[PATCH] x86/asm/entry/64: better check for canonical address

[Denys Vlasenko]

This change makes the check exact (no more false positives
on "negative" addresses).

It isn't really important to be fully correct here -
almost all addresses we'll ever see will be userspace ones,
but OTOH it looks to be cheap enough:
the new code uses two more ALU ops but preserves %rcx,
allowing to not reload it from pt_regs->cx again.
On disassembly level, the changes are:

cmp %rcx,0x80(%rsp) -> mov 0x80(%rsp),%r11; cmp %rcx,%r11
shr $0x2f,%rcx      -> shl $0x10,%rcx; sar $0x10,%rcx; cmp %rcx,%r11
mov 0x58(%rsp),%rcx -> (eliminated)

On 03/26/2015 07:45 PM, Andy Lutomirski wrote:
> I suspect that the two added ALU ops are free for all practical
> purposes, and the performance of this path isn't *that* critical.
>
> If anyone is running with vsyscall=native because they need the
> performance, then this would be a big win.  Otherwise I don't have a
> real preference.  Anyone else have any thoughts here?
>
> Let me just run through the math quickly to make sure I believe all the numbers:
>
> Canonical addresses either start with 17 zeros or 17 ones.
>
> In the old code, we checked that the top (64-47) = 17 bits were all
> zero.  We did this by shifting right by 47 bits and making sure that
> nothing was left.
>
> In the new code, we're shifting left by (64 - 48) = 16 bits and then
> signed shifting right by the same amount, this propagating the 17th
> highest bit to all positions to its left.  If we get the same value we
> started with, then we're good to go.
>
> So it looks okay to me.
>
> IOW, the new code extends the optimization correctly to one more case
> (native vsyscalls or the really weird corner case of returns to
> emulated vsyscalls, although that should basically never happen) at
> the cost of two probably-free ALU ops.

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
---

Changes since last submission: expanded commit message with Andy's reply
as requested by Ingo.

 arch/x86/kernel/entry_64.S | 23 ++++++++++++-----------
 1 file changed, 12 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 3bdfdcd..e952f6b 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -410,26 +410,27 @@ syscall_return:
 	 * a completely clean 64-bit userspace context.
 	 */
 	movq RCX(%rsp),%rcx
-	cmpq %rcx,RIP(%rsp)		/* RCX == RIP */
+	movq RIP(%rsp),%r11
+	cmpq %rcx,%r11			/* RCX == RIP */
 	jne opportunistic_sysret_failed
 
 	/*
 	 * On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP
 	 * in kernel space.  This essentially lets the user take over
-	 * the kernel, since userspace controls RSP.  It's not worth
-	 * testing for canonicalness exactly -- this check detects any
-	 * of the 17 high bits set, which is true for non-canonical
-	 * or kernel addresses.  (This will pessimize vsyscall=native.
-	 * Big deal.)
+	 * the kernel, since userspace controls RSP.
 	 *
-	 * If virtual addresses ever become wider, this will need
+	 * If width of "canonical tail" ever becomes variable, this will need
 	 * to be updated to remain correct on both old and new CPUs.
 	 */
 	.ifne __VIRTUAL_MASK_SHIFT - 47
 	.error "virtual address width changed -- SYSRET checks need update"
 	.endif
-	shr $__VIRTUAL_MASK_SHIFT, %rcx
-	jnz opportunistic_sysret_failed
+	/* Change top 16 bits to be the sign-extension of 47th bit */
+	shl	$(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
+	sar	$(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
+	/* If this changed %rcx, it was not canonical */
+	cmpq	%rcx, %r11
+	jne	opportunistic_sysret_failed
 
 	cmpq $__USER_CS,CS(%rsp)	/* CS must match SYSRET */
 	jne opportunistic_sysret_failed
@@ -466,8 +467,8 @@ syscall_return:
 	 */
 syscall_return_via_sysret:
 	CFI_REMEMBER_STATE
-	/* r11 is already restored (see code above) */
-	RESTORE_C_REGS_EXCEPT_R11
+	/* rcx and r11 are already restored (see code above) */
+	RESTORE_C_REGS_EXCEPT_RCX_R11
 	movq RSP(%rsp),%rsp
 	USERGS_SYSRET64
 	CFI_RESTORE_STATE
-- 
1.8.1.4

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