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

[Brian Gerst]

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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