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Re: [PATCH 1/2] MIPS: ptrace: protect watchpoint handling code from sett

To: James Hogan <james.hogan@imgtec.com>
Subject: Re: [PATCH 1/2] MIPS: ptrace: protect watchpoint handling code from setting watchpoints
From: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Date: Fri, 13 Jan 2017 12:33:26 +0100
Cc: Ralf Baechle <ralf@linux-mips.org>, <linux-mips@linux-mips.org>
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Hi James,

Thanks for your comments

On 13.01.2017 12:11, James Hogan wrote:
Hi Marcin,

On Fri, Jan 13, 2017 at 08:44:46AM +0100, Marcin Nowakowski wrote:
With certain EVA configurations it is possible for the kernel address
space to overlap user address space, which allows the user to set
watchpoints on kernel addresses via ptrace.

If a watchpoint is set in the watch exception handling code (after
exception level has been cleared) then the system will hang in an
infinite loop when hitting a watchpoint while trying to process it.

To prevent that place all watch exception entry/exit code in a single
named section and disallow placing watchpoints in that area.

An interesting approach. If I understand correctly, any watch placed on
watch exception entry code up to when watchpoints are disabled in
do_watch and interrupts re-enabled is disallowed, as well as the code
after watchpoints are restored.

That's the intention. In theory it could be relaxed so that we can allow watchpoints to be placed _after_ watchpoint registers are cleared and before they are restored, but that would bring extra complexity and no benefit (at least I cannot see any).

Should mips_install_watch_registers() also move into that section?

It probably should, as we may end up hitting the watchpoint as soon as they are enabled again, and the same goes for the resume() method as well, which is called in switch_to() after mips_install_watch_registers().


Should do_watch be made notrace too, so we don't get calls into
unprotected ftrace code?

Good point.


Signed-off-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
---
 arch/mips/kernel/entry.S       |  2 +-
 arch/mips/kernel/genex.S       |  2 ++
 arch/mips/kernel/ptrace.c      | 14 ++++++++++++++
 arch/mips/kernel/traps.c       |  2 ++
 arch/mips/kernel/vmlinux.lds.S |  8 ++++++++
 5 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/arch/mips/kernel/entry.S b/arch/mips/kernel/entry.S
index 7791840..ef69a64 100644
--- a/arch/mips/kernel/entry.S
+++ b/arch/mips/kernel/entry.S
@@ -24,7 +24,7 @@
 #define __ret_from_irq ret_from_exception
 #endif

-       .text
+       .section .text..no_watch
        .align  5
 #ifndef CONFIG_PREEMPT
 FEXPORT(ret_from_exception)
diff --git a/arch/mips/kernel/genex.S b/arch/mips/kernel/genex.S
index dc0b296..102a9e8 100644
--- a/arch/mips/kernel/genex.S
+++ b/arch/mips/kernel/genex.S
@@ -433,6 +433,7 @@ NESTED(nmi_handler, PT_SIZE, sp)
        BUILD_HANDLER ftlb ftlb none silent             /* #16 */
        BUILD_HANDLER msa msa sti silent                /* #21 */
        BUILD_HANDLER mdmx mdmx sti silent              /* #22 */
+.section .text..no_watch
 #ifdef CONFIG_HARDWARE_WATCHPOINTS
        /*
         * For watch, interrupts will be enabled after the watch
@@ -442,6 +443,7 @@ NESTED(nmi_handler, PT_SIZE, sp)
 #else
        BUILD_HANDLER watch watch sti verbose           /* #23 */
 #endif
+.previous
        BUILD_HANDLER mcheck mcheck cli verbose         /* #24 */
        BUILD_HANDLER mt mt sti silent                  /* #25 */
        BUILD_HANDLER dsp dsp sti silent                /* #26 */
diff --git a/arch/mips/kernel/ptrace.c b/arch/mips/kernel/ptrace.c
index c8ba260..1c8d75c 100644
--- a/arch/mips/kernel/ptrace.c
+++ b/arch/mips/kernel/ptrace.c
@@ -235,6 +235,17 @@ int ptrace_get_watch_regs(struct task_struct *child,
        return 0;
 }

+static int ptrace_watch_in_watch_code_region(unsigned long addr)
+{
+       extern unsigned long __nowatch_text_start, __nowatch_text_end;
+       unsigned long start, end;
+
+       start = (((unsigned long)&__nowatch_text_start) & ~MIPS_WATCHLO_IRW);
+       end = (((unsigned long)&__nowatch_text_end) & ~MIPS_WATCHLO_IRW);
+
+       return addr >= start && addr < end;
+}
+
 int ptrace_set_watch_regs(struct task_struct *child,
                          struct pt_watch_regs __user *addr)
 {
@@ -262,6 +273,9 @@ int ptrace_set_watch_regs(struct task_struct *child,
                                return -EINVAL;
                }
 #endif
+               if (ptrace_watch_in_watch_code_region(lt[i]))
+                       return -EINVAL;

This would apparently permit userland to probe for the address of the
kernel on EVA kernels, regardless of KASLR (assuming that works with
EVA). Perhaps in that case we should be more restrictive and disallow
any watchpoints in the same segment as the kernel.

That's an unfortunate side-effect.

Actually for stable kernel purposes, it might be better to do that
first, i.e. disallow any to the same segments as kernel (tagged for
stable), then relax it as you've done here.

That sounds like a good idea.
What do you think about making my 'relaxed' approach depend on !KASLR?

+
                __get_user(ht[i], &addr->WATCH_STYLE.watchhi[i]);
                if (ht[i] & ~MIPS_WATCHHI_MASK)
                        return -EINVAL;
diff --git a/arch/mips/kernel/traps.c b/arch/mips/kernel/traps.c
index 6c7f9d7..b86ce85 100644
--- a/arch/mips/kernel/traps.c
+++ b/arch/mips/kernel/traps.c
@@ -1509,6 +1509,8 @@ asmlinkage void do_mdmx(struct pt_regs *regs)
  * Called with interrupts disabled.
  */
 asmlinkage void do_watch(struct pt_regs *regs)
+       __attribute__((section(".text..no_watch")));

maybe worth a macro, especially if any other functions do need this.

Will do

is a separate declaration really needed? could it go after void in the
definition like notrace does?

I need to check that. I had always thought that when attributes are added then a separate declaration is required, but if that's not the case then I will happily remove it, as it doesn't look too good.


Cheers
James

+asmlinkage void do_watch(struct pt_regs *regs)
 {
        siginfo_t info = { .si_signo = SIGTRAP, .si_code = TRAP_HWBKPT };
        enum ctx_state prev_state;
diff --git a/arch/mips/kernel/vmlinux.lds.S b/arch/mips/kernel/vmlinux.lds.S
index d5de675..f76f481 100644
--- a/arch/mips/kernel/vmlinux.lds.S
+++ b/arch/mips/kernel/vmlinux.lds.S
@@ -32,6 +32,13 @@ PHDRS {
        jiffies  = jiffies_64;
 #endif

+#define NOWATCH_TEXT                                                   \
+               ALIGN_FUNCTION();                                       \
+               VMLINUX_SYMBOL(__nowatch_text_start) = .;               \
+               *(.text..no_watch)                                      \
+               VMLINUX_SYMBOL(__nowatch_text_end) = .;
+
+
 SECTIONS
 {
 #ifdef CONFIG_BOOT_ELF64
@@ -54,6 +61,7 @@ SECTIONS
        _text = .;      /* Text and read-only data */
        .text : {
                TEXT_TEXT
+               NOWATCH_TEXT
                SCHED_TEXT
                CPUIDLE_TEXT
                LOCK_TEXT
--
2.7.4


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