x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue

[karo-tx-linux.git] / arch / x86 / ia32 / ia32entry.S

diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
index a821b1cd4fa7a2748cce60ba986ef63d27efb131..72bf2680f819ad06d8c0461f6ad7ac6d12042166 100644 (file)
--- a/arch/x86/ia32/ia32entry.S
+++ b/arch/x86/ia32/ia32entry.S
@@ -427,6 +427,13 @@ sysretl_from_sys_call:
         * cs and ss are loaded from MSRs.
         * (Note: 32bit->32bit SYSRET is different: since r11
         * does not exist, it merely sets eflags.IF=1).
+        *
+        * NB: On AMD CPUs with the X86_BUG_SYSRET_SS_ATTRS bug, the ss
+        * descriptor is not reinitialized.  This means that we must
+        * avoid SYSRET with SS == NULL, which could happen if we schedule,
+        * exit the kernel, and re-enter using an interrupt vector.  (All
+        * interrupt entries on x86_64 set SS to NULL.)  We prevent that
+        * from happening by reloading SS in __switch_to.
         */
        USERGS_SYSRET32