From: Peter Zijlstra <peterz@infradead.org>
To: Yian Chen <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: Tue, 10 Jan 2023 22:04:39 +0100 [thread overview]
Message-ID: <Y73S56t/wDIGEPlK@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20230110055204.3227669-4-yian.chen@intel.com>
On Mon, Jan 09, 2023 at 09:52:00PM -0800, Yian Chen wrote:
> Most of the kernel is mapped at virtual addresses
> in the upper half of the address range. But kernel
> deliberately initialized a temporary mm area
> within the lower half of the address range
> for text poking, see commit 4fc19708b165
> ("x86/alternatives: Initialize temporary mm
> for patching").
>
> LASS stops access to a lower half address in kernel,
> and this can be deactivated if AC bit in EFLAGS
> register is set. Hence use stac and clac instructions
> around access to the address to avoid triggering a
> LASS #GP fault.
>
> Kernel objtool validation warns if the binary calls
> to a non-whitelisted function that exists outside of
> the stac/clac guard, or references any function with a
> dynamic function pointer inside the guard; see section
> 9 in the document tools/objtool/Documentation/objtool.txt.
>
> For these reasons, also considering text poking size is
> usually small, simple modifications have been done
> in function text_poke_memcpy() and text_poke_memset() to
> avoid non-whitelisted function calls inside the stac/clac
> guard.
>
> Gcc may detect and replace the target with its built-in
> functions. However, the replacement would break the
> objtool validation criteria. Hence, add compiler option
> -fno-builtin for the file.
Please reflow to 72 characters consistently, this is silly.
> Co-developed-by: Tony Luck <tony.luck@intel.com>
> Signed-off-by: Tony Luck <tony.luck@intel.com>
> Signed-off-by: Yian Chen <yian.chen@intel.com>
> ---
> arch/x86/include/asm/smap.h | 13 +++++++++++++
> arch/x86/kernel/Makefile | 2 ++
> arch/x86/kernel/alternative.c | 21 +++++++++++++++++++--
> tools/objtool/arch/x86/special.c | 2 ++
> 4 files changed, 36 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/smap.h b/arch/x86/include/asm/smap.h
> index bab490379c65..6f7ac0839b10 100644
> --- a/arch/x86/include/asm/smap.h
> +++ b/arch/x86/include/asm/smap.h
> @@ -39,6 +39,19 @@ static __always_inline void stac(void)
> alternative("", __ASM_STAC, X86_FEATURE_SMAP);
> }
>
> +/* 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. Also if you look at bit 63 as a sign bit,
it's actively wrong since -1 is lower than 0.
> +
> static __always_inline unsigned long smap_save(void)
> {
> unsigned long flags;
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index 96d51bbc2bd4..f8a455fc56a2 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -7,6 +7,8 @@ extra-y += vmlinux.lds
>
> CPPFLAGS_vmlinux.lds += -U$(UTS_MACHINE)
>
> +CFLAGS_alternative.o += -fno-builtin
> +
> ifdef CONFIG_FUNCTION_TRACER
> # Do not profile debug and lowlevel utilities
> CFLAGS_REMOVE_tsc.o = -pg
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 7d8c3cbde368..4de8b54fb5f2 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -1530,14 +1530,31 @@ __ro_after_init unsigned long poking_addr;
>
> static void text_poke_memcpy(void *dst, const void *src, size_t len)
> {
> - memcpy(dst, src, len);
> + const char *s = src;
> + char *d = dst;
> +
> + /* The parameter dst ends up referencing to the global variable
> + * poking_addr, which is mapped to the low half address space.
> + * In kernel, accessing the low half address range is prevented
> + * by LASS. So relax LASS prevention while accessing the memory
> + * range.
> + */
> + low_addr_access_begin();
> + while (len-- > 0)
> + *d++ = *s++;
> + low_addr_access_end();
> }
>
> static void text_poke_memset(void *dst, const void *src, size_t len)
> {
> int c = *(const int *)src;
> + char *d = dst;
>
> - memset(dst, c, len);
> + /* The same comment as it is in function text_poke_memcpy */
> + low_addr_access_begin();
> + while (len-- > 0)
> + *d++ = c;
> + low_addr_access_end();
> }
This is horrific tinkering :/
Also, what about the EFI mm? IIRC EFI also lives in the user address
space.
next prev parent reply other threads:[~2023-01-10 21:05 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 [this message]
2023-01-11 1:01 ` Chen, Yian
2023-01-11 9:10 ` Peter Zijlstra
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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