Re: [PATCH v2 3/8] x86/vsyscall: Show something useful on a read fault

[Kees Cook <keescook@chromium.org>]

On Wed, Jun 26, 2019 at 09:45:04PM -0700, Andy Lutomirski wrote:
> Signed-off-by: Andy Lutomirski <luto@kernel.org>

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
>  arch/x86/entry/vsyscall/vsyscall_64.c | 19 ++++++++++++++++++-
>  arch/x86/include/asm/vsyscall.h       |  6 ++++--
>  arch/x86/mm/fault.c                   | 11 +++++------
>  3 files changed, 27 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c b/arch/x86/entry/vsyscall/vsyscall_64.c
> index fedd7628f3a6..9c58ab807aeb 100644
> --- a/arch/x86/entry/vsyscall/vsyscall_64.c
> +++ b/arch/x86/entry/vsyscall/vsyscall_64.c
> @@ -117,7 +117,8 @@ static bool write_ok_or_segv(unsigned long ptr, size_t size)
>  	}
>  }
>  
> -bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
> +bool emulate_vsyscall(unsigned long error_code,
> +		      struct pt_regs *regs, unsigned long address)
>  {
>  	struct task_struct *tsk;
>  	unsigned long caller;
> @@ -126,6 +127,22 @@ bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
>  	long ret;
>  	unsigned long orig_dx;
>  
> +	/* Write faults or kernel-privilege faults never get fixed up. */
> +	if ((error_code & (X86_PF_WRITE | X86_PF_USER)) != X86_PF_USER)
> +		return false;
> +
> +	if (!(error_code & X86_PF_INSTR)) {
> +		/* Failed vsyscall read */
> +		if (vsyscall_mode == EMULATE)
> +			return false;
> +
> +		/*
> +		 * User code tried and failed to read the vsyscall page.
> +		 */
> +		warn_bad_vsyscall(KERN_INFO, regs, "vsyscall read attempt denied -- look up the vsyscall kernel parameter if you need a workaround");
> +		return false;
> +	}
> +
>  	/*
>  	 * No point in checking CS -- the only way to get here is a user mode
>  	 * trap to a high address, which means that we're in 64-bit user code.
> diff --git a/arch/x86/include/asm/vsyscall.h b/arch/x86/include/asm/vsyscall.h
> index b986b2ca688a..ab60a71a8dcb 100644
> --- a/arch/x86/include/asm/vsyscall.h
> +++ b/arch/x86/include/asm/vsyscall.h
> @@ -13,10 +13,12 @@ extern void set_vsyscall_pgtable_user_bits(pgd_t *root);
>   * Called on instruction fetch fault in vsyscall page.
>   * Returns true if handled.
>   */
> -extern bool emulate_vsyscall(struct pt_regs *regs, unsigned long address);
> +extern bool emulate_vsyscall(unsigned long error_code,
> +			     struct pt_regs *regs, unsigned long address);
>  #else
>  static inline void map_vsyscall(void) {}
> -static inline bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
> +static inline bool emulate_vsyscall(unsigned long error_code,
> +				    struct pt_regs *regs, unsigned long address)
>  {
>  	return false;
>  }
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 46df4c6aae46..288a5462076f 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1369,16 +1369,15 @@ void do_user_addr_fault(struct pt_regs *regs,
>  
>  #ifdef CONFIG_X86_64
>  	/*
> -	 * Instruction fetch faults in the vsyscall page might need
> -	 * emulation.  The vsyscall page is at a high address
> -	 * (>PAGE_OFFSET), but is considered to be part of the user
> -	 * address space.
> +	 * Faults in the vsyscall page might need emulation.  The
> +	 * vsyscall page is at a high address (>PAGE_OFFSET), but is
> +	 * considered to be part of the user address space.
>  	 *
>  	 * The vsyscall page does not have a "real" VMA, so do this
>  	 * emulation before we go searching for VMAs.
>  	 */
> -	if ((hw_error_code & X86_PF_INSTR) && is_vsyscall_vaddr(address)) {
> -		if (emulate_vsyscall(regs, address))
> +	if (is_vsyscall_vaddr(address)) {
> +		if (emulate_vsyscall(hw_error_code, regs, address))
>  			return;
>  	}
>  #endif
> -- 
> 2.21.0
> 

-- 
Kees Cook