"sysenter_fix_flags" detour does not need to be convoluted anymore,
straigten it up. However, we still use this:
jnz 2f
jmp sysenter_jumps_here
2: ...
instead of this:
jz sysenter_jumps_here
...
because "cold" conditional forward branch is predicted not taken
by most CPUs - exactly what we want. Latter version would get it wrong.
Reinstate "why we use SYSRETL instead of SYSEXIT" comment.
Signed-off-by: Denys Vlasenko <dvlas...@redhat.com>
CC: Ingo Molnar <mi...@kernel.org>
CC: Linus Torvalds <torva...@linux-foundation.org>
CC: Krzysztof A. Sobiecki <sob...@gmail.com>
CC: Steven Rostedt <rost...@goodmis.org>
CC: Borislav Petkov <b...@alien8.de>
CC: "H. Peter Anvin" <h...@zytor.com>
CC: Andy Lutomirski <l...@amacapital.net>
CC: Oleg Nesterov <o...@redhat.com>
CC: Frederic Weisbecker <fweis...@gmail.com>
CC: Alexei Starovoitov <a...@plumgrid.com>
CC: Will Drewry <w...@chromium.org>
CC: Kees Cook <keesc...@chromium.org>
CC: x...@kernel.org
CC: linux-kernel@vger.kernel.org
---
arch/x86/entry/entry_64_compat.S | 26 +++++++++++++++++---------
1 file changed, 17 insertions(+), 9 deletions(-)
diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 73b56a5..bd3664f 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -98,20 +98,28 @@ ENTRY(entry_SYSENTER_compat)
ASM_CLAC
/*
- * Sysenter doesn't filter flags, so we need to clear NT
- * ourselves. To save a few cycles, we can check whether
- * NT was set instead of doing an unconditional popfq.
+ * Sysenter doesn't filter flags, so we need to clear NT ourselves.
*/
testl $X86_EFLAGS_NT, EFLAGS(%rsp)
- jnz sysenter_fix_flags
-sysenter_flags_fixed:
+ jnz 2f
jmp sysenter_jumps_here
-
-sysenter_fix_flags:
+2:
pushq $(X86_EFLAGS_IF|X86_EFLAGS_FIXED)
popfq
- jmp sysenter_flags_fixed
-
+ jmp sysenter_jumps_here
+ /*
+ * SYSEXIT insn is not obviously safe for 64-bit kernels --
+ * an NMI between STI and SYSEXIT has poorly specified behavior,
+ * and NMI followed by an IRQ with usergs is fatal.
+ * So we just pretend we're using SYSEXIT but we really use
+ * SYSRETL instead. (Yes, SYSRETL works even on Intel CPUs.)
+ * We do that by reusing the entire SYSCALL code path:
+ * the jump above takes us there.
+ *
+ * The difference of SYSENTER 32-bit ABI versus SYSCALL
+ * is that SYSENTER ABI does not promise to preserve EDX and EBP,
+ * SYSCALL does.
+ */
ENDPROC(entry_SYSENTER_compat)
/*
--
1.8.1.4
--
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