From: Peter Zijlstra <peterz@infradead.org>
To: "Chen, Yian" <yian.chen@intel.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
Andy Lutomirski <luto@kernel.org>,
Dave Hansen <dave.hansen@linux.intel.com>,
Ravi Shankar <ravi.v.shankar@intel.com>,
Tony Luck <tony.luck@intel.com>,
Sohil Mehta <sohil.mehta@intel.com>,
Paul Lai <paul.c.lai@intel.com>
Subject: Re: [PATCH 3/7] x86/cpu: Disable kernel LASS when patching kernel alternatives
Date: Wed, 11 Jan 2023 10:10:24 +0100 [thread overview]
Message-ID: <Y759AJ/0N9fqwDED@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <34600873-3716-eedd-84d0-dada88dc1a70@intel.com>
On Tue, Jan 10, 2023 at 05:01:59PM -0800, Chen, Yian wrote:
> > > +/* Deactivate/activate LASS via AC bit in EFLAGS register */
> > > +static __always_inline void low_addr_access_begin(void)
> > > +{
> > > + /* Note: a barrier is implicit in alternative() */
> > > + alternative("", __ASM_STAC, X86_FEATURE_LASS);
> > > +}
> > > +
> > > +static __always_inline void low_addr_access_end(void)
> > > +{
> > > + /* Note: a barrier is implicit in alternative() */
> > > + alternative("", __ASM_CLAC, X86_FEATURE_LASS);
> > > +}
> >
> > Can't say I like the name.
> Indeed, there are alternative ways to name the functions. for example,
> enable_kernel_lass()/disable_kernel_lass(), or simply keep no change to use
> stac()/clac().
>
> I choose this name because it is straight forward to the purpose and helps
> in understanding when to use the functions.
Given we normally refer to the kernel address space as negative, it is
somewhat confusing.
lass_access_{begin,end}()
or something might be better names.
> Also if you look at bit 63 as a sign bit,
> > it's actively wrong since -1 is lower than 0.
> This could be a trade-off choice. While considering address manipulation
> and calculation, it is likely an unsigned. I would be happy to get input for
> better naming.
Note that Documentation/x86/x86_64/mm.rst likes to refer to the kernel
range as negative.
Also things like making a canonical address use sign-extention.
> > This is horrific tinkering :/
> >
> This part seems difficult to have a perfect solution since function call or
> function pointer inside the guard of instruction stac and clac will trigger
> objtool warning (stated the reasons in the commit msg)
Yeah, I'm familiar with that objtool warning, I wrote that particular check :-)
Still, this is a horrific solution. Adding something like
__inline_mem{set,cpy}() is a much saner option.
Something a little like the completely untested below.
---
diff --git a/arch/x86/include/asm/string_64.h b/arch/x86/include/asm/string_64.h
index 888731ccf1f6..f43fc2d9b182 100644
--- a/arch/x86/include/asm/string_64.h
+++ b/arch/x86/include/asm/string_64.h
@@ -23,6 +23,16 @@ extern void *memcpy(void *to, const void *from, size_t len);
#endif
extern void *__memcpy(void *to, const void *from, size_t len);
+static __always_inline void *__inline_memcpy(void *to, const void *from, size_t len)
+{
+ void *ret = to;
+
+ asm volatile ("rep movsb"
+ : "+D" (to), "+S" (from), "+c" (len)
+ : : "memory");
+ return ret;
+}
+
#define __HAVE_ARCH_MEMSET
#if defined(__SANITIZE_MEMORY__) && defined(__NO_FORTIFY)
extern void *__msan_memset(void *s, int c, size_t n);
@@ -33,6 +43,17 @@ void *memset(void *s, int c, size_t n);
#endif
void *__memset(void *s, int c, size_t n);
+static __always_inline void *__inline_memset(void *s, int v, size_t n)
+{
+ void *ret = s;
+
+ asm volatile("rep stosb"
+ : "+D" (s), "+c" (n)
+ : "a" ((uint8_t)v)
+ : "memory");
+ return ret;
+}
+
#define __HAVE_ARCH_MEMSET16
static inline void *memset16(uint16_t *s, uint16_t v, size_t n)
{
next prev parent reply other threads:[~2023-01-11 9:15 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-10 5:51 [PATCH 0/7] Enable LASS (Linear Address space Separation) Yian Chen
2023-01-10 5:51 ` [PATCH 1/7] x86/cpu: Enumerate LASS CPUID and CR4 bits Yian Chen
2023-01-10 20:14 ` Sohil Mehta
2023-01-11 0:13 ` Dave Hansen
2023-01-11 23:23 ` Chen, Yian
2023-01-12 0:06 ` Luck, Tony
2023-01-12 0:15 ` Chen, Yian
2023-01-11 19:21 ` Chen, Yian
2023-01-10 5:51 ` [PATCH 2/7] x86: Add CONFIG option X86_LASS Yian Chen
2023-01-10 21:05 ` Sohil Mehta
2023-01-12 0:13 ` Chen, Yian
2023-01-10 5:52 ` [PATCH 3/7] x86/cpu: Disable kernel LASS when patching kernel alternatives Yian Chen
2023-01-10 21:04 ` Peter Zijlstra
2023-01-11 1:01 ` Chen, Yian
2023-01-11 9:10 ` Peter Zijlstra [this message]
2023-01-10 22:41 ` Sohil Mehta
2023-01-12 0:27 ` Chen, Yian
2023-01-12 0:37 ` Dave Hansen
2023-01-12 18:36 ` Chen, Yian
2023-01-12 18:48 ` Dave Hansen
2023-02-01 2:25 ` Sohil Mehta
2023-02-01 18:20 ` Dave Hansen
2023-02-01 2:10 ` Sohil Mehta
2023-01-10 5:52 ` [PATCH 4/7] x86/vsyscall: Setup vsyscall to compromise LASS protection Yian Chen
2023-01-11 0:34 ` Sohil Mehta
2023-01-12 1:43 ` Chen, Yian
2023-01-12 2:49 ` Sohil Mehta
2023-01-21 4:09 ` Andy Lutomirski
2023-01-10 5:52 ` [PATCH 5/7] x86/cpu: Enable LASS (Linear Address Space Separation) Yian Chen
2023-01-11 22:22 ` Sohil Mehta
2023-01-12 17:56 ` Chen, Yian
2023-01-12 18:17 ` Dave Hansen
2023-01-13 1:17 ` Sohil Mehta
2023-01-13 19:39 ` Sohil Mehta
2023-01-10 5:52 ` [PATCH 6/7] x86/cpu: Set LASS as pinning sensitive CR4 bit Yian Chen
2023-01-10 5:52 ` [PATCH 7/7] x86/kvm: Expose LASS feature to VM guest Yian Chen
2023-02-07 3:21 ` Wang, Lei
2023-02-09 17:18 ` Sean Christopherson
2023-01-10 19:48 ` [PATCH 0/7] Enable LASS (Linear Address space Separation) Sohil Mehta
2023-01-10 22:57 ` Dave Hansen
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