Merge branch 'linus' into x86/asm, before applying dependent patch

[cascardo/linux.git] / arch / x86 / kernel / entry_64.S

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 60705b0..7423e3e 100644 (file)
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -295,6 +295,15 @@ system_call_fastpath:
         * rflags from r11 (but RF and VM bits are forced to 0),
         * cs and ss are loaded from MSRs.
         * Restoration of rflags re-enables interrupts.
+        *
+        * NB: On AMD CPUs with the X86_BUG_SYSRET_SS_ATTRS bug, the ss
+        * descriptor is not reinitialized.  This means that we should
+        * 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.  (Actually
+        * detecting the failure in 64-bit userspace is tricky but can be
+        * done.)
         */
        USERGS_SYSRET64