Re: [PATCH v4 07/10] x86: narrow out of bounds syscalls to sys_read under speculation

[Luis Henriques]

On Thu, Jan 18, 2018 at 04:02:21PM -0800, Dan Williams wrote:
> The syscall table base is a user controlled function pointer in kernel
> space. Like, 'get_user, use 'MASK_NOSPEC' to prevent any out of bounds
> speculation. While retpoline prevents speculating into the user
> controlled target it does not stop the pointer de-reference, the concern
> is leaking memory relative to the syscall table base.

This patch seems to cause a regression.  An easy way to reproduce what
I'm seeing is to run the samples/statx/test-statx.  Here's what I see
when I have this patchset applied:

# ./test-statx /tmp
statx(/tmp) = -1
/tmp: Bad file descriptor

Reverting this single patch seems to fix it.

Cheers,
--
Luís

> 
> Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
>  arch/x86/entry/entry_64.S   |    2 ++
>  arch/x86/include/asm/smap.h |    9 ++++++++-
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 4f8e1d35a97c..2320017077d4 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -35,6 +35,7 @@
>  #include <asm/asm.h>
>  #include <asm/smap.h>
>  #include <asm/pgtable_types.h>
> +#include <asm/smap.h>
>  #include <asm/export.h>
>  #include <asm/frame.h>
>  #include <asm/nospec-branch.h>
> @@ -264,6 +265,7 @@ entry_SYSCALL_64_fastpath:
>  	cmpl	$__NR_syscall_max, %eax
>  #endif
>  	ja	1f				/* return -ENOSYS (already in pt_regs->ax) */
> +	MASK_NOSPEC %r11 %rax			/* sanitize syscall_nr wrt speculation */
>  	movq	%r10, %rcx
>  
>  	/*
> diff --git a/arch/x86/include/asm/smap.h b/arch/x86/include/asm/smap.h
> index 2b4ad4c6a226..3b5b2cf58dc6 100644
> --- a/arch/x86/include/asm/smap.h
> +++ b/arch/x86/include/asm/smap.h
> @@ -35,7 +35,14 @@
>   * this directs the cpu to speculate with a NULL ptr rather than
>   * something targeting kernel memory.
>   *
> - * assumes CF is set from a previous 'cmp TASK_addr_limit, %ptr'
> + * In the syscall entry path it is possible to speculate past the
> + * validation of the system call number. Use MASK_NOSPEC to sanitize the
> + * syscall array index to zero (sys_read) rather than an arbitrary
> + * target.
> + *
> + * assumes CF is set from a previous 'cmp' i.e.:
> + *     cmp TASK_addr_limit, %ptr
> + *     cmp __NR_syscall_max, %idx
>   */
>  .macro MASK_NOSPEC mask val
>  	sbb \mask, \mask
> 
>