Re: [PATCH v2 4/8] x86/vsyscall: Document odd SIGSEGV error code for vsyscalls

[Kees Cook <keescook@chromium.org>]

On Wed, Jun 26, 2019 at 09:45:05PM -0700, Andy Lutomirski wrote:
> Even if vsyscall=none, we report uer page faults on the vsyscall
> page as though the PROT bit in the error code was set.  Add a
> comment explaining why this is probably okay and display the value
> in the test case.
> 
> While we're at it, explain why our behavior is correct with respect
> to PKRU.
> 
> This also modifies the selftest to print the odd error code so that
> you can run the selftest and see that the behavior is odd.
> 
> If anyone really cares about more accurate emulation, we could
> change the behavior.
> 
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Kernel Hardening <kernel-hardening@lists.openwall.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Andy Lutomirski <luto@kernel.org>

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

-Kees

> ---
>  arch/x86/mm/fault.c                         | 7 +++++++
>  tools/testing/selftests/x86/test_vsyscall.c | 9 ++++++++-
>  2 files changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 288a5462076f..58e4f1f00bbc 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -710,6 +710,10 @@ static void set_signal_archinfo(unsigned long address,
>  	 * To avoid leaking information about the kernel page
>  	 * table layout, pretend that user-mode accesses to
>  	 * kernel addresses are always protection faults.
> +	 *
> +	 * NB: This means that failed vsyscalls with vsyscall=none
> +	 * will have the PROT bit.  This doesn't leak any
> +	 * information and does not appear to cause any problems.
>  	 */
>  	if (address >= TASK_SIZE_MAX)
>  		error_code |= X86_PF_PROT;
> @@ -1375,6 +1379,9 @@ void do_user_addr_fault(struct pt_regs *regs,
>  	 *
>  	 * The vsyscall page does not have a "real" VMA, so do this
>  	 * emulation before we go searching for VMAs.
> +	 *
> +	 * PKRU never rejects instruction fetches, so we don't need
> +	 * to consider the PF_PK bit.
>  	 */
>  	if (is_vsyscall_vaddr(address)) {
>  		if (emulate_vsyscall(hw_error_code, regs, address))
> diff --git a/tools/testing/selftests/x86/test_vsyscall.c b/tools/testing/selftests/x86/test_vsyscall.c
> index 0b4f1cc2291c..4c9a8d76dba0 100644
> --- a/tools/testing/selftests/x86/test_vsyscall.c
> +++ b/tools/testing/selftests/x86/test_vsyscall.c
> @@ -183,9 +183,13 @@ static inline long sys_getcpu(unsigned * cpu, unsigned * node,
>  }
>  
>  static jmp_buf jmpbuf;
> +static volatile unsigned long segv_err;
>  
>  static void sigsegv(int sig, siginfo_t *info, void *ctx_void)
>  {
> +	ucontext_t *ctx = (ucontext_t *)ctx_void;
> +
> +	segv_err =  ctx->uc_mcontext.gregs[REG_ERR];
>  	siglongjmp(jmpbuf, 1);
>  }
>  
> @@ -416,8 +420,11 @@ static int test_vsys_r(void)
>  	} else if (!can_read && should_read_vsyscall) {
>  		printf("[FAIL]\tWe don't have read access, but we should\n");
>  		return 1;
> +	} else if (can_read) {
> +		printf("[OK]\tWe have read access\n");
>  	} else {
> -		printf("[OK]\tgot expected result\n");
> +		printf("[OK]\tWe do not have read access: #PF(0x%lx)\n",
> +		       segv_err);
>  	}
>  #endif
>  
> -- 
> 2.21.0
> 

-- 
Kees Cook