[PATCH v5] arm64: errata: Fix exec handling in erratum 1418040 workaround

[Catalin Marinas]

On Wed, Dec 22, 2021 at 08:12:27AM -0800, D Scott Phillips wrote:
> Catalin Marinas <catalin.marinas at arm.com> writes:
> > On Mon, Dec 20, 2021 at 03:41:14PM -0800, D Scott Phillips wrote:
> >> The erratum 1418040 workaround enables CNTVCT_EL1 access trapping in EL0
> >> when executing compat threads. The workaround is applied when switching
> >> between tasks, but the need for the workaround could also change at an
> >> exec(), when a non-compat task execs a compat binary or vice versa. Apply
> >> the workaround in arch_setup_new_exec().
> >> 
> >> This leaves a small window of time between SET_PERSONALITY and
> >> arch_setup_new_exec where preemption could occur and confuse the old
> >> workaround logic that compares TIF_32BIT between prev and next. Instead, we
> >> can just read cntkctl to make sure it's in the state that the next task
> >> needs. I measured cntkctl read time to be about the same as a mov from a
> >> general-purpose register on N1. Update the workaround logic to examine the
> >> current value of cntkctl instead of the previous task's compat state.
> >> 
> >> Fixes: d49f7d7376d0 ("arm64: Move handling of erratum 1418040 into C code")
> >> Signed-off-by: D Scott Phillips <scott at os.amperecomputing.com>
> >> Cc: <stable at vger.kernel.org> # 5.4.x
> >
> > Why 5.4? I think the fixed commit is 5.9.
> 
> d49f7d7376d0 got backported into v5.4.62's 82b05f0838aa. That looks like
> that's the farthest it's made it back. Is this the correct way to handle
> fixing the backported change, or should a separate backport be sent to
> 5.4 for the fix?

If it applies cleanly to 5.4.62, I'll just tweak the fixes line back to
5.4 (I changed it to 5.9 as I have a git hook that adjusts the Fixes
line automatically when applying). If it doesn't apply cleanly, you can
send it separately.

-- 
Catalin