On 24/08/2020 11:14, pet...@infradead.org wrote:
> The WARN added in commit 3c73b81a9164 ("x86/entry, selftests: Further
> improve user entry sanity checks") unconditionally triggers on my IVB
> machine because it does not support SMAP.
>
> For !SMAP hardware we patch out CLAC/STAC instructions and thus if
> userspace sets AC, we'll still have it set after entry.
Technically, you don't patch in, rather than patch out.
>
> Fixes: 3c73b81a9164 ("x86/entry, selftests: Further improve user entry sanity
> checks")
> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
> Acked-by: Andy Lutomirski <l...@kernel.org>
> ---
> arch/x86/include/asm/entry-common.h | 11 +++++++++--
> 1 file changed, 9 insertions(+), 2 deletions(-)
>
> --- a/arch/x86/include/asm/entry-common.h
> +++ b/arch/x86/include/asm/entry-common.h
> @@ -18,8 +18,15 @@ static __always_inline void arch_check_u
> * state, not the interrupt state as imagined by Xen.
> */
> unsigned long flags = native_save_fl();
> - WARN_ON_ONCE(flags & (X86_EFLAGS_AC | X86_EFLAGS_DF |
> - X86_EFLAGS_NT));
> + unsigned long mask = X86_EFLAGS_DF | X86_EFLAGS_NT;
> +
> + /*
> + * For !SMAP hardware we patch out CLAC on entry.
> + */
> + if (boot_cpu_has(X86_FEATURE_SMAP))
> + mask |= X86_EFLAGS_AC;
The Xen PV ABI clears AC on entry for 64bit guests, because Linux is
actually running in Ring 3, and therefore susceptible to #AC's which
wouldn't occur natively.
~Andrew
