[PATCH] x86/entry: Clarify the RF saving/restoring situation with SYSCALL/SYSRET

[Borislav Petkov]

From: Borislav Petkov <bp@xxxxxxx>

Clarify why exactly RF cannot be restored properly by SYSRET to avoid
confusion.

No functionality change.

Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
---
 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