On Thu, Feb 24, 2022 at 07:16:17AM -0800, Dave Hansen wrote: > On 2/15/22 11:22, Brian Geffon wrote: > > When eagerly switching PKRU in switch_fpu_finish() it checks that > > current is not a kernel thread as kernel threads will never use PKRU. > > It's possible that this_cpu_read_stable() on current_task > > (ie. get_current()) is returning an old cached value. To resolve this > > reference next_p directly rather than relying on current. > > > > As written it's possible when switching from a kernel thread to a > > userspace thread to observe a cached PF_KTHREAD flag and never restore > > the PKRU. And as a result this issue only occurs when switching > > from a kernel thread to a userspace thread, switching from a non kernel > > thread works perfectly fine because all that is considered in that > > situation are the flags from some other non kernel task and the next fpu > > is passed in to switch_fpu_finish(). > > > > This behavior only exists between 5.2 and 5.13 when it was fixed by a > > rewrite decoupling PKRU from xstate, in: > > commit 954436989cc5 ("x86/fpu: Remove PKRU handling from switch_fpu_finish()") > > > > Unfortunately backporting the fix from 5.13 is probably not realistic as > > it's part of a 60+ patch series which rewrites most of the PKRU handling. > > > > Fixes: 0cecca9d03c9 ("x86/fpu: Eager switch PKRU state") > > Signed-off-by: Brian Geffon <bgeffon@xxxxxxxxxx> > > Signed-off-by: Willis Kung <williskung@xxxxxxxxxx> > > Tested-by: Willis Kung <williskung@xxxxxxxxxx> > > Cc: <stable@xxxxxxxxxxxxxxx> # v5.4.x > > Cc: <stable@xxxxxxxxxxxxxxx> # v5.10.x > > I don't like forking the stable code from mainline. But I also think > that backporting the FPU reworking that changed the PKRU handling is > likely to cause more bugs in stable than it fixes. > > This fix is at least isolated to the protection keys code. > > Acked-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> now queued up, thanks. greg k-h