleen wrote:
> > > > Handling it like you expect would require to disassemble
> > > > the function in the page fault handler and it's probably not
> > > > worth doing that for this weird case.
> > > Just wondering, is this case really that weird?
> > > In fact, the check against %esp that the kernel
> > > does, looks strange. I realize that it can catch a
> > > (very rare) user-space bug of accessing below %esp, but
> > > other than that it looks redundant (IMHO) and as soon as
> > > it triggers the false-positives, what is it really good for?
> >
> > I can't get a SIGSEGV on any native i386 kernel, not even when
> > running on AMD64. It only happens on native x86_64 kernels.
> I reproduced the original SIGSEGV on several i386 kernels.

OK, I got SIGSEGV on a 2.6.9 i386 kernel in addition to ia32 mode on x86_64.
But it doesn't happen on any recent 2.6, even with "enter $65535,$0".
Digging, I found the stack vma is 22 pages (88k) on recent i386
kernels while it's only 8k on 2.6.9 and 12k in x86_64 ia32 emulation.
So you have to go deeper into the stack before you will hit this on
recent i386 kernels.

> > Intel says nothing about a write check. Is that a mistake in the manual
> > or is that something only AMD64 does, and then only in long mode?
> In 98+% of all cases when Intel and AMD documentation differ
> in subtle detail it's a documentation bug.

Yeah, that's why it's good to have both on hand. But sometimes it can be
hard to tell which one is wrong. :)

BTW one easy way to fix this bug would be to enlarge the window for
access below the stack pointer to allow for the largest possible enter
instruction, i.e. "enter $65535,$31". On x86_64 that would be 65536+256
instead of the current 128 bytes.

"The x86 isn't all that complex -- it just doesn't make a lot of sense."
-- Mike Johnson
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