[PATCH 1/3] x86_64, entry: Remove a bogus ret_from_fork optimization

From: Andy Lutomirski <luto@amacapital.net>
To: x86@kernel.org, linux-kernel@vger.kernel.org
Cc: Denys Vlasenko <dvlasenk@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	Andy Lutomirski <luto@amacapital.net>
Subject: [PATCH 1/3] x86_64, entry: Remove a bogus ret_from_fork optimization
Date: Tue, 24 Feb 2015 16:01:37 -0800	[thread overview]
Message-ID: <1b78c9e7b89e1c9481cd096fe57c2fb653eb9679.1424822291.git.luto@amacapital.net> (raw)
In-Reply-To: <cover.1424822291.git.luto@amacapital.net>
In-Reply-To: <cover.1424822291.git.luto@amacapital.net>

ret_from_fork checks TIF_IA32 to determine whether pt_regs and the
related state make sense for ret_from_sys_call.  This is entirely
the wrong check.  TS_COMPAT would make a little more sense, but
there's really no point in keeping this optimization at all.

This fixes a return to the wrong user CS if we came from int 0x80 in
a 64-bit task.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
 arch/x86/kernel/entry_64.S | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 9ebaf63ba182..4ee9a2315f80 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -334,11 +334,14 @@ ENTRY(ret_from_fork)
 	testl $3, CS-ARGOFFSET(%rsp)		# from kernel_thread?
 	jz   1f
 
-	testl $_TIF_IA32, TI_flags(%rcx)	# 32-bit compat task needs IRET
-	jnz  int_ret_from_sys_call
-
-	RESTORE_TOP_OF_STACK %rdi, -ARGOFFSET
-	jmp ret_from_sys_call			# go to the SYSRET fastpath
+	/*
+	 * By the time we get here, we have no idea whether our pt_regs,
+	 * ti flags, and ti status came from the 64-bit SYSCALL fast path,
+	 * the slow path, or one of the ia32entry paths.
+	 * Use int_ret_from_sys_call to return, since it can safely handle
+	 * all of the above.
+	 */
+	jmp  int_ret_from_sys_call
 
 1:
 	subq $REST_SKIP, %rsp	# leave space for volatiles
-- 
2.3.0

