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Subject[tip:x86/urgent] x86/asm/entry/64: Remove a bogus 'ret_from_fork' optimization
Commit-ID:  956421fbb74c3a6261903f3836c0740187cf038b
Gitweb: http://git.kernel.org/tip/956421fbb74c3a6261903f3836c0740187cf038b
Author: Andy Lutomirski <luto@amacapital.net>
AuthorDate: Thu, 5 Mar 2015 01:09:44 +0100
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 5 Mar 2015 01:12:23 +0100

x86/asm/entry/64: Remove a bogus 'ret_from_fork' optimization

'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>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/r/4710be56d76ef994ddf59087aad98c000fbab9a4.1424989793.git.luto@amacapital.net
[ Backported from tip:x86/asm. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
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 10074ad..1d74d16 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -269,11 +269,14 @@ 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

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