Re: [PATCH 13/15] x86/fsgsbase/64: With FSGSBASE, compare GS bases on paranoid_entry

[Bae, Chang Seok]

On 3/20/18, 07:58, "Andy Lutomirski" <luto@xxxxxxxxxx> wrote:
 
>> On Mar 19, 2018, at 10:49 AM, Chang S. Bae <chang.seok.bae@xxxxxxxxx> wrote:
>>
>> When FSGSBASE is enabled, SWAPGS needs if and only if (current)
>> GS base is not the kernel's.
>>
>> FSGSBASE instructions allow user to write any value on GS base;
>> even negative. Sign check on the current GS base is not
>> sufficient. Fortunately, reading GS base is fast. Kernel GS
>> base is also known from the offset table with the CPU number.
    
> The original version of these patches (mine and Andiâs) didnât have
>  this comparison, didnât need RDMSR, and didnât allow malicious user
>  programs to cause the kernel to run decently large chunks of code with
>  the reverse of the expected GS convention. Why did you change it?
    
>    I really really don't like having a corner case like this that can and
>    will be triggered by malicious user code but that is hard to write a
>    self-test for because it involves guessing a 64-bit magic number.
>    Untestable corner cases in the x86 entry code are bad.

Originally, I took it. But since it keeps kernel GS base on the (IST) stack,
it is thought as fragile [1] AFAIK. If you really don't like this "comparison"
then GS base can be written (unconditionally) while the original GS
base stitched (like the original approach did) IMO.
    
[1] https://marc.info/?l=linux-kernel&m=151725088506036&w=3