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Date: Wed, 3 Aug 2016 12:19:24 +0200 From: Borislav Petkov <bp@...en8.de> To: X86 ML <x86@...nel.org> Cc: LKML <linux-kernel@...r.kernel.org>, Andy Lutomirski <luto@...capital.net> Subject: [PATCH] x86/entry: Clarify the RF saving/restoring situation with SYSCALL/SYSRET From: Borislav Petkov <bp@...e.de> Clarify why exactly RF cannot be restored properly by SYSRET to avoid confusion. No functionality change. Signed-off-by: Borislav Petkov <bp@...e.de> Cc: Andy Lutomirski <luto@...capital.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
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