Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue

From: Brian Gerst
Date: Fri Apr 24 2015 - 13:58:05 EST

On Fri, Apr 24, 2015 at 1:41 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, Apr 24, 2015 at 10:33 AM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
>> To clarify, I was thinking of the CONFIG_PREEMPT case. A nested
>> interrupt wouldn't change SS, and IST interrupts can't schedule.
> It has absolutely nothing to do with nested interrupts or CONFIG_PREEMPT.
> The problem happens simply because
> - process A does a system call SS=__KERNEL_DS
> - the system call sleeps for whatever reason. SS is still __KERNEL_DS
> - process B runs, returns to user space, and takes an interrupt. Now SS=0
> - process B is about to return to user space (where the interrupt
> happened), but we schedule as part of that regular user-space return.
> SS=0
> - process A returns to user space using sysret, the SS selector
> becomes __USER_DS, but the cached descriptor remains non-present
> Notice? No nested interrupts, no CONFIG_PREEMPT, nothing special at all.
> The reason Luto's patch fixes the problem is that now the scheduling
> from B back to A will reload SS, making it __KERNEL_DS, but more
> importantly, fixing the cached descriptor to be the usual present flag
> one, which is what the AMD sysret instruction needs.
> Or do I misunderstand what you are talking about?
> Linus

Your explanation is correct. I meant that this can happen even if
CONFIG_PREEMPT is disabled. I just took "preemption" to mean kernel
preemption, not normal scheduling.

Brian Gerst
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