Re: [PATCH] x86/smap: Fix the smap_save() asm

[Andy Lutomirski]

> On Sep 15, 2020, at 2:24 PM, Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
> 
> On Tue, Sep 15, 2020 at 1:56 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>> 
>> The old smap_save() code was:
>> 
>>  pushf
>>  pop %0
>> 
>> with %0 defined by an "=rm" constraint.  This is fine if the
>> compiler picked the register option, but it was incorrect with an
>> %rsp-relative memory operand.
> 
> It is incorrect because ... (I think mentioning the point about the
> red zone would be good, unless there were additional concerns?)

This isn’t a red zone issue — it’s a just-plain-wrong issue.  The popf is storing the result in the wrong place in memory — it’s RSP-relative, but RSP is whatever the compiler thinks it should be minus 8, because the compiler doesn’t know that pushfq changed RSP.

> 
> This is something we should fix.  Bill, James, and I are discussing
> this internally.  Thank you for filing a bug; I owe you a beer just
> for that.

I’m looking forward to the day that beers can be exchanged in person again :)

> 
>> 
>> Fixes: e74deb11931f ("x86/uaccess: Introduce user_access_{save,restore}()")
>> Cc: stable@xxxxxxxxxxxxxxx
>> Reported-by: Bill Wendling <morbo@xxxxxxxxxx> # I think
> 
> LOL, yes, the comment can be dropped...though I guess someone else may
> have reported the problem to Bill?

The “I think” is because I’m not sure whether Bill reported this particular issue. But I’m fine with dropping it.