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Message-ID: <201906271028.00EE29E9E@keescook>
Date: Thu, 27 Jun 2019 10:28:44 -0700
From: Kees Cook <keescook@...omium.org>
To: Andy Lutomirski <luto@...nel.org>
Cc: x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
Florian Weimer <fweimer@...hat.com>, Jann Horn <jannh@...gle.com>,
Borislav Petkov <bp@...en8.de>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 4/8] x86/vsyscall: Document odd SIGSEGV error code for
vsyscalls
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@...omium.org>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Kernel Hardening <kernel-hardening@...ts.openwall.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
Reviewed-by: Kees Cook <keescook@...omium.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
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