From: Borislav Petkov <b...@suse.de>
Clarify why exactly RF cannot be restored properly by SYSRET to avoid
confusion.
No functionality change.
Signed-off-by: Borislav Petkov <b...@suse.de>
Cc: Andy Lutomirski <l...@amacapital.net>
---
arch/x86/entry/entry_64.S | 14 +++++++++-----
1 file changed, 9 insertions(+), 5 deletions(-)
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 8956eae04c25..80ad6d0fe38b 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -288,11 +288,15 @@ return_from_SYSCALL_64:
jne opportunistic_sysret_failed
/*
- * SYSRET can't restore RF. SYSRET can restore TF, but unlike IRET,
- * restoring TF results in a trap from userspace immediately after
- * SYSRET. This would cause an infinite loop whenever #DB happens
- * with register state that satisfies the opportunistic SYSRET
- * conditions. For example, single-stepping this user code:
+ * SYSCALL clears RF when it saves rFLAGS in R11 so SYSRET cannot
+ * restore RF properly. If the slowpath sets it for whatever reason, we
+ * need to restore it correctly.
+ *
+ * SYSRET can restore TF, but unlike IRET, restoring TF results in a
+ * trap from userspace immediately after SYSRET. This would cause an
+ * infinite loop whenever #DB happens with register state that satisfies
+ * the opportunistic SYSRET conditions. For example, single-stepping
+ * this user code:
*
* movq $stuck_here, %rcx
* pushfq
--
2.8.4
