On 24.08.20 17:58, Andrew Cooper wrote:
On 24/08/2020 16:21, pet...@infradead.org wrote:
On Mon, Aug 24, 2020 at 03:22:06PM +0100, Andrew Cooper wrote:
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.
True.
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.
So do you then want it to be something like:
if (boot_cpu_has(X86_FEATURE_SMAP) ||
(IS_ENABLED(CONFIG_64_BIT) && boot_cpu_has(X86_FEATURE_XENPV)))
? Or are you fine with the proposed?
Dealers choice, but this option would be slightly better overall.
(Are there any other cases where Linux will be running in Ring 3? I
haven't been paying attention to recent changes in PVOps.)
I'm not aware of any other case running kernel code in ring 3.
Juergen
