[PATCH] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)

From: Denys Vlasenko
Date: Mon Mar 09 2015 - 10:06:24 EST

Old code was trying to avoid having three branch insns,
but instead it has a chain of six insns where each insn
depends on previos one.

And it was touching PT_OLDSS(%esp) unconditionally, even when it may
contain bogus data. Elsewhere we have to jump thru hoops
just to make sure here PT_OLDSS(%esp) is at least in a valid page.

All this just to have one branch instead of three?

The new code simply checks each condition.
All three checks can run in parallel on an out-of-order CPU.
Most of the time, none of branches will be taken.

Comparison of object code:
1e6: 8b 44 24 38 mov 0x38(%esp),%eax
1ea: 8a 64 24 40 mov 0x40(%esp),%ah
1ee: 8a 44 24 34 mov 0x34(%esp),%al
1f2: 25 03 04 02 00 and $0x20403,%eax
1f7: 3d 03 04 00 00 cmp $0x403,%eax
1fc: 74 0f je 20d <ldt_ss>
1e6: 0f ba 64 24 38 11 btl $0x11,0x38(%esp)
1ec: 72 0e jb 1fc <restore_nocheck>
1ee: f6 44 24 34 03 testb $0x3,0x34(%esp)
1f3: 74 07 je 1fc <restore_nocheck>
1f5: f6 44 24 40 04 testb $0x4,0x40(%esp)
1fa: 75 0f jne 20b <ldt_ss>

Patch is run-tested.

Signed-off-by: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
CC: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: Borislav Petkov <bp@xxxxxxxxx>
CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
CC: Oleg Nesterov <oleg@xxxxxxxxxx>
CC: Frederic Weisbecker <fweisbec@xxxxxxxxx>
CC: Alexei Starovoitov <ast@xxxxxxxxxxxx>
CC: Will Drewry <wad@xxxxxxxxxxxx>
CC: Kees Cook <keescook@xxxxxxxxxxxx>
CC: x86@xxxxxxxxxx
CC: linux-kernel@xxxxxxxxxxxxxxx
arch/x86/kernel/entry_32.S | 17 ++++++++---------
1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index e33ba51..0a4996b 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -516,16 +516,15 @@ restore_all:
#ifdef CONFIG_X86_ESPFIX32
- movl PT_EFLAGS(%esp), %eax # mix EFLAGS, SS and CS
- # Warning: PT_OLDSS(%esp) contains the wrong/random values if we
- # are returning to the kernel.
- # See comments in process.c:copy_thread() for details.
- movb PT_OLDSS(%esp), %ah
- movb PT_CS(%esp), %al
- andl $(X86_EFLAGS_VM | (SEGMENT_TI_MASK << 8) | SEGMENT_RPL_MASK), %eax
- cmpl $((SEGMENT_LDT << 8) | USER_RPL), %eax
- je ldt_ss # returning to user-space with LDT SS
+ btl $X86_EFLAGS_VM_BIT,PT_EFLAGS(%esp)
+ jc restore_nocheck # VM set, not it
+ testb $3,PT_CS(%esp)
+ jz restore_nocheck # CPL0, not it
+ # Note: we access PT_OLDSS only when we know it exists.
+ # If PT_CS is from CPL0, it does not.
+ jnz ldt_ss # returning to user-space with LDT SS
RESTORE_REGS 4 # skip orig_eax/error_code

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