On 3/20/18, 07:58, "Andy Lutomirski" <l...@kernel.org> wrote:
>> On Mar 19, 2018, at 10:49 AM, Chang S. Bae <chang.seok....@intel.com> 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 Andi’s) didn’t have
> this comparison, didn’t need RDMSR, and didn’t 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
