On Tue, Feb 15, 2022 at 2:45 PM Greg KH gregkh@linuxfoundation.org wrote:
On Tue, Feb 15, 2022 at 11:22:33AM -0800, 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@google.com Signed-off-by: Willis Kung williskung@google.com Tested-by: Willis Kung williskung@google.com Cc: stable@vger.kernel.org # v5.4.x Cc: stable@vger.kernel.org # v5.10.x
arch/x86/include/asm/fpu/internal.h | 13 ++++++++----- arch/x86/kernel/process_32.c | 6 ++---- arch/x86/kernel/process_64.c | 6 ++---- 3 files changed, 12 insertions(+), 13 deletions(-)
So this is ONLY for 5.4.y and 5.10.y? I'm really really loath to take non-upstream changes as 95% of the time (really) it goes wrong.
That's correct, this bug was introduced in 5.2 and that code was completely refactored in 5.13 indirectly fixing it.
How was this tested, and what do the maintainers of this subsystem think? And will you be around to fix the bugs in this when they are found?
This has been trivial to reproduce, I've used a small repro which I've put here: https://gist.github.com/bgaff/9f8cbfc8dd22e60f9492e4f0aff8f04f , I also was able to reproduce this using the protection_keys self tests on a 11th Gen Core i5-1135G7. I'm happy to commit to addressing any bugs that may appear. I'll see what the maintainers say, but there is also a smaller fix that just involves using this_cpu_read() in switch_fpu_finish() for this specific issue, although that approach isn't as clean.
And finally, what's wrong with 60+ patches to backport to fix a severe issue? What's preventing that from happening? Did you try it and see what exactly is involved?
It was quite a substantial rewrite of that code with fixes layered on since.
thanks,
greg k-h