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: Borislav Petkov <bp@suse.de> Acked-by: Andy 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: Ingo Molnar <mingo@kernel.org>
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@ -288,11 +288,15 @@ return_from_SYSCALL_64:
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jne opportunistic_sysret_failed
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/*
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* SYSRET can't restore RF. SYSRET can restore TF, but unlike IRET,
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* restoring TF results in a trap from userspace immediately after
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* SYSRET. This would cause an infinite loop whenever #DB happens
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* with register state that satisfies the opportunistic SYSRET
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* conditions. For example, single-stepping this user code:
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* SYSCALL clears RF when it saves RFLAGS in R11 and SYSRET cannot
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* restore RF properly. If the slowpath sets it for whatever reason, we
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* need to restore it correctly.
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*
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* SYSRET can restore TF, but unlike IRET, restoring TF results in a
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* trap from userspace immediately after SYSRET. This would cause an
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* infinite loop whenever #DB happens with register state that satisfies
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* the opportunistic SYSRET conditions. For example, single-stepping
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* this user code:
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*
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* movq $stuck_here, %rcx
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* pushfq
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