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Commit 3e035305 authored by Borislav Petkov's avatar Borislav Petkov Committed by Ingo Molnar
Browse files

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



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

No functionality change.

Signed-off-by: default avatarBorislav Petkov <bp@suse.de>
Acked-by: default avatarAndy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20160803171429.GA2590@nazgul.tnic


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 5cf0791d

arch/x86/entry/entry_64.S

+9 −5
Original line number Diff line number Diff line
@@ -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 and 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